OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 2

1 mark · Medium difficulty · Multiple Choice

Identify why hydrogen reacts more readily with alkenes than with alkanes from multiple-choice options concerning sigma and pi bonds.

Practise this question

Question

Multiple choice question asking why hydrogen reacts more readily with alkenes than with alkanes. Option A states alkenes are polar molecules whereas alkanes are not. Option B states all atoms in an alkane have a full outer shell of electrons. Option C states the bond enthalpy of C-C sigma bonds is higher than that of pi bonds. Option D states the bond enthalpy of C-C sigma bonds is lower than that of pi bonds.
Question text

2 Hydrogen reacts much more readily with alkenes than with alkanes.

Why is this?

A Alkenes are polar molecules whereas alkanes are not.

B All atoms in an alkane have a full outer shell of electrons.

C The bond enthalpy of C–C σσ bonds is higher than that of ππ bonds.

D The bond enthalpy of C–C σσ bonds is lower than that of ππ bonds.

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C with 1 mark.

2 C 1

How to answer it

Reactivity of Alkenes vs Alkanes with Hydrogen

📌 What this question tests

This question assesses your understanding of bonding in hydrocarbons, specifically comparing the strength and bond enthalpies of sigma (σ) bonds found in alkanes versus pi (π) bonds found in the double bonds of alkenes, and how this relates to their chemical reactivity.

Question 2: Multiple Choice Analysis

Exam Part: Multiple Choice Selection [1 Mark]

✅ Correct Answer: C

The correct option is C: The bond enthalpy of C–C σ bonds is higher than that of π bonds.

💡 Key Knowledge

  • Alkanes contain only strong C–C and C–H σ (sigma) bonds formed by the direct overlap of orbitals.
  • Alkenes contain a double bond made of one strong σ bond and one weaker π (pi) bond formed by the sideways overlap of adjacent p-orbitals.
  • Because π bonds have a lower bond enthalpy (they are weaker and electron density is exposed above and below the plane), they require less energy to break, making alkenes far more reactive towards electrophiles and hydrogen addition.

🧠 Exam Technique

Read multiple-choice statements extremely carefully, especially words that look similar like "higher" vs "lower". Eliminate obviously incorrect statements first (such as alkanes being polar or having unfulfilled outer shells).

❌ Common Errors

Students sometimes select D by confusing the relative strengths of sigma and pi bonds, or incorrectly believing that sigma bonds are weaker because they break during substitution reactions.

Mark Scheme Award: 1 mark for selecting letter C.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.