OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify which experimental error caused the calculated enthalpy change of combustion of methanol to be more exothermic than the data book value.

Practise this question

Question

Multiple-choice question 12 about an enthalpy of combustion experiment for methanol. The student's calculated enthalpy change was more exothermic than data book values, and four options A to D provide potential errors: A suggests methanol evaporation, B suggests using the molar mass of ethanol instead of methanol, C suggests incomplete combustion, and D suggests the water boiled for 5 minutes. A box is provided for the student's answer.
Question text

12 A student carried out an experiment to measure the enthalpy change of combustion of methanol.

The energy from the combustion of methanol was used to heat a beaker containing water.

The student’s calculated enthalpy change of combustion was more exothermic than the value in

data books.

Which error could have caused this difference?

A Some methanol had evaporated from the wick before the final weighing.

B In the calculation, the student used the molar mass of ethanol instead of methanol.

C There was incomplete combustion.

D The water boiled for 5 minutes before the final temperature was taken.

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 12 showing the correct answer as B, worth 1 mark.

12 B 1

How to answer it

Evaluating Errors in Enthalpy of Combustion Experiments

OCR AS Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your understanding of calorimetry experiments used to find enthalpy changes of combustion (CH₃OH). Specifically, it tests how experimental errors, calculation mistakes, and heat losses mathematically impact the final calculated value of enthalpy change (delta H), forcing you to distinguish between experimental flaws and calculation-based errors.

Question Analysis & Solution

Question 12 (Multiple Choice)

✅ Correct Answer: Option B

Using the molar mass of ethanol ( 46.0 g mol⁻¹ ) instead of methanol ( 32.0 g mol⁻¹ ) results in a calculated enthalpy change that appears artificially more exothermic.

💡 Key Knowledge

  • Enthalpy of combustion is calculated using: q = m c delta T
  • Number of moles = mass / molar mass
  • Enthalpy change per mole = -q / moles
  • Methanol formula: CH₃OH (Mr = 32.0)
  • Ethanol formula: C₂H₅OH (Mr = 46.0)

📐 Step-by-Step Mathematical Proof

  1. Step 1: Calculate energy released ( q ). Let's assume a fixed heat energy released of 1000 J .
  2. Step 2: Assume the mass of alcohol burned gives 0.100 mol of methanol.
  3. Step 3 (Correct calculation): -1000 J / 0.100 mol = -10,000 J mol⁻¹ ( -10.0 kJ mol⁻¹ ).
  4. Step 4 (Using ethanol's larger Mr): Using the larger molar mass means the calculated number of moles appears smaller for the same mass burned (e.g., 0.070 mol ).
  5. Step 5 (Impact): -1000 J / 0.070 mol = -14,285 J mol⁻¹ ( -14.3 kJ mol⁻¹ ) — a larger negative number, meaning more exothermic!

❌ Common Errors & Distractor Analysis

  • Option A (Evaporation): If methanol evaporates from the wick, the final mass recorded is smaller than it should be. Mass burned = initial - final, so mass burned appears larger. Larger moles mean a smaller enthalpy change (less exothermic).
  • Option C (Incomplete combustion): Releases less heat energy ( q is smaller), making the calculated enthalpy change less exothermic.
  • Option D (Water boiling): Heat is lost to the surroundings after boiling starts, or temperature readings stop early, reducing the recorded delta T and making it less exothermic.

🧠 Exam Technique & Examiner Insight

Examiners note that students frequently confuse the direction of enthalpy changes. Remember: more exothermic means a larger negative value (further down the number line, e.g., -500 kJ mol⁻¹ is more exothermic than -200 kJ mol⁻¹). Always trace back how a larger or smaller denominator (moles) or numerator (energy, q) affects the final division.

Mark Allocation: [1 mark] awarded for selecting option B.

Topics

Module 3: Periodic table and energy · Practical Activity Groups · 3.2 Physical chemistry · PAG 3: Enthalpy determination

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.