Physical properties
2 marks · HardA student claims that all alkenes have the same general formula. State this general formula and explain how it shows that alkenes are unsaturated.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
General formula: CnH2n. It shows alkenes are unsaturated because they have fewer hydrogen atoms than the corresponding alkane (CnH2n+2) and contain a C=C double bond.
Also accept: General formula: CnH2n. Alkenes are unsaturated since they contain a double bond and have two fewer hydrogen atoms than alkanes.
Common mistakes: Giving alkane formula; Saying 'alkenes are saturated'
Show mark scheme and worked solution
Marking points
B1 · 1 mark · States the general formula for alkenes: CnH2n.
B2 · 1 mark · Explains that alkenes are unsaturated due to the double bond or having fewer hydrogens than alkanes.
Worked solution
The general formula for alkenes is CnH2n.
This shows that alkenes are unsaturated because they have a C=C double bond, meaning they contain two fewer hydrogen atoms than the corresponding alkane (which is CnH2n+2).