AS
Cambridge (CIE)
Chemistry
17 mins
17 questions
Multiple Choice Questions
Enthalpy Change, ΔH
Enthalpy Change, ΔH / Reaction Pathway Diagrams / Standard Enthalpy Change /
Enthalpy & Bond Energies
Easy (6 questions)
/6
Medium (6 questions)
/6
Hard (5 questions)
/5
Total Marks
/17
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Easy Questions
1 Which equation below can represent both an enthalpy change of formation and
combustion?
A. CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
B. 4Na (s) + O2 (g) → 2Na2O (s)
C. HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
D. CO (g) + C (s) → CO2 (g)
(1 mark)
2 The reaction pathway for a reversible reaction is shown below:
Which statement is correct?
A. The activation energy of the reverse reaction is +90 kJ mol–1
B. The activation energy of the forward reaction is +20 kJ mol–1
C. The activation of the reverse reaction is +20 kJ mol–1
D. The enthalpy change of forwards reaction is - 70 kJ mol–1
(1 mark)
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3 The reaction pathway for a reversible reaction is shown below.
Which statements are correct?
1. The forward reaction is endothermic.
2. The enthalpy change for the backward reaction is –30 kJ mol–1.
3. The activation energy for the forward reaction is +90 kJ mol–1.
A. 1 only
B. 1 and 2
C. 2 and 3
D. 1, 2 and 3
(1 mark)
4 A student carried out an experiment to determine the enthalpy change for the
combustion of ethanol.
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The following results were obtained by the student. The specific heat capacity of water is
4.18 J g−1 ˚C−1.
Start temperature of the water
21 °C
Final temperature of the water
54 °C
Mass of alcohol burner before burning
259.75 g
Mass of alcohol burner after burning
259.18 g
Mass of glass beaker plus water
150.00 g
Mass of glass beaker
50.0 g
How much of the heat energy produced by the burning of ethanol went into the water?
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A. 6897 J
B. 20691 J
C. 22572 J
D. 13794 J
(1 mark)
5 PCl5 (g) dissociates as follows.
PCl5 (g) → PCl3 (g) + Cl2 (g)
The yield of the products increases as the temperature is raised from 80oC to 110oC.
Which row correctly describes the formation of PCl3 (g)?
Shape of PCl3 molecule
The reaction is
A
Pyramidal
Endothermic
B
Pyramidal
Exothermic
C
Trigonal
Endothermic
D
Trigonal
Exothermic
(1 mark)
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5
6 Hydrogen atoms bond covalently to Iodine atoms to form hydrogen iodide as shown in
the equation below:
H2 (g) + I2 (g) → 2HI (g)
Which statement best describes what is meant by the average HI bond enthalpy?
A. The energy stored in a covalent bond.
B. The energy required to break one covalent bond in the gas phase.
C. The energy required to break one mole of the HI bonds in the gas phase.
D. The energy released when two atoms form a covalent bond.
(1 mark)
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Medium Questions
1 A student mixed 30.0 cm3 of 0.0250 mol dm-3 potassium hydroxide solution with 30.0
cm3 of 0.0250 mol dm-3 nitric acid. The temperature rose by 0.50 oC. Assume no heat was
lost to the surroundings.
The final mixture had a specific heat capacity of 4.18 J cm-3 K-1.
What is the molar enthalpy change for the reaction?
A. -84.0 kJ mol-1
B. -83.6 kJ mol-1
C. 167.2 kJ mol-1
D. -167.2 kJ mol-1
(1 mark)
2 An experiment was carried out to determine the approximate value for the molar
enthalpy change of neutralisation.
75 cm3 of 3.00 mol dm–3 hydrochloric acid was placed in a polystyrene beaker of
negligible heat capacity. Its temperature was recorded, and then 75 cm3 of 3.00 mol
dm−3 potassium hydroxide at the same temperature was quickly added, and the solution
was stirred.
The temperature rose by 14 °C. The resulting solution may be considered to have a
specific heat capacity of 4.18 J g-1 K-1.
Calculate the molar enthalpy change of neutralisation.
