Edexcel A-Level Chemistry AS Paper 2, June 2023: Question 8

7 marks · Medium difficulty · Calculations

State standard conditions, draw a Hess's law cycle, and calculate the enthalpy change of formation of gaseous water using provided thermochemical data.

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Question

Question text

8 A data book gave the following information.

H (g) + ½O (g) → H O(l) ΔH d = –285.8 kJ mol–1

22 2

(a) State the two conditions denoted by the standard symbol d for this reaction.

(2)

(b) (i) Draw a Hess’s Law cycle that would enable you to use the data provided to

calculate the enthalpy change of formation of gaseous water, ΔrH.

(2)

H2(g) + ½O2(g) → H2O(g)

Data:

H (g) + ½O (g) → H O(l) ΔH d = –285.8 kJ mol–1

22 2 1

H O(l) → H O(g) ΔH d = +2.261 kJ g–1

22 2

(ii) Calculate the enthalpy change for the reaction shown.

H2(g) + ½O2(g) → H2O(g)

(3)

(Total for Question 8 = 7 marks)

Mark scheme

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8(a) An answer that refers to the following points: (2)

• pressure = 1 atm / 100 kPa / 100000 Pa / 1000000 N m−2 / 1 bar (1) Allow 101 kPa / 101000 Pa

• stated temperature (1) Allow any stated temperature in K or °C

273−298 K / 0−25°C inclusive

Ignore room temperature

Question

Acceptable Answer Additional Guidance Mark

Number

8(b)(i) An answer that makes reference to the following points: Example of Hess’s Law diagram (2)

• correct species and states at each corner of the triangle (1)

• all arrows labelled and pointing in the correct direction (1) Allow H , H or figures ( H = −285.8 kJ mol-1,

12 1

H = +2.261 kJ g-1) with correct sign

Allow reversed arrows provided the H1, H2 or

figures also have reversed signs

Question

Acceptable Answer Additional Guidance Mark

Number

8(b)(ii) Example of calculation (3)

• convert energy change for H O(l) H O(g) from (1) 2.261 × 18.0 (+)40698 (J mol−1) = (+)40.698 (kJ mol−1)

kJ g−1 to J mol−1 / kJ mol−1

conversion of −285.8 kJ mol-1 to J g-1/kJ g-1 −285.8 ÷ 18.0 −15.878 kJ g−1/ −15878 J g−1

• correct use of Hess’s Law (1) rH = H1 + H2

COMMENT

An incorrect Hess cycle in (b)(i) cannot score this mark

(1) −285.8 + (+40.698) = −245.10 / −245.1 / −245 (kJ mol−1)

• calculate numerical value of energy change ( rH)

Accept final answer in kJ mol-1, J mol-1 (−245102),

kJ g-1 (−13.617) or J g-1 (−13617)

Ignore SF except 1 SF

Correct answer with no working scores 3 marks

Allow TE throughout (b)(ii)

Ignore units even if incorrect

If the Hess cycle is repeated in (b)(ii) or only seen (b)(ii)

then it can be credited for part (b)(i)

(Total for Question 8 = 7 marks)

(Total for Paper = 80 marks)

How to answer it

Thermochemistry & Hess's Law Study Guide

What this question tests

This question assesses your understanding of standard enthalpy conditions, construction of Hess's Law thermochemical cycles, and multi-step enthalpy calculations involving unit conversions (specifically handling kJ g⁻¹ to kJ mol⁻¹ ).

Question Part (a)

Standard Conditions Definition

✅ Correct Answer

Pressure = 1 atm , 100 kPa , 100000 Pa , or 1 bar
A stated temperature (e.g., 298 K , 25°C , or any explicit value in K or °C between 273–298 K)

💡 Key Knowledge

The standard state symbol (⦵) indicates specific baseline conditions so chemists worldwide can compare enthalpy changes fairly.

❌ Common Errors

Writing vague terms like "room temperature" or "normal conditions" will lose you the temperature mark. You must specify a numerical value or standard range (e.g., 298 K).

Marks: 2 marks (1 mark per condition)

Question Part (b)(i)

Hess's Law Cycle Construction

✅ Correct Answer

Construct a thermochemical triangle linking reactants, liquid water, and gaseous water:

Top line: H₂(g) + 0.5O₂(g) ➔ H₂O(g) with direct enthalpy change ( ΔH or Δr H )
Downward/diagonal path to intermediate state: H₂O(l) labelled with ΔH₁
Arrow pointing from H₂O(l) to H₂O(g) labelled with ΔH₂

🧠 Exam Technique

Always include physical state symbols (g) and (l) for every chemical species in a Hess cycle. Arrow directions are vital: if you reverse a given data arrow, remember to flip its sign during calculations.

❌ Common Errors

Omitting state symbols or drawing ambiguous arrowheads loses marks immediately. Make sure arrows clearly point from start to finish of each pathway.

Marks: 2 marks (1 for correct species & states at all corners, 1 for correctly directed & labelled arrows)

Question Part (b)(ii)

Enthalpy Calculation & Unit Conversion

📐 Step-by-Step Calculation

Convert units for ΔH₂: Data gives ΔH₂ = +2.261 kJ g⁻¹ . Multiply by the molar mass of water ( 18.0 g mol⁻¹ ) to get kJ mol⁻¹ :
+2.261 × 18.0 = +40.698 kJ mol⁻¹
Apply Hess's Law: Δr H = ΔH₁ + ΔH₂
Substitute values and evaluate:
Δr H = -258.8 + (+40.698) = -245.102 kJ mol⁻¹

✅ Final Answer

-245.1 kJ mol⁻¹ (Acceptable range: -245 to -245.1 kJ mol⁻¹ )

❌ Calculation Traps

Forgetting to multiply the vaporization enthalpy by the molar mass ( 18.0 ) is the most frequent place students lose marks. Ensure your units match ( kJ mol⁻¹ across all terms) before adding them together!

Marks: 3 marks (1 for unit conversion, 1 for correct application of Hess's Law, 1 for correct numerical final value)

Topics

Physical Chemistry · Topic 8: Energetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.