A LevelChemistryCIETopic Questions5. Chemical Energetics5.1 Enthalpy Change, ΔH

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First teaching 2023

First exams 2025

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Enthalpy Change, ΔH (CIE A Level Chemistry)

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11 mark

Which equation below can represent both an enthalpy change of formation and combustion?

  • CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

  • 4Na (s) + O2 (g) → 2Na2O (s)

  • HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

  • CO (g) + C (s) → CO2 (g)

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21 mark

The reaction pathway for a reversible reaction is shown below:

1-5-diagram-q4e

Which statement is correct?

  • The activation energy of the reverse reaction is +90 kJ mol–1 

  • The activation energy of the forward reaction is +20 kJ mol–1

  • The activation of the reverse reaction is +20 kJ mol–1

  • The enthalpy change of forwards reaction is - 70 kJ mol–1

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31 mark

The reaction pathway for a reversible reaction is shown below.

1-5-diagram-q5e

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.

  • 1 only

  • 1 and 2

  • 2 and 3

  • 1, 2 and 3

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41 mark

A student carried out an experiment to determine the enthalpy change for the combustion of ethanol.

The following results were obtained by the student. The specific heat capacity of water is 4.18 J g−1 ˚C−1.

1-5-diagram-q7e

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?

  • 6897 J

  • 20691 J

  • 22572 J

  • 13794 J

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51 mark

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

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61 mark

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?

  • The energy stored in a covalent bond.

  • The energy required to break one covalent bond in the gas phase.

  • The energy required to break one mole of the HI bonds in the gas phase.

  • The energy released when two atoms form a covalent bond.

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11 mark

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)

Cl – Cl 

328

241

What is the enthalpy change in the decomposition of the reaction?

  • – 415 kJ mol–1

  • + 415 kJ mol–1

  • + 95 kJ mol–1

  • – 95 kJ mol–1

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21 mark

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?

  • 25 284 J

  • 63 210 J

  • 119 827 J

  • 301 000 J

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31 mark

Which equation correctly shows how the bond energy for the covalent bond Y-Z can be calculated by dividing ΔH by n?

  • n YZ (g) → n Y (g) + n over 2 Z2 (g)

  • Z (g) + Y Zn-1 (g) → YZn (g)

  • 2 YZn (g) → 2 YZn-1 (g) + Y2 (g)

  • YZn (g) → Y (g) + nZ (g)

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41 mark

The diagram shows the skeletal formula of cyclobutane.

1-5-h-5

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.

What is the average bond enthalpy of the C–C bond in cyclobutane, rounded to the nearest whole number?

  • 236 kJ mol–1

  • 315 kJ mol–1

  • 342 kJ mol–1

  • 700 kJ mol–1

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51 mark

Some bond energy values are listed below.

Bond

Bond energy / kJ mol-1

  Br – Br  

Cl – Cl

C – H

C – Cl

194

247

412

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

What is the correct order of enthalpy changes of the above reactions from most negative to most positive?

  • Y → Z → W → X

  • Z → W → X → Y

  • Y → X → W → Z

  • X → Y → Z → W

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11 mark

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?

  • -84.0 kJ mol-1

  • -83.6 kJ mol-1

  • 167.2 kJ mol-1

  • -167.2 kJ mol-1

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21 mark

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.

  • -4876.7 J mol-1

  • -39 013.3 J mol-1

  • -39.0 J mol-1

  • -11 843.3 J mol-1

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31 mark

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?

  • - 592

  • + 592

  • + 296

  • - 296

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41 mark

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

  • + 1307.5 kJ mol–1

  • - 1307.5 kJ mol–1

  • + 1390 kJ mol–1

  • - 1390 kJ mol–1

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51 mark

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?

1-5-m-7

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61 mark

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?

 

Formation

Combustion

Neutralisation

A

+

+

+ / –

B

+ / –

+

+ / –

C

+ / –

–

–

D

–

–

+

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