OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 11

1 mark · Medium difficulty · Multiple Choice

Calculate the standard enthalpy change of combustion of hydrazine using given standard enthalpy changes of formation data.

Practise this question

Question

Multiple choice question 11 showing the combustion equation for hydrazine, N2H4(l) + 3O2(g) -> 2NO2(g) + 2H2O(l), alongside a table of standard enthalpies of formation for each substance. Four multiple choice options A, B, C, and D give numerical values for the enthalpy of combustion, followed by a box for the final answer and a mark allocation of 1.
Question text

11 Combustion of hydrazine, N2H4, produces NO2 and H2O as in the equation below.

N2H4(l) + 3O2(g) → 2NO2(g) + 2H2O(l)

The table shows standard enthalpy changes of formation, ∆ H o .

f

Substance ∆ H o / kJ mol−1

f

N2H4(l) +50.6

O2(g) 0

NO2(g) +33.2

H2O(l) –285.8

What is the enthalpy change of combustion, in kJ mol–1, for hydrazine, N H (l)?

A –555.8

B –303.2

C +303.2

D +555.8

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct multiple choice answer for question 11 is A, worth 1 mark.

11 A 1 AO2.2

How to answer it

Calculating Enthalpy of Combustion from Enthalpies of Formation

What this question tests

This question assesses your ability to apply Hess's Law using standard enthalpy changes of formation (ΔfHꙇ) to calculate an enthalpy change of combustion (ΔcHꙇ). It tests stoichiometric balancing ratios, correct handling of state symbols, algebraic sign conventions, and multi-term summation.

Question 11 (Multiple Choice)

Calculating ΔcHꙇ of Hydrazine, N₂H₄(l)

✅ Correct Answer

A: -555.8 kJ mol⁻¹

Mark Awarded: 1 mark (AO2.2 - Application of quantitative chemical concepts)

💡 Key Knowledge

  • Definition: Enthalpy of formation (ΔfHꙇ) is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
  • Elements: Elements in their standard states, such as O₂(g), always have a standard enthalpy of formation of exactly 0 kJ mol⁻¹ .
  • Hess's Law Cycle Rule: When calculating enthalpy changes of reaction/combustion from formation data, use the formula:
    ΔH = ΣΔfHꙇ(products) − ΣΔfHꙇ(reactants)

🧠 Exam Technique

  • Always multiply every individual formation value by its corresponding stoichiometric coefficient from the balanced equation before adding them together.
  • Double-check positive and negative signs carefully; failing to distribute the negative sign across the reactant bracket is a leading cause of dropped marks.

❌ Common Errors

  • Inverted subtraction: Calculating Reactants − Products instead of Products − Reactants , resulting in option D (+555.8 kJ mol⁻¹).
  • Ignoring stoichiometry: Forgetting to multiply the formation value of NO₂ by 2 (matching the coefficient in the balanced equation).
  • Misinterpreting signs: Dropping negative signs on exothermic values like H₂O(l) ( −285.8 ).

📐 Step-by-Step Calculation Guide

  1. Write out the formula using the given data:
    ΔcHꙇ = [2 × ΔfHꙇ(NO₂) + 2 × ΔfHꙇ(H₂O)] − [1 × ΔfHꙇ(N₂H₄) + 3 × ΔfHꙇ(O₂)]
  2. Substitute the numerical values from the table:
    ΔcHꙇ = [ (2 × +33.2) + (2 × −285.8) ] − [ (1 × +50.6) + (3 × 0) ]
  3. Calculate the products term:
    [ +66.4 + (−571.6) ] = −505.2 kJ mol⁻¹
  4. Calculate the reactants term:
    [ +50.6 + 0 ] = +50.6 kJ mol⁻¹
  5. Subtract reactants from products:
    ΔcHꙇ = (−505.2) − (+50.6) = −555.8 kJ mol⁻¹ (Matches option A)

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.