Good morning students, welcome to today's science class. I am very happy to be teaching you this important chapter on carbon and its compounds. This is one of the most fascinating chapters in your science textbook because carbon is the element that forms the basis of all life on Earth. In fact, you and I are literally made up of carbon compounds! So let's begin our journey into the world of carbon together.
In the previous chapter, you studied about many compounds that are important to us. Today, we will learn about some more interesting compounds and their properties. More importantly, we will learn about carbon, an element which is of immense significance to us in both its elemental form and in its combined form.
Let me start with an activity that will help you understand why we are studying carbon.
Activity 4.1 - Let's do this together. I want you to make a list of ten things you have used or consumed since this morning. Think about your breakfast, the clothes you are wearing, the books you read, the medicines you might have taken, and so on. Now, let's compile this list with your classmates and sort the items into a table with three columns: things made of metal, things made of glass or clay, and other things.
Now look at the items that come in the last column - the "others" column. Your teacher will tell you that most of these are made up of compounds of carbon. Can you think of a method to test this? What would be the product if a compound containing carbon is burnt? Do you know of any test to confirm this?
Let me tell you something interesting. Food, clothes, medicines, books, or many of the things that you listed are all based on this versatile element carbon. In addition, all living structures are carbon based. The amount of carbon present in the earth's crust and in the atmosphere is quite meagre. The earth's crust has only 0.02% carbon in the form of minerals like carbonates, hydrogen-carbonates, coal and petroleum, and the atmosphere has 0.03% of carbon dioxide. In spite of this small amount of carbon available in nature, the importance of carbon seems to be immense. In this chapter, we will know about the properties of carbon which make carbon so important to us.
Now let's begin with the most fundamental concept - how carbon forms bonds.
## 4.1 Bonding in Carbon - The Covalent Bond
In the previous chapter, we studied the properties of ionic compounds. We saw that ionic compounds have high melting and boiling points and conduct electricity in solution or in the molten state. We also saw how the nature of bonding in ionic compounds explains these properties. Let us now study the properties of some carbon compounds.
Most carbon compounds are poor conductors of electricity as we have seen in Chapter 2. From the data given in Table 4.1 on the boiling and melting points of the carbon compounds, we find that these compounds have low melting and boiling points as compared to ionic compounds. We can conclude that the forces of attraction between the molecules are not very strong. Since these compounds are largely non-conductors of electricity, we can conclude that the bonding in these compounds does not give rise to any ions.
Look at Table 4.1 carefully. Acetic acid has a melting point of 290 K and boiling point of 391 K. Chloroform melts at 209 K and boils at 334 K. Ethanol melts at 156 K and boils at 351 K. Methane melts at just 90 K and boils at 111 K. Compare these with common salt, sodium chloride, which melts at 1074 K and boils at 1686 K. The difference is huge! This tells us that carbon compounds have much weaker intermolecular forces compared to ionic compounds.
Now, let's understand why this happens. In Class IX, we learnt about the combining capacity of various elements and how it depends on the number of valence electrons. Let us now look at the electronic configuration of carbon. The atomic number of carbon is 6. What would be the distribution of electrons in various shells of carbon? The first shell can hold 2 electrons, and the second shell can hold 8 electrons. So carbon has 2 electrons in the first shell and 4 electrons in the second shell. How many valence electrons will carbon have? That's right, carbon has 4 valence electrons in its outermost shell.
We know that the reactivity of elements is explained as their tendency to attain a completely filled outer shell, that is, attain noble gas configuration. Elements forming ionic compounds achieve this by either gaining or losing electrons from the outermost shell. In the case of carbon, it has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration. If it were to gain or lose electrons, let's see what happens.
First, it could gain four electrons forming a C⁴⁻ anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons. The positive nucleus would not be able to hold the negatively charged electrons strongly enough.
Second, it could lose four electrons forming a C⁴⁺ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons. This would also be very unstable.
So carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements. Not just carbon, but many other elements form molecules by sharing electrons in this manner. The shared electrons 'belong' to the outermost shells of both the atoms and lead to both atoms attaining the noble gas configuration.
This type of bonding where electrons are shared between atoms is called covalent bonding, and the bonds formed are called covalent bonds. Let me explain this with some examples.
The simplest molecule formed in this manner is that of hydrogen. As you have learnt earlier, the atomic number of hydrogen is 1. Hence hydrogen has one electron in its K shell and it requires one more electron to fill the K shell. So two hydrogen atoms share their electrons to form a molecule of hydrogen, H₂. This allows each hydrogen atom to attain the electronic configuration of the nearest noble gas, helium, which has two electrons in its K shell.
The shared pair of electrons is said to constitute a single covalent bond between the two hydrogen atoms. A single covalent bond is also represented by a line between the two atoms.
Now let's look at chlorine. The atomic number of chlorine is 17. Its electronic configuration is 2, 8, 7. So chlorine has 7 valence electrons and needs one more electron to complete its octet. So two chlorine atoms share their electrons to form a diatomic molecule, Cl₂. Each chlorine atom contributes one electron to the shared pair, and both atoms achieve the noble gas configuration of argon.
Now let's look at oxygen. An atom of oxygen has six electrons in its L shell because its atomic number is 8. It requires two more electrons to complete its octet. So each atom of oxygen shares two electrons with another atom of oxygen to form a double bond. The two electrons contributed by each oxygen atom give rise to two shared pairs of electrons. This is said to constitute a double bond between the two atoms.
Now, what about water? Can you depict a molecule of water showing the nature of bonding between one oxygen atom and two hydrogen atoms? Oxygen has 6 valence electrons. It needs 2 more electrons. Each hydrogen provides 1 electron. So oxygen shares one electron with each of the two hydrogen atoms. This gives us two single bonds - one with each hydrogen atom. The molecule has single bonds, not double bonds.
Now let's think about nitrogen. Nitrogen has the atomic number 7. Its electronic configuration is 2, 5. So it has 5 valence electrons and needs 3 more electrons to complete its octet. In a molecule of nitrogen, each nitrogen atom contributes three electrons giving rise to three shared pairs of electrons. This is said to constitute a triple bond between the two atoms. So N₂ has a triple bond.
Now let's look at ammonia. A molecule of ammonia has the formula NH₃. Nitrogen has 5 valence electrons and needs 3 more. Each hydrogen provides 1 electron. So nitrogen shares one electron with each of the three hydrogen atoms, forming three single bonds. All four atoms achieve noble gas configuration - nitrogen achieves octet, and each hydrogen achieves duplet.
Now let us take a look at methane, which is a compound of carbon. Methane is widely used as a fuel and is a major component of biogas and Compressed Natural Gas, CNG. It is also one of the simplest compounds formed by carbon. Methane has a formula CH₄. Hydrogen, as you know, has a valency of 1. Carbon is tetravalent because it has four valence electrons. In order to achieve noble gas configuration, carbon shares these electrons with four atoms of hydrogen.
