Chemistry questions, chapter 3

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Thermochemistry

46 marks. Use Practice to check each answer as you go, or Exam to answer everything against the clock first.

Section A: objective questions

12 marks

Choose the best answer for each question. Use c = 4.2 J g⁻¹ °C⁻¹ and density of solution = 1 g cm⁻³ where needed.

Question 1

Which describes an exothermic reaction?

Heat is absorbed and the temperature of the mixture falls
Heat is released to the surroundings and the temperature of the mixture rises
Heat is released and the temperature of the mixture falls
There is no heat change

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An exothermic reaction releases heat to the surroundings, so the mixture becomes hotter and ΔH is negative.

For the teacher: Remembering, low difficulty, standard 3.1.1

Textbook p. 116

Question 2

Which of these is an endothermic reaction?

Neutralisation of an acid by an alkali
Combustion of petrol
Photosynthesis
Respiration

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Photosynthesis absorbs energy. The other three release heat.

For the teacher: Remembering, low difficulty, standard 3.1.1

Textbook p. 116

Question 3

In a reaction, ΔH = −85 kJ mol⁻¹. What does the negative sign show?

The products have less energy than the reactants and heat is released
The products have more energy than the reactants and heat is absorbed
The reaction needs a catalyst
The reaction is very slow

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ΔH = H(products) − H(reactants). A negative value means the products are lower in energy, so heat is released (exothermic).

For the teacher: Understanding, low difficulty, standard 3.1.2

Textbook p. 118

Question 4

The heat of precipitation is the heat change when

1 mole of water is formed from an acid and an alkali
1 mole of a metal is displaced from its salt solution
1 mole of a substance burns completely in excess oxygen
1 mole of precipitate is formed from its ions in aqueous solution

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A is the heat of neutralisation, B displacement and C combustion.

For the teacher: Remembering, low difficulty, standard 3.2.1

Textbook p. 121

Question 5

Diagram 1 shows an energy level diagram. Which statement is correct?

Energy level diagram with the products higher than the reactants — Diagram 1

The reaction is exothermic
ΔH is negative
The temperature of the mixture decreases
The reactants have more energy than the products

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The products are on the higher level, so heat is absorbed: endothermic, ΔH positive, and the temperature falls.

For the teacher: Applying, medium difficulty, standard 3.1.2

Textbook p. 118

Question 6

A reaction is exothermic. What is true about the energy changes of the bonds?

Heat released when bonds form is greater than heat absorbed when bonds break
Heat absorbed when bonds break is greater than heat released when bonds form
No heat is absorbed when bonds break
Heat is released when bonds break

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In an exothermic reaction, forming the new bonds releases more heat than breaking the old bonds absorbs.

For the teacher: Understanding, medium difficulty, standard 3.1.2

Textbook p. 119

Question 7

The heat of neutralisation of ethanoic acid with sodium hydroxide is lower than that of hydrochloric acid with sodium hydroxide. Why?

Ethanoic acid is more concentrated
Ethanoic acid produces less water
Ethanoic acid is a diprotic acid
Some heat is used to ionise ethanoic acid completely, because it is a weak acid

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Ethanoic acid ionises only partially. Part of the heat released is absorbed to ionise the remaining molecules, so less heat is given out.

For the teacher: Understanding, medium difficulty, standard 3.2.3

Textbook p. 130

Question 8

The reaction H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O releases 114 kJ. What is the heat of neutralisation?

−114 kJ mol⁻¹
−57 kJ mol⁻¹
−28.5 kJ mol⁻¹
−228 kJ mol⁻¹

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Two moles of water form, so per mole of water = 114 ÷ 2 = 57 kJ, ΔH = −57 kJ mol⁻¹.

For the teacher: Applying, medium difficulty, standard 3.2.3

Textbook p. 131

Question 9

50 g of a solution is heated so that its temperature rises by 6.0 °C. What is the heat absorbed?

1260 J
50.4 J
2520 J
300 J

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Q = mcθ = 50 × 4.2 × 6.0 = 1260 J.

