AQA A-Level Chemistry Paper 3, 2017: Question 27

1 mark · Medium difficulty · Multiple Choice

Calculate the new pH of a reaction mixture when the rate of an acid-catalysed reaction decreases to one quarter of its original value.

Practise this question

Question

Question 27 describes an acid-catalysed reaction between iodine and propanone with the rate equation: rate = k [H+][C3H6O]. The rate of reaction was measured at pH = 0.70. In a second mixture, the sulfuric acid concentration is changed while propanone and iodine concentrations remain the same, causing the rate to become one quarter of the original rate. Students must choose the new pH from four options: A (1.00), B (1.30), C (1.40), and D (2.80).
Question text

27 The rate equation for the acid-catalysed reaction between iodine and propanone is:

rate = k [H+] [C H O]

The rate of reaction was measured for a mixture of iodine, propanone and sulfuric acid

at pH = 0.70

In a second mixture the concentration of the sulfuric acid was different but the

concentrations of iodine and propanone were unchanged. The new rate of reaction

was a quarter of the original rate.

What was the pH of the second mixture?

[1 mark]

A 1.00

B 1.30

C 1.40

D 2.80

Mark scheme

Show the mark scheme Mark scheme table row showing that question 27 corresponds to correct answer option B.

27 B

How to answer it

Reaction Kinetics & pH Calculations

📋 What this question tests

This question tests your ability to link two core physical chemistry concepts:

  • Rate Equations & Reaction Order: Deducing how changing reactant concentration affects rate (first order with respect to H⁺).
  • Logarithmic pH Calculations: Converting between [H⁺] and pH using pH = -log₁₀[H⁺] and [H⁺] = 10⁻ᵖᴴ .
  • Mathematical Log Rules (Shortcut): Recognising that changing concentration by a factor directly shifts pH by -log₁₀(factor) .
Question 27 • Multiple Choice [1 mark]

Determining the New pH of the Acid Mixture

AQA A-Level Chemistry • Rates of Reaction and Acids & Bases

✅ Correct Answer

B — 1.30

Mark Scheme: Award 1 mark for option B. No working needs to be shown in multiple-choice questions, but fast accuracy is essential.

💡 Key Knowledge

  • Rate Equation: rate = k [H⁺][C₃H₆O] . The reaction is first order with respect to H⁺.
  • Because it is first order and [C₃H₆O] is unchanged, if the rate becomes ¼ of the original rate, the new [H⁺] must also be ¼ of the original [H⁺] .
  • pH Definition: pH = -log₁₀[H⁺] and [H⁺] = 10⁻ᵖᴴ .

📐 Step-by-Step Calculation

There are two reliable ways to solve this question:

Method 1: Direct Calculation (Standard Exam Route)

  1. Find original [H⁺] from initial pH:
    [H⁺]initial = 10-0.70 = 0.1995 mol dm⁻³
  2. Determine the new [H⁺]:
    Because the rate is first order with respect to [H⁺], when rate is multiplied by 0.25 (a quarter), [H⁺] is also multiplied by 0.25:
    [H⁺]new = 0.1995 × 0.25 = 0.04988 mol dm⁻³
  3. Calculate the new pH:
    pHnew = -log₁₀(0.04988) = 1.302 ≈ 1.30

Method 2: Log Rules (Top-Tier Speed Shortcut)

pHnew = -log₁₀([H⁺]initial × 0.25)
pHnew = -log₁₀([H⁺]initial) - log₁₀(0.25)
pHnew = pHinitial - log₁₀(0.25)
pHnew = 0.70 - (-0.602) = 0.70 + 0.60 = 1.30

❌ Common Errors & Distractor Traps

  • Selecting D (2.80): Multiplying the pH directly by 4 ( 0.70 × 4 = 2.80 ). pH is a logarithmic scale, not a linear scale!
  • Selecting C (1.40): Doubling the pH ( 0.70 × 2 = 1.40 ), confusing halving/quartering with doubling pH.
  • Overthinking Sulfuric Acid Diprotic Nature: Some students worry about whether H₂SO₄ completely dissociates its second proton. Notice the rate equation depends directly on [H⁺], not [H₂SO₄]. You do not need the acid formula or Ka2 at all.

🧠 Exam Technique & Examiner Insight

  • Spot the Rate Order First: Always verify powers in the rate equation. If it were second order ( [H⁺]² ), a quarter rate would mean halving the concentration. Here it is clearly power of 1.
  • Check Direction of Change: Lower rate → lower [H⁺] → higher pH. pH must increase from 0.70. All options are > 0.70, but keeping this principle in mind prevents simple negative-sign errors on your calculator.
  • Speed Tip: Using new pH = old pH + log₁₀(dilution factor) saves precious time in Section A/Multiple Choice. Here, dilution factor is 4 → 0.70 + log₁₀(4) = 0.70 + 0.60 = 1.30 .

Topics

Physical Chemistry · 3.1.9 Rate Equations · 3.1.12 Acids and Bases

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.