Such bonds which are formed by the sharing of an electron pair between two atoms are known as covalent bonds. Covalently bonded molecules are seen to have strong bonds within the molecule, but intermolecular forces are weak. This gives rise to the low melting and boiling points of these compounds. Since the electrons are shared between atoms and no charged particles are formed, such covalent compounds are generally poor conductors of electricity.
So let me recap what we learned in this section. Carbon has four valence electrons. It cannot lose or gain electrons easily because that would be energetically unfavorable. Instead, it shares electrons with other atoms to form covalent bonds. These covalent bonds can be single bonds (one shared pair), double bonds (two shared pairs), or triple bonds (three shared pairs). Covalent compounds have low melting and boiling points because the intermolecular forces are weak, and they are generally poor conductors of electricity because no ions are formed.
Now let's answer the questions based on this section.
Question 1: What would be the electron dot structure of carbon dioxide which has the formula CO₂?
Let me draw this for you. Carbon has 4 valence electrons, oxygen has 6 valence electrons each. Carbon needs 4 more electrons, and each oxygen needs 2 more electrons. The only way this works is if carbon forms double bonds with both oxygen atoms. So we have O=C=O. Each oxygen shares two electrons with carbon, and carbon shares two electrons with each oxygen. This gives each oxygen atom 8 electrons in its outer shell, and carbon also has 8 electrons in its outer shell. The electron dot structure would show two pairs of electrons shared between carbon and each oxygen, with each oxygen also having two lone pairs of electrons.
Question 2: What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? The hint says the eight atoms of sulphur are joined together in the form of a ring.
Sulphur has atomic number 16, so its electronic configuration is 2, 8, 6. It has 6 valence electrons and needs 2 more to complete its octet. In a ring of eight sulphur atoms, each sulphur atom is bonded to two other sulphur atoms. Each sulphur atom shares two electrons with each of its two neighboring sulphur atoms, forming single bonds. Each sulphur atom also has two lone pairs of electrons. So the structure is a ring of eight sulphur atoms, each connected to its neighbours by single covalent bonds.
Now let's move on to learn about allotropes of carbon.
## Allotropes of Carbon
The element carbon occurs in different forms in nature with widely varying physical properties. Both diamond and graphite are formed by carbon atoms, but the difference lies in the manner in which the carbon atoms are bonded to one another.
In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure. This creates a very strong network of carbon atoms, making diamond the hardest substance known to us.
In graphite, each carbon atom is bonded to three other carbon atoms in the same plane giving a hexagonal array. One of these bonds is a double-bond, and thus the valency of carbon is satisfied. Graphite structure is formed by the hexagonal arrays being placed in layers one above the other. Because the layers can slide over each other easily, graphite feels slippery and is used as a lubricant. Also, since one electron from each carbon atom is free to move, graphite is a very good conductor of electricity, unlike other non-metals.
These two different structures result in diamond and graphite having very different physical properties even though their chemical properties are the same. Diamond is the hardest substance known while graphite is smooth and slippery. Graphite is also a very good conductor of electricity.
Diamonds can be synthesised by subjecting pure carbon to very high pressure and temperature. These synthetic diamonds are small but are otherwise indistinguishable from natural diamonds.
Fullerenes form another class of carbon allotropes. The first one to be identified was C-60 which has carbon atoms arranged in the shape of a football. Since this looked like the geodesic dome designed by the US architect Buckminster Fuller, the molecule was named fullerene. There are also other fullerenes like C-70, C-76, and so on.
Now let's move on to the next section to understand why carbon forms such a huge number of compounds.
## 4.2 Versatile Nature of Carbon
We have seen the formation of covalent bonds by the sharing of electrons in various elements and compounds. We have also seen the structure of a simple carbon compound, methane. In the beginning of the chapter, we saw how many things we use contain carbon. In fact, we ourselves are made up of carbon compounds. The numbers of carbon compounds whose formulae are known to chemists was recently estimated to be in millions! This outnumbers by a large margin the compounds formed by all the other elements put together.
Why is it that this property is seen in carbon and no other element?
The nature of the covalent bond enables carbon to form a large number of compounds. Two factors noticed in the case of carbon are:
First, carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation. These compounds may have long chains of carbon, branched chains of carbon or even carbon atoms arranged in rings. In addition, carbon atoms may be linked by single, double or triple bonds. Compounds of carbon which are linked by only single bonds between the carbon atoms are called saturated compounds. Compounds of carbon having double or triple bonds between their carbon atoms are called unsaturated compounds.
No other element exhibits the property of catenation to the extent seen in carbon compounds. Silicon forms compounds with hydrogen which have chains of up to seven or eight atoms, but these compounds are very reactive. The carbon-carbon bond is very strong and hence stable. This gives us the large number of compounds with many carbon atoms linked to each other.
Second, since carbon has a valency of four, it is capable of bonding with four other atoms of carbon or atoms of some other mono-valent element. Compounds of carbon are formed with oxygen, hydrogen, nitrogen, sulphur, chlorine and many other elements giving rise to compounds with specific properties which depend on the elements other than carbon present in the molecule.
Again the bonds that carbon forms with most other elements are very strong making these compounds exceptionally stable. One reason for the formation of strong bonds by carbon is its small size. This enables the nucleus to hold on to the shared pairs of electrons strongly. The bonds formed by elements having bigger atoms are much weaker.
So the two main reasons for the huge number of carbon compounds are: first, catenation - the ability of carbon to form bonds with other carbon atoms, and second, tetravalency - the ability of carbon to form bonds with four other atoms.
Now let's learn about organic compounds.
### Organic Compounds
The two characteristic features seen in carbon, that is, tetravalency and catenation, put together give rise to a large number of compounds. Many have the same non-carbon atom or group of atoms attached to different carbon chains. These compounds were initially extracted from natural substances and it was thought that these carbon compounds or organic compounds could only be formed within a living system. That is, it was postulated that a 'vital force' was necessary for their synthesis. Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate. But carbon compounds, except for carbides, oxides of carbon, carbonate and hydrogencarbonate salts continue to be studied under organic chemistry.
This is a very important historical point. For a long time, scientists believed that organic compounds could only be produced by living organisms through some mysterious "vital force." But in 1828, Friedrich Wöhler accidentally synthesized urea, a waste product excreted in urine, from ammonium cyanate, which is an inorganic compound. This experiment proved that organic compounds can be synthesized in the laboratory without any vital force, and this marked the beginning of modern organic chemistry.
Now let's learn about saturated and unsaturated carbon compounds.
### 4.2.1 Saturated and Unsaturated Carbon Compounds
We have already seen the structure of methane. Another compound formed between carbon and hydrogen is ethane with a formula of C₂H₆.
In order to arrive at the structure of simple carbon compounds, the first step is to link the carbon atoms together with a single bond and then use the hydrogen atoms to satisfy the remaining valencies of carbon. For example, the structure of ethane is arrived in the following steps.