For the teacher: Applying, medium difficulty, standard 3.2

Textbook p. 120

Question 10

What is the unit of fuel value?

kJ mol⁻¹
J °C⁻¹
kJ dm⁻³
kJ g⁻¹

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Fuel value is the heat released per gram of fuel burnt, in kJ g⁻¹.

For the teacher: Remembering, low difficulty, standard 3.3.2

Textbook p. 137

Question 11

The heat of combustion of ethanol, C₂H₅OH, is −1376 kJ mol⁻¹. [Relative atomic mass: H = 1, C = 12, O = 16] What is its fuel value?

0.03 kJ g⁻¹
29.9 kJ g⁻¹
63 296 kJ g⁻¹
1330 kJ g⁻¹

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M = 46 g mol⁻¹. Fuel value = 1376 ÷ 46 = 29.9 kJ g⁻¹.

For the teacher: Applying, medium difficulty, standard 3.3.2

Textbook p. 137

Question 12

The heat of combustion increases from methanol to ethanol to propanol. Which is the best explanation?

Heavier alcohols are more soluble in water
Heavier alcohols have a lower boiling point
Each molecule has more carbon and hydrogen atoms, so more carbon dioxide and water form
Larger alcohol molecules need less oxygen

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More C and H atoms per molecule means more CO₂ and H₂O molecules form and more bonds are made, so more heat is released.

For the teacher: Analysing, high difficulty, standard 3.2.4

Textbook p. 134

Section B: structured questions

34 marks

Answer all questions. Write your answers in the spaces provided. Use c = 4.2 J g⁻¹ °C⁻¹ and density of solution = 1 g cm⁻³.

Question 13

State whether each process is exothermic or endothermic.

[1 mark] (a)Dissolving solid ammonium nitrate in water

AExothermicBEndothermic

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Endothermic

Ammonium salts dissolving absorb heat.

[1 mark] (b)Neutralisation of hydrochloric acid with sodium hydroxide

AExothermicBEndothermic

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Exothermic

Neutralisation releases heat.

[1 mark] (c)Heating copper(II) carbonate until it decomposes

AExothermicBEndothermic

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Endothermic

Decomposition of a metal carbonate on heating absorbs heat.

For the teacher: Understanding, low difficulty, standard 3.1.1

Textbook p. 116

Question 14

Diagram 2 shows the energy level diagram for a reaction in which P forms Q.

Energy level diagram with P on the higher level and Q on the lower level — Diagram 2

[2 marks] (a)Is the reaction exothermic or endothermic? Give a reason.

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Exothermic [1]1
Energy content of the product Q is lower than that of reactant P / heat is released [1]1

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[1 mark] (b)State the sign of ΔH.

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Negative1

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For the teacher: Applying, medium difficulty, standard 3.1.2

Textbook p. 118

Question 15

The equation is H₂(g) + Br₂(g) → 2HBr(g). Bond energies: H–H 436, Br–Br 193, H–Br 366 kJ mol⁻¹.

[1 mark] (a)Calculate the heat absorbed to break the bonds in the reactants.

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436 + 193 = 629 kJ1

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[1 mark] (b)Calculate the heat released when the bonds in the product form.

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2 × 366 = 732 kJ1

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[2 marks] (c)Calculate ΔH and state whether the reaction is exothermic or endothermic.

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ΔH = 629 − 732 = −103 kJ mol⁻¹ [1]1
Exothermic (ΔH negative) [1]1

Tick each point you got.

For the teacher: Applying, medium difficulty, standard 3.1.2

Textbook p. 119

Question 16

50 cm³ of 2.0 mol dm⁻³ calcium chloride, CaCl₂, is added to 50 cm³ of 2.0 mol dm⁻³ sodium carbonate, Na₂CO₃. The temperature of the mixture falls from 27.0 °C to 23.0 °C.

[1 mark] (a)Write the ionic equation for the precipitation of calcium carbonate.

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Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)1

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[1 mark] (b)Calculate the number of moles of calcium carbonate formed.

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2.0 × 50 / 1000 = 0.1 mol1

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[1 mark] (c)Calculate the heat absorbed.

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Q = 100 × 4.2 × 4.0 = 1680 J (1.68 kJ)1

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[2 marks] (d)Calculate the heat of precipitation of calcium carbonate.

