OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 23
10 marks · Medium difficulty · Structured Questions
Write the combustion equation for heptane, complete an enthalpy profile diagram for the reaction of CO and NO, explain the role of a catalyst using the diagram, state standard conditions, and calculate the standard enthalpy of formation of carbon dioxide.
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Question text
23 This question is about enthalpy changes.
(a) In a petrol engine, alkanes undergo combustion.
(i) Heptane is one of the alkanes in petrol.
Write the equation for the complete combustion of heptane.
State symbols are not required.
… [2]
(ii) In a petrol engine, polluting gases such as CO and NO are formed. These are mostly
removed before being emitted from the exhaust.
The equation for the removal of CO and NO is shown below.
2CO(g) + 2NO(g) 2CO (g) + N (g) ΔH = –746 kJ mol–1
Complete the enthalpy profile diagram in Fig. 23.1 for this reaction.
On your diagram:
• Label the enthalpy change of reaction, ΔH.
• Include the formulae of the reactants and products.
• Label the activation energy, Ea.
Fig. 23.1
enthalpy
progress of reaction
[2]
(iii) CO and NO are removed by use of a catalyst.
Explain the role of the catalyst.
Refer to your enthalpy profile diagram in Fig. 23.1 in your answer.
… [2]
(b) Iron(III) oxide reacts with carbon monoxide as shown:
Fe O (s) + 3CO(g) 2Fe(s) + 3CO (g) ΔH = –25 kJ mol–1
23 2
Standard enthalpy changes of formation, Δ Ho, are given in the table.
f
Substance Δ Ho / kJ mol–1
f
Fe2O3(s) –824
CO(g) –111
(i) State the conditions of temperature and pressure for standard enthalpy changes.
Temperature …
Pressure … [1]
(ii) Calculate the standard enthalpy change of formation for CO2(g).
Δ Ho (CO (g)) = … kJ mol–1 [3]
f 2
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
23 (a) (i) C7H16 + 11O2 → 7CO2 + 8H2O 2 AO2.6 ALLOW multiples
2 IGNORE state symbols
Correct species ✓
Balanced ✓ For heptane formula, ALLOW any combination of
skeletal OR structural OR displayed formula as long
as unambiguous
ALLOW 1 mark for balanced combustion equation
for a different alkane (ECF)
e.g. C6H14 + 9½O2 → 6CO2 + 7H2O
AO
element
(ii) 2 ANNOTATE ANSWER WITH TICKS AND
CROSSES ETC
Ea IGNORE state symbols
2CO(g) + 2NO(g)
enthalpy
∆H
2CO2(g) + N2(g)
progress of reaction
Reactants, products and ∆H ∆H DO NOT ALLOW –∆H
2CO + 2NO on LHS DO NOT ALLOW double headed arrow on ∆H
AND
2CO2 + N2 on RHS ALLOW ∆H arrow even with small gap at the
AND top and bottom, i.e. line does not quite reach
∆H labelled with products below reactants reactant or product line.
AND
Arrow downwards AO2.1 ALLOW –746 for ∆H
Ea (independent of ∆H) Ea ALLOW AE OR AE
curve with arrow from reactants to top of curve
AND ALLOW 2 arrowheads at each end of Ea line
Ea labelled AO1.2 OR no arrowhead
BUT DO NOT ALLOW arrowhead down
IF endothermic diagram shown,
ALLOW ECF for Ea using MS criteria Ea line must reach maximum (or near to
maximum) on curve
AO
15 element
(iii) Catalyst lowers activation energy 2 AO1.2 ALLOW 2nd labelled curve on profile diagram in
OR 2 23(a)(ii) with lower activation energy/Ec with catalyst
Catalyst increases rate without itself changing ✓
ALLOW Ec needs less energy to start reaction
ALLOW Ec curve is lower than Ea curve
Reaction proceeds via a different route/pathway IGNORE ‘shorter route’ for alternative route
OR
More molecules/particles exceed activation energy ✓ IGNORE more successful collisions
(b) (i) 298 K/25ºC 1 AO1.1 ALLOW ‘a stated temperature’
AND To accept that other standard temperatures can be
100 kPa ✓ used and 298 should strictly be added as ∆H o
IF a temperature is seen, it must be 298/25ºC
ALLOW 1 105 Pa, 101 kPa, 1.01 105 Pa,
1 atm, 1 bar
AO
16 element
(ii) FIRST CHECK THE ANSWER ON ANSWER LINE 3 AO2.6 FULL ANNOTATIONS MUST BE USED
If answer = –394 (kJ mol–1) award 3 marks 3
ALLOW ECF throughout
Use of ∆fH values and balancing numbers
± (–824) seen anywhere COMMON ERRORS
AND –1182 omission of ÷3 for ∆fH(CO2) 2 marks