Step 1: We connect the two carbon atoms with a single bond: C—C
Step 2: Three valencies of each carbon atom remain unsatisfied, so each is bonded to three hydrogen atoms giving: H—C—C—H with three hydrogens on each carbon.
The electron dot structure of ethane shows each carbon sharing electrons with three hydrogen atoms and one carbon atom.
Now, can you draw the structure of propane, which has the molecular formula C₃H₈ in a similar manner? You will see that the valencies of all the atoms are satisfied by single bonds between them. Such carbon compounds are called saturated compounds. These compounds are normally not very reactive.
However, another compound of carbon and hydrogen has the formula C₂H₄ and is called ethene. How can this molecule be depicted? We follow the same step-wise approach as above.
Step 1: Carbon-carbon atoms linked together with a single bond: C—C
Step 2: We see that one valency per carbon atom remains unsatisfied. This can be satisfied only if there is a double bond between the two carbons.
Step 3: So we get H—C=C—H with each carbon having two hydrogens.
The electron dot structure for ethene shows a double bond between the two carbon atoms.
Yet another compound of hydrogen and carbon has the formula C₂H₂ and is called ethyne. Can you draw the electron dot structure for ethyne? How many bonds are necessary between the two carbon atoms in order to satisfy their valencies? That's right, we need a triple bond: H—C≡C—H
Such compounds of carbon having double or triple bonds between the carbon atoms are known as unsaturated carbon compounds and they are more reactive than the saturated carbon compounds.
So to summarize: Saturated carbon compounds have only single bonds between carbon atoms and are less reactive. Unsaturated carbon compounds have double or triple bonds between carbon atoms and are more reactive.
Now let's learn about chains, branches and rings.
### 4.2.2 Chains, Branches and Rings
In the earlier section, we mentioned the carbon compounds methane, ethane and propane, containing respectively 1, 2 and 3 carbon atoms. Such 'chains' of carbon atoms can contain many more carbon atoms. The names and structures of six of these are given in Table 4.2.
Let me explain the table to you:
For 1 carbon atom: Methane, CH₄. Structure: one carbon with four hydrogens.
For 2 carbon atoms: Ethane, C₂H₆. Structure: two carbons bonded to each other, each with three hydrogens.
For 3 carbon atoms: Propane, C₃H₈. Structure: three carbons in a chain, with hydrogens on each carbon.
For 4 carbon atoms: Butane, C₄H₁₀. Structure: four carbons in a chain, with hydrogens on each carbon.
For 5 carbon atoms: Pentane, C₅H₁₂. Structure: five carbons in a chain.
For 6 carbon atoms: Hexane, C₆H₁₄. Structure: six carbons in a chain.
But, let us take another look at butane. If we make the carbon 'skeleton' with four carbon atoms, we see that two different possible 'skeletons' are:
First, a straight chain: C—C—C—C
Second, a branched chain: C—C(C)—C where one carbon branches off from the second carbon.
Filling the remaining valencies with hydrogen gives us two different structures. We see that both these structures have the same formula C₄H₁₀. Such compounds with identical molecular formula but different structures are called structural isomers.
This is very important! Structural isomers are compounds that have the same molecular formula but different structural arrangements of atoms. Butane has two structural isomers: n-butane (straight chain) and isobutane (branched chain).
In addition to straight and branched carbon chains, some compounds have carbon atoms arranged in the form of a ring. For example, cyclohexane has the formula C₆H₁₂ and has a ring structure with each carbon bonded to two other carbons and two hydrogens.
Can you draw the electron dot structure for cyclohexane? Each carbon in the ring shares one electron with each of its two neighboring carbons, and also shares one electron with a hydrogen atom. Each carbon also has one lone pair of electrons.
Straight chain, branched chain and cyclic carbon compounds, all may be saturated or unsaturated. For example, benzene, C₆H₆, has a ring structure with alternating double bonds.
All these carbon compounds which contain only carbon and hydrogen are called hydrocarbons. Among these, the saturated hydrocarbons are called alkanes. The unsaturated hydrocarbons which contain one or more double bonds are called alkenes. Those containing one or more triple bonds are called alkynes.
So we have: - Alkanes: saturated hydrocarbons with single bonds (general formula CₙH₂ₙ₊₂) - Alkenes: unsaturated hydrocarbons with double bonds (general formula CₙH₂ₙ) - Alkynes: unsaturated hydrocarbons with triple bonds (general formula CₙH₂ₙ₋₂)
Now let's learn about functional groups.
### 4.2.3 Will You Be My Friend?
Carbon seems to be a very friendly element. So far we have been looking at compounds containing carbon and hydrogen only. But carbon also forms bonds with other elements such as halogens, oxygen, nitrogen and sulphur. In a hydrocarbon chain, one or more hydrogens can be replaced by these elements, such that the valency of carbon remains satisfied. In such compounds, the element replacing hydrogen is referred to as a heteroatom. These heteroatoms are also present in some groups.
Let me give you Table 4.3 which shows some functional groups in carbon compounds:
If the hetero atom is chlorine or bromine, we have halo-alkanes with the functional group —Cl or —Br.
If the hetero atom is oxygen, we have different classes of compounds: - Alcohol: functional group —OH - Aldehyde: functional group —CHO (shown as —C=O with H attached) - Ketone: functional group —C=O (carbonyl group) - Carboxylic acid: functional group —COOH (carbonyl group with OH)
These heteroatoms and the group containing these confer specific properties to the compound, regardless of the length and nature of the carbon chain and hence are called functional groups. The functional group is attached to the carbon chain through this valency by replacing one hydrogen atom or atoms.
So a functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical behavior of that molecule. For example, all alcohols contain the —OH group and show similar chemical properties, regardless of the size of the carbon chain.
Now let's learn about homologous series.
### 4.2.4 Homologous Series
You have seen that carbon atoms can be linked together to form chains of varying lengths. These chains can be branched also. In addition, hydrogen atom or other atoms on these carbon chains can be replaced by any of the functional groups that we saw above. The presence of a functional group such as alcohol decides the properties of the carbon compound, regardless of the length of the carbon chain. For example, the chemical properties of CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH are all very similar. Hence, such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.
Let us look at the homologous series that we saw earlier in Table 4.2. If we look at the formulae of successive compounds: CH₄ and C₂H₆ — these differ by a —CH₂— unit C₂H₆ and C₃H₈ — these differ by a —CH₂— unit
What is the difference between the next pair – propane and butane (C₄H₁₀)? They also differ by a —CH₂— unit.
Can you find out the difference in molecular masses between these pairs? The atomic mass of carbon is 12 u and the atomic mass of hydrogen is 1 u. So the difference in molecular mass between CH₄ (12 + 4 = 16 u) and C₂H₆ (24 + 6 = 30 u) is 14 u, which is exactly the mass of a —CH₂— group (12 + 2 = 14 u). Similarly, the difference between C₂H₆ and C₃H₈ is also 14 u.
