OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 8
1 mark · Medium difficulty · Multiple Choice
Calculate the enthalpy change for the reaction of hydrogen sulfide with oxygen using given mean bond enthalpies
Practise this questionQuestion
Question text
8 Bond enthalpies are given in the table.
Bond H–S O=O S=O O–H
Bond enthalpy
–1 +346 +498 +531 +464
/ kJ mol
What is the enthalpy change, in kJ mol–1, for the reaction below?
2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(g)
A –174
B –1102
C –1794
D –2098
Your answer [1]
Mark scheme
Show the mark scheme
8 B 1 AO2.2
How to answer it
Calculating Enthalpy Change from Bond Enthalpies
What this question tests
This question assesses your ability to apply mean bond enthalpies to calculate the enthalpy change (delta H) for a combustion/oxidation reaction. It tests your stoichiometry, your understanding that bond breaking is endothermic (positive) and bond making is exothermic (negative), and your care with molar coefficients.
Enthalpy Change Calculation: 2H₂S(g) + 3O₂→ 2SO₂(g) + 2H₂O(g)
✅ Correct Answer
B (-1102 kJ mol⁻¹)
Applying the formula delta H = Bonds broken - Bonds made yields -1102 kJ mol⁻¹.
💡 Key Knowledge
- Bond Breaking: Endothermic process requiring energy input (positive values).
- Bond Making: Exothermic process releasing energy (negative values).
- Always multiply individual bond enthalpies by the total number of moles of that specific bond shown in the balanced equation.
🧠 Exam Technique
- Draw out or mentally visualize the covalent structures: H-S-H for H₂S, O=O for O₂, O=S=O for SO₂, and H-O-H for H₂O.
- Count every single bond carefully to avoid coefficient multipliers dropping out.
❌ Common Errors
- Sign Inversion: Forgetting that bonds formed must be subtracted (or treating them as positive).
- Miscounting Bonds: Overlooking coefficients (e.g., forgetting that 3 molecules of O₂ contain three O=O double bonds, or that 2 molecules of H₂S contain 4 H-S bonds).
📐 Step-by-Step Calculation
- Identify bonds broken in reactants:
- 2 molecules of H₂S contain 4 x (H–S) bonds → 4 × (+346) = +1384 kJ mol⁻¹
- 3 molecules of O₂ contain 3 x (O=O) bonds → 3 × (+498) = +1494 kJ mol⁻¹
- Total energy input (bonds broken) = 1384 + 1494 = +2878 kJ mol⁻¹
- Identify bonds formed in products:
- 2 molecules of SO₂ contain 4 x (S=O) bonds (2 double bonds per SO₂ molecule) → 4 × (+531) = +2124 kJ mol⁻¹
- 2 molecules of H₂O contain 4 x (O–H) bonds (2 single bonds per H₂O molecule) → 4 × (+464) = +1856 kJ mol⁻¹
- Total energy output (bonds made) = 2124 + 1856 = +3980 kJ mol⁻¹
- Calculate overall enthalpy change:
- delta H = (Energy to break bonds) - (Energy released making bonds)
- delta H = 2878 - 3980 = -1102 kJ mol⁻¹
Topics
Module 3: Periodic table and energy · 3.2 Physical chemistry
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.