Energetics (Edexcel International A Level Chemistry)

Exam Questions

1a
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8 marks

This question is about enthalpy changes.

An experiment was carried out to determine the enthalpy change of combustion for ethanol.

C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l)

1.19 g of ethanol was burned in a spirit burner. The heat energy from this combustion raised the temperature of 100 g of water from 21.6°C to 63.9 °C.

i)

Calculate the number of moles of ethanol in 1.19 g.

[Molar mass of ethanol = 46.0 g mol−1 ]

(1)

ii)

Calculate the heat energy required to raise the temperature of 100g of water from 21.6 °C to 63.9 °C.

[Specific heat capacity of water = 4.18 J g−1 °C−1]

(2)

iii)
Use your answers to (a)(i) and (ii) to calculate a value for the enthalpy change of combustion of ethanol.
Give your answer to an appropriate number of significant figures and include a sign and units.

(3)

iv)

The value of the enthalpy change of combustion from this experiment was very inaccurate.

Give two reasons why this value was so inaccurate, apart from heat loss.

(2)

1b
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5 marks

Mean bond enthalpies can be used to calculate a value for the enthalpy change of combustion of a compound.

i)
Give the meaning of the term ‘mean bond enthalpy’.

(2)

ii)
Calculate a value for the enthalpy change of combustion of methanol, using the information in the table and the equation shown.

(3)

CH3OH + 1½O2 → CO2 + 2H2O

  C–H C–O O–H O=O C=O
Mean bond enthalpy / kJ mol−1 413 358 464 498 805
1c
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3 marks

Enthalpy changes of combustion can be used to calculate the enthalpy change of formation of a compound.

Substance Standard enthalpy change of combustion, cbold italic H to the power of bold ⦵/ kJ mol−1
C (s,graphite) – 394
H2 (g) – 286
CH3CHO (g) – 1167


Complete the Hess cycle and use it to calculate the standard enthalpy change of formation for ethanal, CH3CHO.

q19c-paper-2-oct-2021-edexcel-ial-chemistry

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2a2 marks

Magnesium reacts with sulfuric acid in an exothermic reaction.

Write an equation for the reaction.
Include state symbols in your answer.

2b
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3 marks

A student carried out an experiment to determine the enthalpy change of the reaction.

A sample of 0.50 g of magnesium powder was added to 25 cm3 of 0.20 mol dm−3 sulfuric acid.

Calculate the number of moles of magnesium and of sulfuric acid that reacted.
Justify your answer.

2c4 marks
i)
The results obtained are given in the table.

Plot the results on the grid.

(2)

q19c

q22a-paper-1-june-2021-edexcel-ial-chemistry

ii)
Use your graph to determine the maximum change in temperature.

You must show your working on the graph.

(2)
ΔT = ..............................................................

2d
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4 marks

Calculate the standard molar enthalpy change for the reaction, using your answers to (b) and (c)(ii).

Include a sign and units in your answer.

[Specific heat capacity of solution = 4.18 J g–1 °C–1]

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3a
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5 marks

Enthalpy changes of combustion can be determined using calorimetry or calculated using Hess cycles. Apparatus for a calorimetry experiment is shown.

q21-paper-2-jan-2022-edexcel-ial-chemistry

A sample of 2‐methylpropan‐2‐ol was burned in a spirit burner and used to heat 75 g of water. The results are shown.

  At the start At the end Change
Mass of spirit burner / g 267.35 266.78  
Temperature of water / °C 19.5 65.3  

i)
Complete the table.

(1)

ii)

Calculate the enthalpy change of combustion, ΔcH, of 2‐methylpropan‐2‐ol.
Give a sign and units in your answer.
[Specific heat capacity of water = 4.18 Jg−1 °C−1]

(4)

3b
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5 marks

The standard enthalpy change of combustion, ΔcH to the power of ⦵, can be calculated using standard enthalpy changes of formation.

Compound fH to the power of ⦵/ kJ mol−1
2‐methylpropan‐2‐ol –359
carbon dioxide –394
water –286

i)
State why no ∆fH to the power of ⦵value has been given for oxygen.
(1)
ii)
Complete the Hess cycle.

q21b-ii-paper-2-jan-2022-edexcel-ial-chemistry
(2)
iii)
Calculate the standard enthalpy change of combustion of 2‐methylpropan‐2‐ol using the data in the table and the completed Hess cycle.

(2)

3c2 marks

The value for ΔcH obtained in part (a)(ii) is much less exothermic than ΔcH to the power of ⦵calculated in (b)(iii).

Suggest two reasons for this other than non‐standard conditions.

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4a2 marks

This question is about ethanoic acid and some related salts.

A test to confirm the presence of an aqueous acid is adding a small amount of solid sodium carbonate to the solution.
Describe two observations you would see in this test.

4b
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5 marks

Sodium ethanoate is a component of reusable hand warmers.

In use, a supersaturated solution of sodium ethanoate recrystallises to form solid hydrated sodium ethanoate, releasing energy.

CH3COONa (aq) + 3H2O (l) → CH3COONa.3H2O (s)      ΔrH = −19.7 kJ mol−1

A hand warmer has a mass of 63.2 g and forms 20.1 g of hydrated sodium ethanoate on recrystallisation.

Calculate the maximum temperature reached by the hand warmer if its initial temperature is 5.0 °C.

[Specific heat capacity of the hand warmer = 3.0 J °C−1 g−1]

4c
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5 marks

Ammonium ethanoate, CH3COONH4(s), is used to control the pH of foods.
It can be formed by the reaction of pure ethanoic acid, CH3COOH(l), with ammonium carbonate, (NH4)2CO3(s).

Calculate the standard enthalpy change for this reaction by completing the Hess cycle and using the data shown.

q20c-1-paper-2-jan-2021-edexcel-ial-chemistry

Compound Enthalpy change of formation / kJ mol−1
CH3COOH(l) −484.5
(NH4)2CO3(s) −939.9
CH3COONH4(s) −586.3
CO2(g) −393.5
H2O(l) −285.8
4d
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2 marks

Ammonium carbonate, (NH4)2CO3 , is an ingredient in cleaning solutions for camera lenses.

These are aqueous solutions which contain no more than 1.8 g of ammonium carbonate in 100 cm3 of solution.

Calculate the maximum concentration, in mol dm−3, of ammonium carbonate in such a solution.

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