Similarly, take the homologous series for alkenes. The first member of the series is ethene which we have already come across. What is the formula for ethene? C₂H₄. The succeeding members have the formula C₃H₆, C₄H₈ and C₅H₁₀. Do these also differ by a —CH₂— unit? Yes, they do! Do you see any relation between the number of carbon and hydrogen atoms in these compounds? The general formula for alkenes can be written as CₙH₂ₙ, where n = 2, 3, 4.
Can you similarly generate the general formula for alkanes and alkynes? For alkanes, it is CₙH₂ₙ₊₂. For alkynes, it is CₙH₂ₙ₋₂.
As the molecular mass increases in any homologous series, a gradation in physical properties is seen. This is because the melting and boiling points increase with increasing molecular mass. Other physical properties such as solubility in a particular solvent also show a similar gradation. But the chemical properties, which are determined solely by the functional group, remain similar in a homologous series.
Now let's do Activity 4.2.
Activity 4.2: Calculate the difference in the formulae and molecular masses for: a) CH₃OH and C₂H₅OH b) C₂H₅OH and C₃H₇OH c) C₃H₇OH and C₄H₉OH
Let's calculate: a) CH₃OH has formula CH₄O with molecular mass 12 + 4 + 16 = 32 u C₂H₅OH has formula C₂H₆O with molecular mass 24 + 6 + 16 = 46 u Difference: 46 - 32 = 14 u, which is a —CH₂— unit.
b) C₂H₅OH has molecular mass 46 u C₃H₇OH has formula C₃H₈O with molecular mass 36 + 8 + 16 = 60 u Difference: 60 - 46 = 14 u, again a —CH₂— unit.
c) C₃H₇OH has molecular mass 60 u C₄H₉OH has formula C₄H₁₀O with molecular mass 48 + 10 + 16 = 74 u Difference: 74 - 60 = 14 u, again a —CH₂— unit.
Is there any similarity in these three? Yes, they all differ by a —CH₂— unit.
Arrange these alcohols in the order of increasing carbon atoms to get a family. Can we call this family a homologous series? Yes, we can! They are methanol, ethanol, propanol, and butanol. They all contain the —OH functional group and differ from each other by a —CH₂— unit.
Now let's learn about nomenclature of carbon compounds.
### 4.2.5 Nomenclature of Carbon Compounds
The names of compounds in a homologous series are based on the name of the basic carbon chain modified by a "prefix" or "suffix" indicating the nature of the functional group. For example, the names of the alcohols taken in Activity 4.2 are methanol, ethanol, propanol and butanol.
Naming a carbon compound can be done by the following method:
First, identify the number of carbon atoms in the compound. A compound having three carbon atoms would have the name propane.
Second, in case a functional group is present, it is indicated in the name of the compound with either a prefix or a suffix.
Third, if the name of the functional group is to be given as a suffix, and the suffix of the functional group begins with a vowel (a, e, i, o, u), then the name of the carbon chain is modified by deleting the final 'e' and adding the appropriate suffix. For example, a three-carbon chain with a ketone group would be named in the following manner: Propane – 'e' = propan + 'one' = propanone.
Fourth, if the carbon chain is unsaturated, then the final 'ane' in the name of the carbon chain is substituted by 'ene' or 'yne'. For example, a three-carbon chain with a double bond would be called propene and if it has a triple bond, it would be called propyne.
Let me give you Table 4.4 which summarizes the nomenclature of organic compounds:
For halo alkanes, we use the prefix chloro, bromo, etc. Example: Chloropropane.
For alcohols, we use the suffix -ol. Example: Propanol.
For aldehydes, we use the suffix -al. Example: Propanal.
For ketones, we use the suffix -one. Example: Propanone.
For carboxylic acids, we use the suffix -oic acid. Example: Propanoic acid.
For alkenes, we use the suffix -ene. Example: Propene.
For alkynes, we use the suffix -yne. Example: Propyne.
Now let's answer the questions based on sections 4.2.1 to 4.2.5.
Question 1: How many structural isomers can you draw for pentane?
Pentane has the molecular formula C₅H₁₂. The structural isomers are: a) n-pentane: a straight chain of five carbons b) isopentane: a four-carbon chain with one branch c) neopentane: a three-carbon chain with two branches on the central carbon
So there are 3 structural isomers for pentane.
Question 2: What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
The two properties are: a) Catenation - the ability of carbon to form bonds with other carbon atoms b) Tetravalency - the ability of carbon to form bonds with four other atoms
Question 3: What will be the formula and electron dot structure of cyclopentane?
Cyclopentane has the formula C₅H₁₀. It has a ring of five carbon atoms, each carbon bonded to two other carbons in the ring and to two hydrogen atoms. The electron dot structure shows each carbon sharing one electron with each of its two neighboring carbons, and one electron with a hydrogen atom. Each carbon also has one lone pair of electrons.
Question 4: Draw the structures for the following compounds: i) Ethanoic acid ii) Bromopentane iii) Butanone iv) Hexanal
Let me explain each:
i) Ethanoic acid: CH₃COOH. It has two carbons. The first carbon is bonded to three hydrogens and one carbon. The second carbon is double-bonded to one oxygen and single-bonded to an OH group.
ii) Bromopentane: C₅H₁₁Br. It has a chain of five carbons with a bromine atom attached to one of them. There can be different structural isomers depending on where the bromine is attached.
iii) Butanone: C₄H₈O. It has four carbons with a ketone group (C=O) on the second carbon. The structure is CH₃—CO—CH₂—CH₃.
iv) Hexanal: C₆H₁₂O. It has six carbons with an aldehyde group (CHO) at the end. The structure is CH₃—CH₂—CH₂—CH₂—CH₂—CHO.
Are structural isomers possible for bromopentane? Yes, structural isomers are possible for bromopentane because the bromine atom can be attached to different carbon atoms in the pentane chain, giving different structural isomers.
Question 5: How would you name the following compounds?
i) CH₃—CH₂—Br This is bromoethane. It has two carbons with a bromine on the second carbon.
ii) H—C=O with H attached to the carbon This is methanal, also known as formaldehyde.
iii) H—C—C—C—C—C≡C—H with hydrogens on each carbon This is a six-carbon chain with a triple bond at the end. Counting the carbons: there are 6 carbons in a chain with a triple bond between the fifth and sixth carbons. The name is hex-5-yne or 5-hexyne.
Now let's move on to the chemical properties of carbon compounds.
## 4.3 Chemical Properties of Carbon Compounds
In this section we will be studying about some of the chemical properties of carbon compounds. Since most of the fuels we use are either carbon or its compounds, we shall first study combustion.
