AQA A-Level Chemistry AS Paper 2, June 2025: Question 22

1 mark · Medium difficulty · Multiple Choice

Identify the equation that represents the standard enthalpy of formation of liquid iodoethane, C2H5I(l).

Practise this question

Question

Multiple-choice question with four chemical equations representing the formation of iodoethane: Option A: 1/2 I2(l) + 2 C(s) + 5/2 H2(g) gives C2H5I(l). Option B: 1/2 I2(s) + 2 C(s) + 5/2 H2(g) gives C2H5I(l). Option C: I2(l) + 4 C(s) + 5 H2(g) gives 2 C2H5I(l). Option D: I2(s) + 4 C(s) + 5 H2(g) gives 2 C2H5I(l). Each option has a selection box beside it.
Question text

A I2(I) + 2C(s) + H2(g) → C2H5I(I)

B 2I2(s) + 2C(s) + 2H2(g) → C2H5I(I)

C I2(I) + 4C(s) + 5H2(g) → 2C2H5I(I)

D I2(s) + 4C(s) + 5H2(g) → 2C2H5I(I)

Mark scheme

Show the mark scheme Mark scheme table for question 22 showing correct answer B, worth 1 mark (AO2), corresponding to 1/2 I2(s) + 2 C(s) + 5/2 H2(g) gives C2H5I(l).

22 B 1 (AO2) 2I2(s) + 2C(s) + 2 H2(g) → C2H5I(l)

How to answer it

Standard Enthalpy of Formation of Iodoethane

📌 What this question tests

This question evaluates your understanding of the precise definition of standard enthalpy of formation (ΔfH⦵), focusing on:

  • The requirement that exactly one mole of compound is formed.
  • The requirement that all reactant elements must be in their standard states under standard conditions (298 K and 100 kPa).
  • Recall of physical states for Group 7 halogens (specifically iodine) under standard conditions.

Question 22: Identifying the Correct Formation Equation

Multiple Choice | 1 Mark (AO2)

✅ Correct Answer

B: ½I₂(s) + 2C(s) + ⁵⁄₂H₂(g) → C₂H₅I(l)

Award 1 mark for selecting option B.

💡 Key Knowledge

  • Definition: Standard enthalpy of formation (ΔfH⦵) is the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states under standard conditions (100 kPa and 298 K).
  • Standard states at 298 K:
    • Carbon: C(s) (specifically graphite)
    • Hydrogen: H₂(g) (diatomic gas)
    • Iodine: I₂(s) (dark grey / purple-black solid)
    • Bromine is the only liquid halogen: Br₂(l)

🧠 Exam Technique: Two-Step Elimination

  1. Check the stoichiometric coefficient of the product:
    The definition specifies the formation of 1 mole of compound.
    Options C and D produce 2 moles of C₂H₅I(l), so eliminate them immediately.
  2. Check standard states of reactants:
    Between remaining options A and B, observe the state of iodine:
    • Option A uses I₂(l) ❌
    • Option B uses I₂(s) ✅
    At 298 K (25 °C), iodine is a solid. Therefore, B is correct.

❌ Common Student Traps

  • Confusing states of halogens: Students frequently confuse iodine with bromine, mistakenly writing I₂(l) instead of I₂(s) . Remember: Chlorine and fluorine are gases, bromine is liquid, and iodine is solid at 298 K.
  • Avoiding fractions: Students uncomfortable with fractional balancing numbers ( ½ and ⁵⁄₂ ) often pick option D because it uses whole numbers. In ΔfH⦵ equations, fractional coefficients for reactants are completely normal and required to keep the product at 1 mole.

Topics

Physical Chemistry · 3.1.4 Energetics

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.