AQA A-Level Chemistry AS Paper 1, June 2023: Question 16

1 mark · Easy difficulty · Multiple Choice

Identify which chemical species contains chlorine in its highest oxidation state.

Practise this question

Question

Multiple-choice question 16 asks: 'In which species is chlorine in its highest oxidation state? [1 mark]' Four options are provided: A ClF2 minus, B ClO4 minus, C ClO2, and D ClF3, each with a selection oval next to it.
Question text

16 In which species is chlorine in its highest oxidation state?

[1 mark]

A ClF –

B ClO –

C ClO2

D ClF3

Mark scheme

Show the mark scheme Mark scheme table row for question 16 indicating that the correct answer is B (ClO4 minus) for 1 mark, categorized under assessment objective AO1.

16 B 1 (AO1) ClO –

How to answer it

Determining the Highest Oxidation State of Chlorine

📋 What this question tests

This question evaluates your foundational knowledge of redox rules and your ability to assign oxidation states to elements in molecular compounds, interhalogen compounds, and polyatomic oxyanions.

Question 16 • Multiple Choice

Assigning Oxidation Numbers

Identifying maximum oxidation states in chlorine species

✅ Correct Answer

B: ClO₄⁻

In ClO₄⁻ (the perchlorate ion), chlorine has an oxidation state of +7. This is the maximum possible oxidation state for chlorine, as it belongs to Group 7 (Group 17) and possesses 7 valence electrons.

Mark Scheme: Award 1 mark for selecting option B (AO1).

💡 Key Knowledge

  • Fluorine is the most electronegative element; it is always -1 in compounds.
  • Oxygen has an oxidation state of -2 in almost all compounds (except peroxides where it is -1 , and with fluorine).
  • The sum of oxidation states in a neutral molecule equals 0 .
  • The sum of oxidation states in a polyatomic ion equals the overall charge of the ion.
  • Chlorine is in Group 7, so its oxidation states range from -1 to +7.

📐 Step-by-Step Calculations for All Options

Option Species Calculation Oxidation State of Cl
A ClF₂⁻ Cl + 2(-1) = -1  →  Cl - 2 = -1  →  Cl = -1 + 2 +1
B ClO₄⁻ Cl + 4(-2) = -1  →  Cl - 8 = -1  →  Cl = -1 + 8 +7 (Highest)
C ClO₂ Cl + 2(-2) = 0  →  Cl - 4 = 0  →  Cl = 0 + 4 +4
D ClF₃ Cl + 3(-1) = 0  →  Cl - 3 = 0  →  Cl = 0 + 3 +3

🧠 Exam Technique & Speed Tip

In multiple-choice questions, speed is essential:

  • Notice immediately that chlorine is a Group 7 element, so its theoretical maximum oxidation state is +7.
  • Scanning the options, you can see ClO₄⁻ has four oxygen atoms ( 4 × -2 = -8 ) and an overall charge of -1 , which directly yields +7 .
  • Because you cannot exceed +7 for chlorine, B is guaranteed to be the highest without needing to calculate options A, C, and D!

❌ Common Errors & Pitfalls

  • Ignoring overall ionic charge: Setting Cl + 4(-2) = 0 would give +8 , which is chemically impossible for chlorine.
  • Defaulting chlorine to -1: Halogens are only -1 when bonded to less electronegative elements (e.g. metals or hydrogen), not when bonded to more electronegative elements like F or O .
  • Sign errors: Writing numbers without the + sign (e.g., writing 7 instead of +7 ). Oxidation states always require a sign.

Topics

Physical Chemistry · Inorganic Chemistry · 3.1.7 Oxidation, Reduction and Redox Equations · 3.2.3 Group 7(17), The Halogens

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.