### 4.3.1 Combustion
Carbon, in all its allotropic forms, burns in oxygen to give carbon dioxide along with the release of heat and light. Most carbon compounds also release a large amount of heat and light on burning. These are the oxidation reactions that you learnt about in the first chapter:
First reaction: C + O₂ → CO₂ + heat and light
Second reaction: CH₄ + O₂ → CO₂ + H₂O + heat and light
Third reaction: CH₃CH₂OH + O₂ → CO₂ + H₂O + heat and light
Let me balance the latter two reactions for you.
For methane: CH₄ + 2O₂ → CO₂ + 2H₂O
For ethanol: CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O
Now let's do Activity 4.3.
Activity 4.3: Take some carbon compounds like naphthalene, camphor, and alcohol one by one on a spatula and burn them. Observe the nature of the flame and note whether smoke is produced. Place a metal plate above the flame. Is there a deposition on the plate in case of any of the compounds?
You will observe that some compounds burn with a clean flame while others produce smoke. The smoke is actually carbon particles (soot) that did not completely burn.
Now let's do Activity 4.4.
Activity 4.4: Light a bunsen burner and adjust the air hole at the base to get different types of flames. When do you get a yellow, sooty flame? When do you get a blue flame?
When there is insufficient air, we get a yellow, sooty flame. When there is sufficient air, we get a clean blue flame.
Saturated hydrocarbons will generally give a clean flame while unsaturated carbon compounds will give a yellow flame with lots of black smoke. This results in a sooty deposit on the metal plate in Activity 4.3. However, limiting the supply of air results in incomplete combustion of even saturated hydrocarbons giving a sooty flame. The gas or kerosene stove used at home has inlets for air so that a sufficiently oxygen-rich mixture is burnt to give a clean blue flame. If you observe the bottoms of cooking vessels getting blackened, it means that the air holes are blocked and fuel is getting wasted. Fuels such as coal and petroleum have some amount of nitrogen and sulphur in them. Their combustion results in the formation of oxides of sulphur and nitrogen which are major pollutants in the environment.
Now let me tell you something interesting - why do substances burn with or without a flame?
Have you ever observed either a coal or a wood fire? If not, the next time you get a chance, take close note of what happens when the wood or coal starts to burn. You have seen above that a candle or the LPG in the gas stove burns with a flame. However, you will observe the coal or charcoal in an 'angithi' sometimes just glows red and gives out heat without a flame. This is because a flame is only produced when gaseous substances burn. When wood or charcoal is ignited, the volatile substances present vapourise and burn with a flame in the beginning.
A luminous flame is seen when the atoms of the gaseous substance are heated and start to glow. The colour produced by each element is a characteristic property of that element. Try and heat a copper wire in the flame of a gas stove and observe its colour. You have seen that incomplete combustion gives soot which is carbon. On this basis, what will you attribute the yellow colour of a candle flame to? The yellow colour is due to the incandescence of carbon particles (soot) that are heated and glow.
Now let's learn about the formation of coal and petroleum.
Coal and petroleum have been formed from biomass which has been subjected to various biological and geological processes. Coal is the remains of trees, ferns, and other plants that lived millions of years ago. These were crushed into the earth, perhaps by earthquakes or volcanic eruptions. They were pressed down by layers of earth and rock. They slowly decayed into coal. Oil and gas are the remains of millions of tiny plants and animals that lived in the sea. When they died, their bodies sank to the sea bed and were covered by silt. Bacteria attacked the dead remains, turning them into oil and gas under the high pressures they were being subjected to. Meanwhile, the silt was slowly compressed into rock. The oil and gas seeped into the porous parts of the rock, and got trapped like water in a sponge. Can you guess why coal and petroleum are called fossil fuels? Because they are formed from the fossilized remains of ancient plants and animals.
Now let's learn about oxidation.
### 4.3.2 Oxidation
Activity 4.5: Take about 3 mL of ethanol in a test tube and warm it gently in a water bath. Add a 5% solution of alkaline potassium permanganate drop by drop to this solution. Does the colour of potassium permanganate persist when it is added initially? Why does the colour of potassium permanganate not disappear when excess is added?
You will observe that when you add alkaline potassium permanganate to ethanol, the purple colour of potassium permanganate initially disappears because it gets reduced. But when excess is added, the colour persists because all the ethanol has been converted to ethanoic acid, and the excess potassium permanganate remains unreacted.
You have learnt about oxidation reactions in the first chapter. Carbon compounds can be easily oxidised on combustion. In addition to this complete oxidation, we have reactions in which alcohols are converted to carboxylic acids:
Alkaline KMnO₄ + Heat CH₃ – CH₂OH ─────────────────→ CH₃COOH
Or acidified K₂Cr₂O₇ + Heat CH₃ – CH₂OH ─────────────────→ CH₃COOH
We see that some substances are capable of adding oxygen to others. These substances are known as oxidising agents. Alkaline potassium permanganate or acidified potassium dichromate are oxidising agents - they add oxygen to alcohols to form acids.
Now let's learn about addition reaction.
### 4.3.3 Addition Reaction
Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons. Catalysts are substances that cause a reaction to occur or proceed at a different rate without the reaction itself being affected. This reaction is commonly used in the hydrogenation of vegetable oils using a nickel catalyst. Vegetable oils generally have long unsaturated carbon chains while animal fats have saturated carbon chains.
The reaction can be written as: R—CH=CH—R + H₂ → R—CH₂—CH₂—R (in the presence of nickel catalyst)
You must have seen advertisements stating that some vegetable oils are 'healthy'. Animal fats generally contain saturated fatty acids which are said to be harmful for health. Oils containing unsaturated fatty acids should be chosen for cooking. However, when vegetable oils are hydrogenated to make vanaspati or ghee, they become saturated and less healthy.
Now let's learn about substitution reaction.
### 4.3.4 Substitution Reaction
Saturated hydrocarbons are fairly unreactive and are inert in the presence of most reagents. However, in the presence of sunlight, chlorine is added to hydrocarbons in a very fast reaction. Chlorine can replace the hydrogen atoms one by one. It is called a substitution reaction because one type of atom or a group of atoms takes the place of another. A number of products are usually formed with the higher homologues of alkanes.
The reaction is: CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight)
This is called chloromethane or methyl chloride.
Now let's answer the questions based on section 4.3.
Question 1: Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
Because oxygen is added to ethanol to convert it to ethanoic acid. Ethanol (C₂H₅OH) gains an oxygen atom to become ethanoic acid (CH₃COOH). This is oxidation.
Question 2: A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Because air contains only about 21% oxygen, so the combustion would be incomplete. This would produce a sooty, yellow flame with lower temperature, which is not suitable for welding. Pure oxygen gives a clean, hot flame that can melt metals.
Now let's learn about some important carbon compounds - ethanol and ethanoic acid.
## 4.4 Some Important Carbon Compounds – Ethanol and Ethanoic Acid
Many carbon compounds are invaluable to us. But here we shall study the properties of two commercially important compounds – ethanol and ethanoic acid.
