OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 14

1 mark · Easy difficulty · Multiple Choice

Identify what curly arrows always show in reaction mechanisms.

Practise this question

Question

Multiple choice question 14 asking: What do curly arrows always show in reaction mechanisms? The options are A: Movement of one electron., B: Movement of a pair of electrons., C: Movement of a lone pair of electrons., D: Movement of the electrons in a covalent bond.
Question text

14 What do curly arrows always show in reaction mechanisms?

A Movement of one electron.

B Movement of a pair of electrons.

C Movement of a lone pair of electrons.

D Movement of the electrons in a covalent bond.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing question 14 with the correct answer B, worth 1 mark.

14 B 1 AO1.1

How to answer it

Understanding Curly Arrows in Mechanisms

What this question tests

This question assesses fundamental chemical terminology and core understanding of how electrons move during organic reaction mechanisms (Assessment Objective AO1.1: Demonstrate knowledge and understanding of scientific ideas).

Question 14

Exam Breakdown & Correct Answer

✅ Correct Answer

B — Movement of a pair of electrons.

Mark Awarded: 1 / 1 mark

💡 Key Knowledge

  • A standard curly arrow with a double-headed barb (like the ones used in AS and A Level Chemistry) always represents the movement of an electron pair.
  • Single-headed arrows (fishhooks) represent the movement of a single electron, which are used in free radical mechanisms (covered later or in specific extension topics, but standard AS mechanisms use double-headed arrows for pairs).

🧠 Exam Technique

  • Read multiple-choice questions carefully, paying attention to absolute words like always .
  • Do not confuse specific examples (like lone pairs or covalent bonds) with the definition of what the arrow shows globally.

❌ Common Errors

  • Choosing C (Movement of a lone pair): While curly arrows often start from a lone pair, they can also start from a covalent bond. Therefore, lone pairs are not the only source.
  • Choosing D (Movement of electrons in a covalent bond): While arrows can break covalent bonds (moving that pair onto an atom), they can also form new bonds from lone pairs. Option B is the overarching definition that covers both scenarios.

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.