Edexcel A-Level Chemistry Paper 1, June 2018: Question 3

5 marks · Medium difficulty · Calculations

Calculate the enthalpy change of neutralisation for the reaction between nitric acid and sodium hydroxide using experimental temperature and volume data.

Practise this question

Question

Question 3 presents a neutralisation reaction between nitric acid and sodium hydroxide, providing experimental data including volumes and concentrations for both reactants and the resulting temperature rise. Part (a) asks for a reason why excess sodium hydroxide was used (1 mark). Part (b) asks to calculate the enthalpy of neutralisation to an appropriate number of significant figures, given the density and specific heat capacity of the mixture (4 marks).
Question text

3 Nitric acid reacts with sodium hydroxide solution in a neutralisation reaction.

HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l)

In an experiment to determine the enthalpy change of neutralisation, the following

results were obtained.

Volume of 1.00 mol dm−3 HNO = 25.0 cm3

Volume of 1.05 mol dm−3 NaOH = 25.0 cm3

Temperature rise = 6.8°C

(a) Give a reason why excess sodium hydroxide was used.

(1)

(b) Calculate the enthalpy change of neutralisation for the reaction between nitric acid

and sodium hydroxide solution, using the results of the experiment.

Give your answer to an appropriate number of significant figures.

Assume: density of the reaction mixture = 1.0 g cm−3

specific heat capacity of the reaction mixture = 4.18 J g−1 °C−1

(4)

*P52302RA0428*

(Total for Question 3 = 5 marks)

Mark scheme

Show the mark scheme The mark scheme for Question 3(a) accepts answers stating that excess sodium hydroxide ensures all the nitric acid is reacted. The mark scheme for Question 3(b) awards 4 marks across specific steps: calculation of heat produced, moles of nitric acid, enthalpy change per mole, and providing a negative sign with appropriate units and 2 or 1 significant figures.

Question

Acceptable Answer Additional Guidance Mark

Number

3(a) An answer that makes reference to the following point: (1)

+ Allow (nitric acid) / HNO3 is the limiting

to make sure that (all) the (nitric) acid / HNO3 / H has reagent

reacted / been neutralised / is used up

Allow so that 0.025 mol of water / H2O

forms

Ignore to make sure that 1 mol of

water / H2O forms

Ignore just ‘to ensure that reaction is

complete’

Question

Acceptable Answer Additional Guidance Mark

Number

3(b) Example of calculation (4)

calculation of heat produced (1) heat produced = 50.0 x 4.18 x 6.8 = 1421.2( J) /

1.4212 (kJ)

calculation of amount (mol) of HNO3(1) amount HNO3 used = 25.0 x 1.00/1000

= 0.025 / 2.5 x 10−2 (mol)

Ignore moles NaOH and total moles calculated

calculation of enthalpy change (1) enthalpy change = 1421.2 = 56848 (J mol−1)

0.025

or = 1.4212 = 56.848 (kJ mol−1)

0.025

TE on heat produced and amount HNO3

negative sign final answer −57 / −60 kJ mol−1

and or −57 000 / −60 000 J mol−1

units TE on enthalpy change

and

answer to 2 / 1 SF (1) Do not award 3 SF

Correct final answer with sign, units and 2 or 1 SF but no

working scores (4)

Ignore units and sign of enthalpy change in M1 and M3

(Total for Question 3 = 5 marks)

How to answer it

Enthalpy Change of Neutralisation Study Guide

📌 What this question tests

This question assesses practical chemistry, specifically calorimetric data processing for neutralisation reactions. You must understand why excess reagents are used in calorimetry, calculate heat transferred using q = mcΔT , determine moles of a limiting reactant, and correctly apply thermodynamic sign conventions, appropriate units, and significant figure rules.

Question Part (a)

Reason for using excess sodium hydroxide

✅ Correct Answer

To make sure that all of the nitric acid ( HNO₃ / H⁺ ions) has reacted, been neutralised, or been used up.

💡 Key Knowledge

In calorimetry experiments measuring enthalpy change, the reactant whose enthalpy change is being determined (the limiting reagent) must react completely. Adding an excess of the other reactant ensures 100% reaction of the target species.

❌ Common Errors

Generic statements like "to ensure the reaction is complete" are too vague and do not score. You must specify that the acid is the reactant that is fully used up.

Mark: 1 mark
Question Part (b)

Calculation of Enthalpy Change of Neutralisation

📐 Step-by-Step Calculation

  1. Calculate total mass ( m ) of the reaction mixture:
    Volume = 25.0 cm³ + 25.0 cm³ = 50.0 cm³
    Density = 1.0 g cm⁻³ , so mass = 50.0 g
  2. Calculate heat produced ( q ):
    q = m × c × ΔT
    q = 50.0 × 4.18 × 6.8 = 1421.2 J (or 1.4212 kJ )
  3. Calculate moles of limiting reactant ( HNO₃ ):
    Moles = (25.0 / 1000) × 1.00 = 0.0250 mol
    Note: Ignore the concentration of NaOH ; it is in excess!
  4. Calculate enthalpy change per mole ( ΔH ):
    ΔH = -q / moles = -1421.2 / 0.025 = -56848 J mol⁻¹ = -56.848 kJ mol⁻¹

✅ Final Answer Requirements

  • Value: -57 or -60 kJ mol⁻¹ (or -57000 / -60000 J mol⁻¹ )
  • Significant Figures: 2 or 1 SF only (due to ΔT = 6.8 °C having 2 significant figures). Do NOT give 3 SF!
  • Sign & Units: Must include the negative sign ( - ) and correct units ( kJ mol⁻¹ or J mol⁻¹ ).

🧠 Exam Technique & Mark Breakdown

  • M1: Calculation of heat produced ( 1421.2 J ).
  • M2: Calculation of moles of HNO₃ ( 0.025 mol ).
  • M3: Calculation of enthalpy change value.
  • M4: Correct negative sign, correct units, and strict adherence to 2 or 1 SF.

❌ Common Calculation Traps

  • Using the total moles of both solutions or picking the wrong concentration for moles.
  • Forgetting to apply the negative sign (exothermic reactions require a minus sign for enthalpy changes).
  • Rounding to 3 SF because of habit, thereby losing the final mark.
  • Using 25.0 cm³ instead of total volume ( 50.0 cm³ ) for mass in q = mcΔT .
Marks: 4 marks

Topics

Physical Chemistry · Core Practicals · Topic 8: Energetics I · Topic 5: Formulae, Equations and Amounts of Substance · Core Practical 8: Determine the enthalpy change of a reaction using Hess’s law

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.