### 4.4.1 Properties of Ethanol
Ethanol is a liquid at room temperature. Ethanol is commonly called alcohol and is the active ingredient of all alcoholic drinks. In addition, because it is a good solvent, it is also used in medicines such as tincture iodine, cough syrups, and many tonics. Ethanol is also soluble in water in all proportions. Consumption of small quantities of dilute ethanol causes drunkenness. Even though this practice is condemned, it is a socially widespread practice. However, intake of even a small quantity of pure ethanol (called absolute alcohol) can be lethal. Also, long-term consumption of alcohol leads to many health problems.
Now let's look at the reactions of ethanol.
First reaction: Reaction with sodium.
Activity 4.6: Teacher's demonstration. Drop a small piece of sodium, about the size of a couple of grains of rice, into ethanol (absolute alcohol). What do you observe? How will you test the gas evolved?
When sodium reacts with ethanol, hydrogen gas is evolved. You can test this by bringing a burning matchstick near the gas - it will burn with a pop sound, confirming hydrogen.
The reaction is: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂
This produces sodium ethoxide and hydrogen gas.
Can you recall which other substances produce hydrogen on reacting with metals? Acids like hydrochloric acid and sulphuric acid also produce hydrogen gas when reacted with metals like zinc, magnesium, etc.
Second reaction: Reaction to give unsaturated hydrocarbon. Heating ethanol at 443 K with excess concentrated sulphuric acid results in the dehydration of ethanol to give ethene.
The reaction is: CH₃ – CH₂OH → CH₂=CH₂ + H₂O (hot conc. H₂SO₄ acts as dehydrating agent)
The concentrated sulphuric acid can be regarded as a dehydrating agent which removes water from ethanol.
Now let me tell you how alcohols affect living beings.
When large quantities of ethanol are consumed, it tends to slow metabolic processes and to depress the central nervous system. This results in lack of coordination, mental confusion, drowsiness, lowering of the normal inhibitions, and finally stupor. The individual may feel relaxed without realising that his sense of judgement, sense of timing, and muscular coordination have been seriously impaired.
Unlike ethanol, intake of methanol in very small quantities can cause death. Methanol is oxidised to methanal in the liver. Methanal reacts rapidly with the components of cells. It coagulates the protoplasm, in much the same way an egg is coagulated by cooking. Methanol also affects the optic nerve, causing blindness.
Ethanol is an important industrial solvent. To prevent the misuse of ethanol produced for industrial use, it is made unfit for drinking by adding poisonous substances like methanol to it. Dyes are also added to colour the alcohol blue so that it can be identified easily. This is called denatured alcohol.
Now let's learn about alcohol as a fuel.
Sugarcane plants are one of the most efficient convertors of sunlight into chemical energy. Sugarcane juice can be used to prepare molasses which is fermented to give alcohol (ethanol). Some countries now use alcohol as an additive in petrol since it is a cleaner fuel which gives rise to only carbon dioxide and water on burning in sufficient air (oxygen).
### 4.4.2 Properties of Ethanoic Acid
Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic acids. 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles. The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter in cold climates. This gave rise to its name glacial acetic acid.
The group of organic compounds called carboxylic acids are obviously characterised by their acidic nature. However, unlike mineral acids like HCl, which are completely ionised, carboxylic acids are weak acids.
Now let's do Activity 4.7.
Activity 4.7: Compare the pH of dilute acetic acid and dilute hydrochloric acid using both litmus paper and universal indicator. Are both acids indicated by the litmus test? Does the universal indicator show them as equally strong acids?
Both acids will turn blue litmus red, showing they are acids. However, universal indicator will show that hydrochloric acid is a stronger acid (lower pH) than acetic acid (higher pH), because carboxylic acids are weak acids that do not completely ionise in water.
Now let's do Activity 4.8.
Activity 4.8: Take 1 mL ethanol (absolute alcohol) and 1 mL glacial acetic acid along with a few drops of concentrated sulphuric acid in a test tube. Warm in a water-bath for at least five minutes. Pour into a beaker containing 20-50 mL of water and smell the resulting mixture.
You will smell a sweet fragrance. This is because ethanol and ethanoic acid react to form an ester.
Now let's look at the reactions of ethanoic acid.
First reaction: Esterification reaction. Esters are most commonly formed by reaction of an acid and an alcohol. Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester.
The reaction is: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O (acid catalyst)
The ester formed is ethyl acetate. Generally, esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents.
On treating with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.
The reaction is: CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa
Second reaction: Reaction with a base. Like mineral acids, ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water.
The reaction is: NaOH + CH₃COOH → CH₃COONa + H₂O
Third reaction: Reaction with carbonates and hydrogencarbonates.
Activity 4.9: Set up the apparatus as shown in Chapter 2, Activity 2.5. Take a spatula full of sodium carbonate in a test tube and add 2 mL of dilute ethanoic acid. What do you observe? Pass the gas produced through freshly prepared lime-water. What do you observe? Can the gas produced by the reaction between ethanoic acid and sodium carbonate be identified by this test? Repeat this Activity with sodium hydrogencarbonate instead of sodium carbonate.
When ethanoic acid reacts with sodium carbonate or sodium hydrogencarbonate, carbon dioxide gas is evolved. This gas turns lime water milky, confirming it is carbon dioxide.
The reactions are: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
Now let's answer the questions based on section 4.4.
Question 1: How would you distinguish experimentally between an alcohol and a carboxylic acid?
We can distinguish them by: a) Testing with sodium carbonate or sodium bicarbonate: Carboxylic acids produce effervescence (CO₂ gas) while alcohols do not. b) Testing with sodium metal: Both may produce hydrogen gas, but carboxylic acids react more vigorously. c) Testing with litmus: Carboxylic acids turn blue litmus red, while alcohols do not affect litmus.
Question 2: What are oxidising agents?
Oxidising agents are substances that add oxygen to other substances or remove hydrogen from them. Examples are alkaline potassium permanganate and acidified potassium dichromate. They are used to oxidise alcohols to carboxylic acids.
Now let's learn about soaps and detergents.
## 4.5 Soaps and Detergents
Activity 4.10: Take about 10 mL of water each in two test tubes. Add a drop of oil (cooking oil) to both the test tubes and label them as A and B. To test tube B, add a few drops of soap solution. Now shake both the test tubes vigorously for the same period of time. Can you see the oil and water layers separately in both the test tubes immediately after you stop shaking them? Leave the test tubes undisturbed for some time and observe. Does the oil layer separate out? In which test tube does this happen first?
This activity demonstrates the effect of soap in cleaning. Most dirt is oily in nature and as you know, oil does not dissolve in water. The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. The ionic-end of soap interacts with water while the carbon chain interacts with oil. The soap molecules, thus form structures called micelles where one end of the molecules is towards the oil droplet while the ionic-end faces outside. This forms an emulsion in water. The soap micelle thus helps in pulling out the dirt in water and we can wash our clothes clean.