± (3 –111) OR ± (333) OR ±1157 seen anywhere ✓ (+)394 Incorrect subtraction 2 marks
(+)1182 Incorrect subtraction & no ÷3 1 mark
Correct subtraction for 3∆fH(CO) using ∆H
(–25) + (–1157) –1157 = 824 + (3 –111) –320 no 3 for –111 2 marks
= –1182 (kJ mol–1) ✓ –960 no 3 for –111 AND no ÷3 1 mark
Calculation of ∆fH(CO2) formation –377.3 / –377 incorrect subtraction 2 mark
–1182 –1 –1132 incorrect subtraction AND no ÷3 1 mark
∆fH(CO2) = 3 = –394 (kJ mol ) ✓
–303.3 / –303 no 3 for –111 AND incorrect subtraction
1 mark
–385.66… / –385.7 no ∆H (25) and ÷3 2 marks
(+)385.66… / (+)385.7 no ∆H (25) and ÷3 and wrong sign
1 mark
How to answer it
Enthalpy Changes & Combustion Study Guide
What this question tests
This question assesses your understanding of energetic chemical reactions, including writing complete combustion equations, interpreting and constructing enthalpy profile diagrams, explaining catalytic action in terms of activation energy, recalling standard conditions, and performing complex enthalpy cycle/formation calculations using stoichiometry.
Complete Combustion of Heptane
✅ Correct Answer
C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O
💡 Key Knowledge
- Alkanes burn in excess oxygen to produce carbon dioxide and water.
- General formula for combustion balancing: balance carbons first, hydrogens second, and oxygens last.
❌ Common Errors
- Including state symbols when explicitly told they are not required (though examiners ignore them if correct).
- Incorrect balancing ratios for oxygen.
Enthalpy Profile Diagram for Catalytic Removal of CO
✅ Correct Answer Requirements
- Reactants labelled as 2CO(g) + 2NO(g) placed higher than products.
- Products labelled as 2CO₂(g) + N₂(g) placed lower down.
- ΔH clearly indicated with a downward vertical or double-headed arrow between reactant and product energy levels.
- Eₐ arrow clearly pointing from the reactant energy level up to the absolute peak of the curve.
🧠 Exam Technique
- Since ΔH = -746 kJ mol⁻¹ is negative, this is an exothermic reaction. Products must sit lower than reactants.
- Ensure your activation energy ( Eₐ ) line touches or originates from the reactant energy level up to the very top of the curve. Do not start it from the axis!
Explaining the Role of a Catalyst
✅ Correct Answer
The catalyst lowers the activation energy ( Eₐ ) by providing an alternative reaction pathway with a lower activation energy, allowing more molecules/particles to exceed the lower activation energy.
💡 Key Knowledge
- Catalysts are chemically unchanged at the end of the reaction.
- They do not change the enthalpy change ( ΔH ) of the reaction.
Standard Conditions
✅ Correct Answer
Temperature: 298 K (or 25 °C)
Pressure: 100 kPa (or 1 atm / 101 kPa / 1 bar)
❌ Common Errors
- Stating 0 °C instead of 25 °C (confusing standard temperature and pressure with standard freezing point).
- Omitting units for pressure or temperature.
Calculation of Standard Enthalpy of Formation for CO₂
📐 Step-by-Step Calculation
Given Equation:
Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g) ΔH = -25 kJ mol⁻¹
Given Enthalpies of Formation:
ΔₒHf(Fe₂O₃) = -824 kJ mol⁻¹
ΔₒHf(CO) = -111 kJ mol⁻¹
ΔₒHf(Fe) = 0 kJ mol⁻¹ (elements in standard state)
Step 1: Set up the enthalpy change formula
ΔH = ΣΔfH(products) - ΣΔfH(reactants)
Step 2: Substitute known values incorporating stoichiometry
-25 = [2(0) + 3ΔfH(CO₂)] - [(-824) + 3(-111)]
Step 3: Simplify reactant terms
Reactants total = -824 + (-333) = -1157 kJ mol⁻¹
-25 = 3ΔfH(CO₂) - (-1157)
-25 = 3ΔfH(CO₂) + 1157
Step 4: Rearrange and solve for ΔfH(CO₂)
3ΔfH(CO₂) = -25 - 1157 = -1182
ΔfH(CO₂) = -1182 / 3 = -394 kJ mol⁻¹
❌ Common Calculation Traps
- Forgetting stoichiometry multipliers: Forgetting to multiply CO and CO₂ by 3.
- Sign errors: Incorrectly subtracting negative values or mismanaging the overall rearrangement, resulting in +394 or -1182 .
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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.