General formula of alkenes
CₙH₂ₙ.
A pattern in the formulae
Ethene is C₂H₄. The next members of the series are C₃H₆, C₄H₈ and C₅H₁₀.
Look at the numbers of hydrogen atoms: 4, 6, 8 and 10. How do they compare with the numbers of carbon atoms: 2, 3, 4 and 5?
What this lesson covers
The idea
The general formula of alkenes is CnH2n, where n = 2, 3, 4 … is the number of carbon atoms; ethene is the first member.
A pattern in the formulae
Ethene is C₂H₄. The next members of the series are C₃H₆, C₄H₈ and C₅H₁₀.
Look at the numbers of hydrogen atoms: 4, 6, 8 and 10. How do they compare with the numbers of carbon atoms: 2, 3, 4 and 5?
- Hydrogen atoms are twice the carbon atoms
- Hydrogen atoms are the same as the carbon atoms
- Hydrogen atoms are one more than the carbon atoms
Ethene and its relatives
Open every tab.
One formula for the whole family
Put n carbon atoms in place of 2, 3, 4 and so on, and the number of hydrogen atoms is 2n.
The general formula of alkenes is CₙH₂ₙ, where n = 2, 3, 4 … is the number of carbon atoms; ethene is the first member.
- n | Formula
- 2 | C₂H₄ (ethene)
- 3 | C₃H₆
- 4 | C₄H₈
- 5 | C₅H₁₀
Notes
The general formula of alkenes is CₙH₂ₙ, where n = 2, 3, 4 … is the number of carbon atoms. Ethene is the first member.
Check yourself
An alkene has 5 carbon atoms. How many hydrogen atoms does it have?
Answer: 10
CₙH₂ₙ with n = 5 gives 2 × 5 = 10 hydrogen atoms: C₅H₁₀.
An alkene has 12 hydrogen atoms. How many carbon atoms does it have?
Answer: 6
H = 2n, so n = 12 ÷ 2 = 6 carbon atoms: C₆H₁₂.
Which alkene is the first member of the series?
Which formula fits CₙH₂ₙ for n = 4?
- Propene, C₃H₆. Propene has n = 3. The series begins with n = 2, which is ethene.
- Ethene, C₂H₄ — correct. Yes!
- Methane, CH₄. Methane is an alkane with only single bonds. The alkene series begins with ethene.
- C₄H₁₀. C₄H₁₀ has 10 hydrogen atoms, but 2n = 8 for n = 4.
- C₄H₄. C₄H₄ has 4 hydrogen atoms, but 2n = 8 for n = 4.
- C₄H₈ — correct. Yes!