AQA A-Level Chemistry Paper 3, June 2022: Question 10
1 mark · Easy difficulty · Multiple Choice
Identify which of the given substances has the correct oxidation state assigned to the specified element.
Practise this questionQuestion
Question text
10 Which of these oxidation states is correct?
[1 mark]
A Chlorine in Cl2 is –1
B Chromium in K2Cr2O7 is +7
C Fluorine in F2O is –1
D Hydrogen in NaH is +1
Mark scheme
Show the mark scheme
10 C (AO1) 1 Fluorine in F2O is –1
How to answer it
Rules and Exceptions in Assigning Oxidation States
This question evaluates core AO1 recall and application of the fundamental rules for assigning oxidation states, specifically:
- The oxidation state of uncombined elements vs elements in compounds.
- Calculation of transition metal oxidation states in polyatomic compounds (e.g. K₂Cr₂O₇).
- Crucial rule priority exceptions: fluorine is always -1 in compounds, causing oxygen to adopt a positive oxidation state in F₂O.
- Metal hydrides (e.g. NaH), where hydrogen forms a hydride ion (H⁻) with an oxidation state of -1.
Question 10 Analysis
Multiple Choice: Oxidation States in Elements and Compounds
✅ Correct Answer
C: Fluorine in F₂O is -1
Fluorine is the most electronegative element in the periodic table. It always has an oxidation state of -1 in all its compounds. Because the overall compound F₂O is neutral, oxygen takes an unusual oxidation state of +2.
💡 Key Knowledge: Hierarchy of Rules
- Uncombined elements: Oxidation state is always 0 (e.g. Cl₂, O₂, Na).
- Fluorine: Always -1 in all compounds.
- Group 1 metals: Always +1 in compounds.
- Hydrogen: Usually +1, but -1 in metal hydrides (e.g. NaH, CaH₂).
- Oxygen: Usually -2, except in peroxides (-1) and compounds with fluorine such as F₂O (+2).
- The sum of oxidation states in a neutral compound is always 0.
📐 Option-by-Option Breakdown
| Option | Given Statement | Actual Oxidation State | Detailed Step-by-Step Calculation |
|---|---|---|---|
| A | Chlorine in Cl₂ is -1 | 0 (Incorrect) | Cl₂ is an uncombined neutral diatomic element. Uncombined elements always have an oxidation state of 0. |
| B | Chromium in K₂Cr₂O₇ is +7 | +6 (Incorrect) | 1. K is Group 1: 2 × (+1) = +2 2. O is usually -2: 7 × (-2) = -14 3. Total charge = 0: (+2) + 2(Cr) + (-14) = 0 4. 2(Cr) = +12 ⇒ Cr = +6 (not +7; +7 is Mn in KMnO₄) |
| C | Fluorine in F₂O is -1 | -1 (Correct) | 1. F is the most electronegative element: assigned -1. 2. 2 × (-1) = -2. 3. Oxygen is therefore: (+2) to make the neutral sum equal 0. |
| D | Hydrogen in NaH is +1 | -1 (Incorrect) | 1. Sodium is a Group 1 metal: strictly assigned +1. 2. In a neutral compound: (+1) + H = 0 ⇒ H = -1 (hydride ion, H⁻). |
❌ Common Traps & Student Errors
- Confusing F₂O: Students frequently remember "oxygen is -2" as an absolute rule and incorrectly deduce that F must be +1. Fluorine’s rule takes priority over oxygen's rule!
- Confusing Permanganate and Dichromate: Students frequently mix up the +7 oxidation state of manganese in KMnO₄ (or MnO₄⁻) with the +6 state of chromium in K₂Cr₂O₇ (or Cr₂O₇²⁻).
- Defaulting H to +1: Forgetting that hydrogen forms hydrides with reactive metals where H has a negative oxidation state (-1).
- Ignoring elemental state: Assuming halogens are always -1, forgetting that diatomic elements like Cl₂ have an oxidation state of 0.
🧠 Top-Scorer Exam Technique
- Electronegativity Hierarchy: When two non-metals bond, the more electronegative element gets the negative oxidation state. Fluorine (electronegativity 4.0) beats oxygen (3.5).
- Elimination Speedrun: In multiple-choice questions:
• Instantly cross out A (Cl₂ is elemental ⇒ 0).
• Instantly cross out D (Na is +1, so H must be -1).
• Check Cr in K₂Cr₂O₇ quickly: Cr₂O₇²⁻ means 2Cr - 14 = -2 ⇒ Cr = +6.
• This leaves C confirmed in under 20 seconds.
Topics
Physical Chemistry · 3.1.7 Oxidation, Reduction and Redox Equations
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.