OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement regarding the different rates of hydrolysis of haloalkanes RCl and RBr based on bond enthalpy.

Practise this question

Question

Multiple-choice question 1 asking which statement is correct for the different rates of hydrolysis of RCl and RBr, with four options A to D involving electronegativity and bond enthalpies.
Question text

1 Which statement is correct for the different rates of hydrolysis of RCl and RBr?

A RBr is hydrolysed faster because Cl is more electronegative than Br.

B RBr is hydrolysed faster because the C–Cl bond enthalpy is greater than C–Br.

C RCl is hydrolysed faster because Cl is more electronegative than Br.

D RCl is hydrolysed faster because the C–Br bond enthalpy is greater than C–Cl.

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme table indicates the correct answer is B for question 1, worth 1 mark, with AO element AO1.1.

AO

Question Answer Marks Guidance

element

1 B 1 AO1.1

How to answer it

Rates of Hydrolysis of Halogenoalkanes

What this question tests

This question assesses your understanding of halogenoalkane chemistry, specifically how carbon-halogen bond enthalpy dictates the rate of nucleophilic substitution (hydrolysis) reactions rather than bond polarity or electronegativity.

Question 1 (Multiple Choice)

Exam Breakdown

✅ Correct Answer: B

RBr is hydrolysed faster because the C–Cl bond enthalpy is greater than C–Br .

💡 Key Knowledge

  • Down Group 17 (halogens), atomic radius increases, making the carbon-halogen bond longer and weaker.
  • Bond enthalpy decreases from C–F to C–I .
  • Less energy is required to break a C–Br bond than a C–Cl bond.

🧠 Exam Technique

  • Watch out for the common distractors involving electronegativity! While chlorine is more electronegative than bromine, bond enthalpy is the deciding factor for reaction rate.
  • Eliminate incorrect options systematically by evaluating both bond strength and electronegativity claims.

❌ Common Errors

  • Confusing bond polarity with bond enthalpy. Students often assume that a more polar bond reacts faster because the carbon atom is more electron-deficient.
  • Stating that C–Br has a greater bond enthalpy than C–Cl .
Examiner Note: This is a classic AO1 knowledge retrieval question. Top-level responses instantly link the faster rate of hydrolysis in bromoalkanes to the lower bond enthalpy of the C–Br bond compared to C–Cl .

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.