Properties of covalent compounds
Strong bonds inside, weak forces between.
Low temperatures
Methane melts at 90 K and boils at 111 K. Ethanol, chloroform and acetic acid also melt and boil at low temperatures compared with ionic compounds.
What does this tell us about the forces of attraction between their molecules?
What this lesson covers
The idea
Covalent compounds have strong bonds within the molecule but weak inter-molecular forces, so they have low melting and boiling points; as no charged particles are formed, they are generally poor conductors of electricity.
Low temperatures
Methane melts at 90 K and boils at 111 K. Ethanol, chloroform and acetic acid also melt and boil at low temperatures compared with ionic compounds.
What does this tell us about the forces of attraction between their molecules?
- They are not very strong
- They are extremely strong
- There are no forces at all
Inside and between
Press Next step.
Why low and why poor
Molecules are easy to pull away from each other, so melting and boiling points are low. And with no charged particles, electric charge cannot be carried through the compound.
Covalent compounds have strong bonds within the molecule but weak inter-molecular forces, so they have low melting and boiling points. As no charged particles are formed, they are generally poor conductors of electricity.
Notes
Covalent compounds have strong bonds within the molecule but weak inter-molecular forces, so they have low melting and boiling points. With no charged particles, they are generally poor conductors of electricity.
Check yourself
In covalent compounds, which forces are weak?
Why are covalent compounds generally poor conductors of electricity?
According to Table 4.1, which compound has the lowest boiling point?
From Table 4.1, by how many kelvin is the boiling point of ethanol above its melting point?
Answer: 195 K
351 − 156 = 195 K.
- The forces between one molecule and the next — correct. Yes!
- The bonds between atoms inside a molecule. The bonds within the molecule are strong. It is the forces between molecules that are weak.
- The forces between ions. Covalent compounds do not form ions, because no charged particles are formed.
- Their bonds inside the molecule are very strong. Strong bonds inside a molecule do not decide conduction. Electricity needs charged particles, and none are formed.
- No charged particles are formed — correct. Yes!
- They have low melting points. Low melting points come from weak forces between molecules. Poor conduction comes from having no charged particles.
- Ethanol, 351 K. Ethanol boils at 351 K, which is higher than methane at 111 K.
- Acetic acid, 391 K. Acetic acid boils at 391 K, the highest in the table.
- Methane, 111 K — correct. Yes!
- Chloroform, 334 K. Chloroform boils at 334 K, which is higher than methane at 111 K.