Can you draw the structure of the micelle that would be formed if you dissolve soap in a hydrocarbon? In a hydrocarbon solvent, the soap molecules would orient in the opposite way - the ionic ends would be in the center and the hydrocarbon tails would point outwards.
Now let me explain micelles in more detail.
Soaps are molecules in which the two ends have differing properties, one is hydrophilic, that is, it interacts with water, while the other end is hydrophobic, that is, it interacts with hydrocarbons. When soap is at the surface of water, the hydrophobic 'tail' of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon 'tail' protruding out of water. Inside water, these molecules have a unique orientation that keeps the hydrocarbon portion out of the water. Thus, clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster. This formation is called a micelle. Soap in the form of a micelle is able to clean, since the oily dirt will be collected in the centre of the micelle. The micelles stay in solution as a colloid and will not come together to precipitate because of ion-ion repulsion. Thus, the dirt suspended in the micelles is also easily rinsed away. The soap micelles are large enough to scatter light. Hence a soap solution appears cloudy.
Now let's do Activity 4.11.
Activity 4.11: Take about 10 mL of distilled water (or rain water) and 10 mL of hard water (from a tubewell or hand-pump) in separate test tubes. Add a couple of drops of soap solution to both. Shake the test tubes vigorously for an equal period of time and observe the amount of foam formed. In which test tube do you get more foam? In which test tube do you observe a white curdy precipitate?
You will get more foam in distilled water. In hard water, you will observe a white curdy precipitate (scum). This is because hard water contains calcium and magnesium ions which react with soap to form insoluble salts.
Now let's do Activity 4.12.
Activity 4.12: Take two test tubes with about 10 mL of hard water in each. Add five drops of soap solution to one and five drops of detergent solution to the other. Shake both test tubes for the same period. Do both test tubes have the same amount of foam? In which test tube is a curdy solid formed?
Both test tubes will have foam, but the soap test tube will have a curdy solid (scum) while the detergent test tube will not. This is because detergents do not form insoluble precipitates with calcium and magnesium ions.
Have you ever observed while bathing that foam is formed with difficulty and an insoluble substance (scum) remains after washing with water? This is caused by the reaction of soap with the calcium and magnesium salts, which cause the hardness of water. Hence you need to use a larger amount of soap. This problem is overcome by using another class of compounds called detergents as cleansing agents. Detergents are generally sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions, etc. Both have long hydrocarbon chain. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, they remain effective in hard water. Detergents are usually used to make shampoos and products for cleaning clothes.
Now let's answer the questions based on section 4.5.
Question 1: Would you be able to check if water is hard by using a detergent?
No, because detergents work equally well in hard and soft water. To check if water is hard, we need to use soap, which will produce curdy precipitate or less foam in hard water.
Question 2: People use a variety of methods to wash clothes. Usually after adding the soap, they 'beat' the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?
Agitation is necessary to help the soap or detergent molecules come into contact with the dirt particles, form micelles, and emulsify the dirt. Without agitation, the dirt would remain attached to the clothes. Agitation helps physically remove the dirt from the fabric and suspends it in the water as micelles.
Now let's answer all the exercise questions.
## Exercises
Question 1: Ethane, with the molecular formula C₂H₆ has how many covalent bonds?
Let's count: In ethane (C₂H₆), we have: - One C—C single bond (1 covalent bond) - Six C—H bonds (6 covalent bonds)
Total = 7 covalent bonds. So the answer is (b) 7 covalent bonds.
Question 2: Butanone is a four-carbon compound with the functional group: Butanone has the formula C₄H₈O. It has a ketone group (C=O). So the answer is (c) ketone.
Question 3: While cooking, if the bottom of the vessel is getting blackened on the outside, it means that: The bottom of the vessel gets blackened due to incomplete combustion of fuel, which produces soot (carbon). So the answer is (b) the fuel is not burning completely.
Question 4: Explain the nature of the covalent bond using the bond formation in CH₃Cl.
In CH₃Cl (chloromethane), carbon has 4 valence electrons, hydrogen has 1, and chlorine has 7. Carbon shares one electron with each of the three hydrogen atoms and one electron with chlorine. This sharing of electron pairs between carbon and hydrogen, and between carbon and chlorine, forms covalent bonds. Each bond consists of one shared pair of electrons. Chlorine also has three lone pairs of electrons. This shows how covalent bonds are formed by sharing of electrons so that atoms can achieve noble gas configuration.
Question 5: Draw the electron dot structures for: a) Ethanoic acid: CH₃COOH. The structure shows: the first carbon bonded to three hydrogens and one carbon; the second carbon double-bonded to one oxygen and single-bonded to an OH group. Each oxygen has lone pairs.
b) H₂S: Hydrogen has 1 electron, sulphur has 6 valence electrons. Each hydrogen shares one electron with sulphur, forming two single bonds. Sulphur has two lone pairs.
c) Propanone: CH₃COCH₃. It has three carbons. The middle carbon is double-bonded to oxygen. Each end carbon is bonded to three hydrogens. The middle carbon is also bonded to the two end carbons.
d) F₂: Fluorine has 7 valence electrons. Two fluorine atoms share one electron each, forming a single bond. Each fluorine has three lone pairs.
Question 6: What is a homologous series? Explain with an example.
A homologous series is a series of compounds that have the same functional group, similar chemical properties, and differ from each other by a —CH₂— unit. The compounds have the same general formula, and their physical properties show a gradation with increasing molecular mass.
Example: The alkanes - methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), etc. They all have the general formula CₙH₂ₙ₊₂, contain only single bonds (saturated), and differ from each other by a —CH₂— unit. Their boiling points increase with increasing molecular mass.
Question 7: How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Physical properties: - Ethanol has a characteristic smell, while ethanoic acid has a pungent, vinegar-like smell. - Ethanol boils at 351 K, ethanoic acid boils at 391 K.
Chemical properties: - Reaction with sodium carbonate: Ethanoic acid produces CO₂ gas (effervescence), ethanol does not. - Reaction with sodium bicarbonate: Ethanoic acid produces CO₂ gas, ethanol does not. - Litmus test: Ethanoic acid turns blue litmus red (acidic), ethanol does not affect litmus. - Ester formation: Both can form esters, but the reaction conditions are different.
Question 8: Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
Micelle formation takes place because soap molecules have a hydrophilic (water-loving) ionic end and a hydrophobic (water-hating) hydrocarbon tail. When soap is added to water, the hydrophobic tails try to avoid water and cluster together, with the ionic ends facing the water. This forms a micelle - a spherical structure with the hydrocarbon tails inside and ionic ends outside.
In ethanol, which is a polar organic solvent, micelles may not form because ethanol can dissolve both the ionic and hydrocarbon parts of the soap. The soap molecules would be dispersed uniformly in ethanol rather than forming micelles.
Question 9: Why are carbon and its compounds used as fuels for most applications?