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1.68 / 0.1 = 16.8 kJ [1]1
ΔH = +16.8 kJ mol⁻¹ (positive, temperature fell) [1]1

Tick each point you got.

For the teacher: Applying, medium difficulty, standard 3.2.1

Textbook p. 122

Question 17

Excess magnesium powder is added to 50 cm³ of 0.25 mol dm⁻³ iron(II) sulphate solution at 29.0 °C. Mg(s) + Fe²⁺(aq) → Mg²⁺(aq) + Fe(s), ΔH = −80.6 kJ mol⁻¹.

[2 marks] (a)Calculate the heat released.

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n(Fe) = 0.25 × 50 / 1000 = 0.0125 mol [1]1
Heat = 0.0125 × 80.6 = 1.0075 kJ = 1008 J [1]1

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[2 marks] (b)Calculate the highest temperature reached.

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θ = 1007.5 / (50 × 4.2) = 4.8 °C [1]1
Highest temperature = 29.0 + 4.8 = 33.8 °C [1]1

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For the teacher: Applying, high difficulty, standard 3.2.2

Textbook p. 125

Question 18

50 cm³ of 2.0 mol dm⁻³ hydrochloric acid is mixed with 50 cm³ of 2.0 mol dm⁻³ sodium hydroxide. The temperature rises from 30.0 °C to 43.5 °C.

[3 marks] (a)Calculate the heat of neutralisation.

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n(H₂O) = 2.0 × 50 / 1000 = 0.1 mol [1]1
Q = 100 × 4.2 × 13.5 = 5670 J = 5.67 kJ [1]1
ΔH = −5.67 / 0.1 = −56.7 kJ mol⁻¹ [1]1

Tick each point you got.

[1 mark] (b)Write the thermochemical equation.

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HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) ΔH = −56.7 kJ mol⁻¹1

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[2 marks] (c)The experiment is repeated with ethanoic acid of the same volume and concentration. Predict how the temperature rise compares and explain.

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Smaller temperature rise / lower heat of neutralisation [1]1
Ethanoic acid is a weak acid and ionises partially; part of the heat released is used to ionise it completely [1]1

Tick each point you got.

For the teacher: Analysing, medium difficulty, standard 3.2.3

Textbook p. 131

Question 19

The heat of combustion of methanol, CH₃OH, is −725 kJ mol⁻¹. [Relative atomic mass: H = 1, C = 12, O = 16] In an experiment the flame of burning methanol heats 100 cm³ of water by 30.0 °C. Assume no heat loss.

[1 mark] (a)Calculate the heat absorbed by the water.

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Q = 100 × 4.2 × 30.0 = 12 600 J = 12.6 kJ1

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[2 marks] (b)Calculate the mass of methanol burnt.

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n = 12.6 / 725 = 0.0174 mol [1]1
Mass = 0.0174 × 32 = 0.56 g [1]1

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[1 mark] (c)Suggest why the mass of methanol burnt in a real experiment is greater than your answer.

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Some heat is lost to the surroundings / absorbed by the apparatus (copper can) / incomplete combustion1

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For the teacher: Applying, medium difficulty, standard 3.2.4

Textbook p. 135

Question 20

The fuel values of four fuels are: ethanol 30 kJ g⁻¹, petrol 34 kJ g⁻¹, natural gas 50 kJ g⁻¹ and hydrogen 143 kJ g⁻¹.

[1 mark] (a)Which fuel releases the most heat per gram?

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Hydrogen1

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[1 mark] (b)Give one environmental advantage of hydrogen compared with petrol.

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Burns to form only water / no carbon dioxide, soot or toxic gases1

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[1 mark] (c)Give one reason why hydrogen is still not widely used in vehicles.

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Difficult and costly to produce, store or transport safely / flammable gas / needs special engines1

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[2 marks] (d)State two factors, other than fuel value, in choosing a fuel for cooking.

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Cost [1]1
Availability / safety / effect on the environment (soot) [1]1

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For the teacher: Evaluating, high difficulty, standard 3.3.2

Textbook p. 138

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