A. -4876.7 J mol-1
B. -39 013.3 J mol-1
C. -39.0 J mol-1
D. -11 843.3 J mol-1
(1 mark)
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3 The standard enthalpy change ∆Hθ for the below reaction is -1776 kJ.
2N (g) + 6F (g) → 2NF3 (g)
What is the bond energy of the N–F bond?
A. - 592
B. + 592
C. + 296
D. - 296
(1 mark)
4 The complete combustion of ethyne, C2H2, is shown in the equation below.
H−C≡C−H + 2½ O=O → O−H−H + 2O=C=O
Using the average bond enthalpies given in the table, what is the enthalpy change of
combustion of ethyne?
Bond
Average bond enthalpy / kJ mol-1
C−H
390
C≡C
870
O=O
489
C=O
790
O−H
510
O−C
290
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A. + 1307.5 kJ mol–1
B. - 1307.5 kJ mol–1
C. + 1390 kJ mol–1
D. - 1390 kJ mol–1
(1 mark)
5 Compound R into compound T, it was found that the reaction proceeded by way of
compound S, which could be isolated. The following steps were involved.
R → S; ΔH is positive
S → T; ΔH is negative
Which reaction profile fits these data?
(1 mark)
6 The table below discusses three types of enthalpy change:
‘+’ means that this type of standard enthalpy change can only have positive values.
‘–’ means that this type of standard enthalpy change can only have negative values.
‘+ / –’ means that either positive or negative values are possible.
Which row is correct?
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Formation
Combustion
Neutralisation
A
+
+
+/–
B
+/–
+
+/–
C
+/–
–
–
D
–
–
+
(1 mark)
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Hard Questions
1 In the gas phase, phosphorus pentachloride can be thermally decomposed into gaseous
phosphorus trichloride and chlorine.
PCl5 → PCl3 + Cl2
The table below gives the relevant bond energies found in these compounds.
Bond
Bond energy / kJ mol–1
P – Cl (in both chlorides)
328
Cl – Cl
241
What is the enthalpy change in the decomposition of the reaction?
A. – 415 kJ mol–1
B. + 415 kJ mol–1
C. + 95 kJ mol–1
D. – 95 kJ mol–1
(1 mark)
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2 In a calorimetric experiment 2.50 g of a fuel is burnt in oxygen. 30 % of the energy
released during the combustion is absorbed by 500 g of water, the temperature of which
rises from 25 °C to 68 °C.
The specific heat capacity of water is 4.18 J g–1 K–1.
What is the total energy released per gram of fuel burnt?
A. 25 284 J
B. 63 210 J
C. 119 827 J
D. 301 000 J
(1 mark)
3 Which equation correctly shows how the bond energy for the covalent bond Y-Z can be
calculated by dividing ΔH by n?
A. n YZ (g) → n Y (g) +
n
Z (g)
2 2
B. Z (g) + Y Zn-1 (g) → YZn (g)
C. 2 YZn (g) → 2 YZn-1 (g) + Y2 (g)
D. YZn (g) → Y (g) + nZ (g)
(1 mark)
4 The diagram shows the skeletal formula of cyclobutane.
The enthalpy change of formation of cyclobutane is +75.1 kJ mol–1, and the enthalpy
change of atomisation of graphite is +712 kJ mol–1.
The bond enthalpy of C–H is 390 kJ mol–1 and of H–H is 429 kJ mol–1.
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What is the average bond enthalpy of the C–C bond in cyclobutane, rounded to the
nearest whole number?
A. 236 kJ mol–1
B. 315 kJ mol–1
C. 342 kJ mol–1
D. 700 kJ mol–1
(1 mark)
5 Some bond energy values are listed below.
Bond energy / kJ mol-1
Bond
Br – Br
194
Cl – Cl
247
C–H
412
C – Cl
338
These bond energy values relate to the following four reactions:
W
Br2 → 2Br
X
2Cl → Cl2
Y
CH3 + Cl → CH3Cl
Z
CH4 → CH3 + H
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What is the correct order of enthalpy changes of the above reactions from most negative
to most positive?
A. Y → Z → W → X
B. Z → W → X → Y
C. Y → X → W → Z
D. X → Y → Z → W
(1 mark)
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