OCR Chemistry A
1
9 Enthalpy
Exam-style questions
The following reaction represents an exothermic reaction:
A+B→C+D
∆rH = –638 kJ mol–1
The activation energy for this reaction is +345 kJ mol–1.
a i Define the term activation energy.
(1 mark)
ii
Draw an enthalpy profile diagram for this reaction on the axes below and
label both the activation energy, Ea, and the enthalpy change of reaction,
∆rH.
(2 marks)
iii Use your diagram to calculate the activation energy for the reverse
reaction and suggest why it is unlikely that the reverse reaction will occur.
Activation energy =
(2 marks)
b The reaction below is an exothermic reaction called the thermite reaction:
Fe2O3 + 2Al → Al2O3 + 2Fe
When iron(III)oxide reacts with aluminium, the heat generated will melt the
iron produced. A burning splint cannot initiate the reaction, so instead a fuse
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9 Enthalpy
Exam-style questions
OCR Chemistry A
made of magnesium is inserted into the reactant mixture. This fuse is ignited
to initiate the reaction. Suggest why a burning splint cannot start the reaction.
(1 mark)
c
The thermite reaction is exothermic. Explain what this means in terms of
bond breaking and making.
(2 marks)
2
Ethyne gas, C2H2, is used in welding torches as, when ethyne burns in oxygen, the
flame temperature can reach 3300 °C. Table 1 below shows some average bond
enthalpies.
Table 1 Average bond enthalpies
a
Bond
Average bond enthalpy (kJ mol–1)
C≡C
839
C–H
413
O=O
498
C=O
799
O–H
464
Use the bond enthalpies in Table 1 to calculate the enthalpy change for the
following reaction:
C2H2 + 2
O2 → 2CO2 + H2O
In terms of bonds, the reaction is:
H–C≡C–H + 2
O=O → 2O=C=O + H–O–H
Enthalpy change =
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(3 marks)
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OCR Chemistry A
9 Enthalpy
Exam-style questions
b Is this reaction exothermic or endothermic? Explain how you made your
choice.
(2 marks)
c
Suggest a reason why an enthalpy change calculated from average bond
enthalpies might not be accurate.
(1 mark)
d The enthalpy change of reaction, ∆rH, for the reaction given in part a is also
known as ∆cH of ethyne. Explain why.
(2 marks)
e
Using your answer to part a, calculate the enthalpy change if 78.0 g of ethyne
is completely burnt in oxygen.
Enthalpy change =
3
(2 marks)
25.0 cm3 of 2.0 mol dm–3 hydrochloric acid, HCl, was added to 50.0 cm3 of
2.0 mol dm–3 sodium hydroxide, NaOH. The starting temperature was 22 °C and
the maximum temperature reached was 29 °C. Equation 3.1 below can be used
to calculate the enthalpy change of neutralisation:
Equation 3.1 q = mc∆T
where c = 4.18 J g–1 K–1
a
Write an equation for the neutralisation reaction taking place.
(1 mark)
b What does the symbol c represent in Equation 3.1?
(1 mark)
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OCR Chemistry A
c
9 Enthalpy
Exam-style questions
Use Equation 3.1 to calculate q in kJ.
q=
(2 marks)
d Calculate the number of moles of water produced in this reaction.
moles of water =
e
(1 mark)
Use your answers to parts c and d to calculate the enthalpy change of
neutralisation. Give your answer to an appropriate number of significant
figures.
enthalpy change of neutralisation =
f
(2 marks)
The apparatus used in this experiment is shown below. Predict, with a
reason, whether the experimental enthalpy change of neutralisation,
calculated in part e, will be more or less exothermic than the theoretical
enthalpy change of neutralisation.
(1 mark)
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OCR Chemistry A
9 Enthalpy
Exam-style questions
g Suggest two alterations to the apparatus that would improve this experiment
and that would ensure that the experimental enthalpy change value is closer
to the theoretical value.
(2 marks)
4
A Hess Cycle is needed to deduce the enthalpy change of the following reaction,
as it cannot be measured directly.
Reaction 4.1 6C(s) + 6H2(g) + 3O2(g) → C6H12O6(s)
a
Suggest a reason why the enthalpy change of this reaction cannot be
measured directly.
(1 mark)
b Explain why the enthalpy change of this reaction represents the enthalpy
change of formation of glucose, C6H12O6.
(2 marks)
c
Some enthalpy changes are given below:
C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
C(s) + O2(g) → CO2(g)
H2(g) + 1
O2(g) → H2O(l)
∆H = –2801 kJ mol–1
∆H = –394 kJ mol–1
∆H = –286 kJ mol–1
Calculate the enthalpy change of Reaction 4.1.
enthalpy change of reaction =
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(3 marks)
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OCR Chemistry A
5
9 Enthalpy
Exam-style questions
Methanol, CH3OH, can be used as a fuel in camping stoves. The following
equation shows the complete combustion of methanol in oxygen:
∆cHƟ = –727 kJ mol–1
a
The Ɵ sign, in ∆cHƟ, shows that the enthalpy change of combustion was
calculated under standard conditions. State what the standard conditions are.
(1 mark)
b Calculate a value for the enthalpy change of formation of methanol using the
following information:
∆fH of CO2(g) = –394 kJ mol–1
∆fH of H2O(l) = –286 kJ mol–1
enthalpy change of formation =
c
(3 marks)
The enthalpy change of combustion, ∆cH, of methanol can be determined
experimentally using the equipment below:
i
Name each of the liquids labelled X and Y.
(1 mark)
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OCR Chemistry A
ii
9 Enthalpy
Exam-style questions
Suggest, with a reason, an appropriate material that the calorimeter
should be made from.
(1 mark)
iii What measurements need to be recorded from this experiment in order to
calculate the enthalpy change of combustion, ∆cH, of methanol.
(3 marks)
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