Carbon and its compounds are used as fuels because: - They have high calorific value, releasing large amounts of heat on combustion. - They are readily available in nature (coal, petroleum, natural gas). - They burn easily and can be controlled. - Their combustion products (CO₂ and H₂O) are relatively clean and do not leave solid residues (when combustion is complete). - They are easy to store and transport.
Question 10: Explain the formation of scum when hard water is treated with soap.
Hard water contains calcium and magnesium ions (Ca²⁺, Mg²⁺). When soap (sodium stearate) is added to hard water, the calcium and magnesium ions react with the soap to form calcium stearate or magnesium stearate, which is insoluble in water. This appears as a white curdy precipitate called scum. This scum sticks to clothes and bathtubs, reducing the cleaning ability of soap and leaving deposits.
Question 11: What change will you observe if you test soap with litmus paper (red and blue)?
Soap is slightly basic (alkaline) because it is the sodium or potassium salt of a fatty acid. When tested with blue litmus paper, it may turn slightly red or remain blue depending on the strength. When tested with red litmus paper, it will turn blue, indicating the basic nature of soap.
Question 12: What is hydrogenation? What is its industrial application?
Hydrogenation is a chemical reaction in which hydrogen is added to unsaturated compounds (like alkenes) in the presence of a catalyst (like nickel, palladium, or platinum). The double or triple bonds are converted to single bonds.
Industrial applications: - Hydrogenation of vegetable oils to make vanaspati ghee or margarine. - Production of ammonia from nitrogen and hydrogen (Haber process). - Conversion of unsaturated fats to saturated fats.
Question 13: Which of the following hydrocarbons undergo addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄.
Addition reactions occur in unsaturated hydrocarbons (alkenes and alkynes) that have double or triple bonds. Let's analyze each: - C₂H₆ (ethane): This is an alkane with only single bonds - saturated, does not undergo addition. - C₃H₈ (propane): This is also an alkane - saturated, does not undergo addition. - C₃H₆ (propene): This is an alkene with a double bond - unsaturated, undergoes addition. - C₂H₂ (ethyne): This is an alkyne with a triple bond - unsaturated, undergoes addition. - CH₄ (methane): This is an alkane - saturated, does not undergo addition.
So C₃H₆ and C₂H₂ undergo addition reactions.
Question 14: Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.
Bromine test: Add bromine solution (reddish-brown) to the hydrocarbon. Saturated hydrocarbons will not decolourise bromine solution (or will do so very slowly in the presence of sunlight). Unsaturated hydrocarbons (alkenes and alkynes) will rapidly decolourise bromine solution because addition reaction occurs, breaking the double or triple bonds.
Alternatively, we can use alkaline potassium permanganate solution. Unsaturated hydrocarbons decolourise the purple colour of KMnO₄, while saturated hydrocarbons do not.
Question 15: Explain the mechanism of the cleaning action of soaps.
The cleaning action of soaps is based on micelle formation: 1. Soap molecules have a hydrophilic ionic end and a hydrophobic hydrocarbon tail. 2. When soap is applied to dirt (which is often oily), the hydrophobic tails attach to the oil/dirt particles. 3. The ionic ends face outward, towards the water. 4. This forms micelles - spherical structures with dirt trapped in the center. 5. These micelles remain suspended in water (emulsified) because the ionic ends are water-soluble. 6. When rinsed with water, the dirt is washed away along with the micelles. 7. The repulsion between the similarly charged ionic ends of the soap molecules prevents the micelles from coming together and re-depositing on the fabric.
This is how soaps clean clothes, skin, and other surfaces.
Now let's do the group activity.
Group Activity I: Use molecular model kits to make models of the compounds you have learnt in this chapter. This will help you visualize the three-dimensional structures of molecules like methane, ethane, ethene, ethyne, ethanol, ethanoic acid, etc.
Group Activity II: Take about 20 mL of castor oil or cotton seed oil or linseed oil or soyabean oil in a beaker. Add 30 mL of 20% sodium hydroxide solution. Heat the mixture with continuous stirring for a few minutes till the mixture thickens. Add 5-10 g of common salt to this. Stir the mixture well and allow it to cool. You can cut out the soap in fancy shapes. You can also add perfume to the soap before it sets.
This activity demonstrates the making of soap through saponification - the process of hydrolyzing esters (oils/fats) with an alkali (NaOH) to form soap and glycerol.
Now let me give you a complete summary of everything we have learned in this chapter.
## Summary
In this chapter on carbon and its compounds, we have covered the following important topics:
First, we learned about covalent bonding in carbon. Carbon has four valence electrons and cannot lose or gain electrons easily. Instead, it forms covalent bonds by sharing electrons with other atoms. Covalent bonds can be single, double, or triple bonds. Covalent compounds have low melting and boiling points and are generally poor conductors of electricity.
Second, we learned about allotropes of carbon. Carbon exists in different forms called allotropes - diamond, graphite, and fullerenes. Diamond is the hardest substance with a three-dimensional network of carbon atoms. Graphite has layers of carbon atoms and is a good conductor of electricity. Fullerenes are spherical or cylindrical molecules like C-60.
Third, we learned about the versatile nature of carbon. Carbon forms a huge number of compounds due to two main properties: catenation (ability to form bonds with other carbon atoms) and tetravalency (ability to form four bonds). Carbon can form straight chains, branched chains, and rings.
Fourth, we learned about saturated and unsaturated carbon compounds. Saturated compounds have only single bonds (alkanes), while unsaturated compounds have double bonds (alkenes) or triple bonds (alkynes). Unsaturated compounds are more reactive than saturated compounds.
Fifth, we learned about functional groups. Functional groups are specific groups of atoms that give characteristic properties to compounds. We learned about alcohols (—OH), aldehydes (—CHO), ketones (—C=O), carboxylic acids (—COOH), and halogens.
Sixth, we learned about homologous series. A homologous series is a family of compounds with the same functional group that differ by a —CH₂— unit. They have similar chemical properties and show gradation in physical properties.
Seventh, we learned about nomenclature of carbon compounds. The names are based on the number of carbon atoms and the functional group present, using prefixes and suffixes.
Eighth, we learned about chemical properties of carbon compounds, including combustion (burning in oxygen to produce CO₂ and H₂O), oxidation (adding oxygen), addition reactions (adding hydrogen to unsaturated compounds), and substitution reactions (replacing hydrogen with other atoms).
Ninth, we learned about ethanol and ethanoic acid. Ethanol is a colourless liquid used in alcoholic drinks, as a solvent, and as a fuel. Ethanoic acid (acetic acid) is used in vinegar and in making esters. We learned about their properties and reactions, including esterification and reactions with carbonates.
Finally, we learned about soaps and detergents. Soaps are sodium or potassium salts of fatty acids. They clean by forming micelles that emulsify dirt. Hard water contains calcium and magnesium ions that react with soap to form scum. Detergents are synthetic cleansing agents that work even in hard water.
This is a very important chapter that forms the foundation of organic chemistry. I hope you all understood the concepts clearly. Thank you for your attention, and see you in the next class!