Question detail
What does the formula CnH2n show about alkenes compared with alkanes?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Reactions of alkenes and alcohols (chemistry only)
Question
- A. They have two fewer hydrogen atoms than CnH2n+2 alkanes
- B. They have two more hydrogen atoms than alkanes
- C. They contain no hydrogen atoms
- D. They must contain oxygen
Answer
The correct option is They have two fewer hydrogen atoms than CnH2n+2 alkanes. CnH2n shows that an alkene has two fewer hydrogen atoms than the corresponding alkane formula CnH2n+2.
Explanation
The correct option is They have two fewer hydrogen atoms than CnH2n+2 alkanes. The formula comparison is the key GCSE reasoning. For the same number of carbon atoms, an alkene has two fewer hydrogens than the matching saturated alkane. That two-hydrogen difference matches the presence of a carbon-carbon double bond and is why alkenes are described as unsaturated.
Common mistake
Understanding Unsaturation
Students often state that alkenes are unsaturated because they have a double bond, without mentioning the hydrogen atom difference.
Emphasize that alkenes are unsaturated specifically because they contain two fewer hydrogen atoms than the corresponding alkane.
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