Science questions, chapter 5

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Thermochemistry

43 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.

Question 1

What is thermochemistry?

The study of heat changes when chemical reactions occur
The study of the temperature of the atmosphere
The study of how heat flows through solids
The study of the melting points of substances

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Thermochemistry is the study of heat changes that occur when chemical reactions take place (textbook p. 147).

For the teacher: Remembering, low difficulty, standard 5.1.1

Textbook p. 147

Question 2

Which statement describes an exothermic reaction?

Heat is absorbed from the surroundings
There is no change in temperature
Heat is released into the surroundings
Heat is converted into chemical energy

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An exothermic reaction releases heat into the surroundings ('exo' means outside), so the temperature rises.

For the teacher: Remembering, low difficulty, standard 5.1.1

Textbook p. 148

Question 3

The reading of a thermometer drops when two substances are mixed in a polystyrene cup. What does this show?

Heat is released and the reaction is exothermic
Heat is absorbed and the reaction is endothermic
Heat is released and the reaction is endothermic
Heat is absorbed and the reaction is exothermic

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A drop in the thermometer reading means heat is absorbed from the surroundings, which is an endothermic reaction.

For the teacher: Understanding, low difficulty, standard 5.1.2

Textbook p. 149

Question 4

Which of the following is an endothermic reaction?

Respiration
Burning of paper
Neutralisation of an acid with an alkali
Photosynthesis

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Photosynthesis absorbs heat from the surroundings, so it is endothermic. The other three release heat.

For the teacher: Remembering, low difficulty, standard 5.1.4

Textbook p. 154

Question 5

A student records the temperature of a mixture before the reaction as 27 °C and the maximum temperature as 38 °C. Which of the following is correct?

Exothermic; the temperature rises by 11 °C
Endothermic; the temperature rises by 11 °C
Exothermic; the temperature falls by 11 °C
Endothermic; the temperature rises by 65 °C

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38 − 27 = 11 °C. The temperature rises, so heat is released: an exothermic reaction.

For the teacher: Applying, medium difficulty, standard 5.1.2

Textbook p. 151

Question 6

Why can an instant cold pack help to stop bleeding?

It releases heat that makes the blood clot
It increases the lumen of the blood capillaries
It reduces the lumen of the blood capillaries, so blood flows more slowly
It adds water to the wound

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The pack absorbs heat, so the lumen of the blood capillaries becomes smaller, the rate of blood flow is reduced and this helps to stop bleeding.

For the teacher: Understanding, medium difficulty, standard 5.1.5

Textbook p. 147

Question 7

Diagram 1 shows the temperature of two reactions, P and Q, over 5 minutes. Which statement is correct?

Graph of reaction P: temperature rises from 30 to 43 degrees Celsius and then stays constant — Reaction P

Graph of reaction Q: temperature falls from 31 to 22 degrees Celsius and then stays constant — Reaction P

Q releases heat into the surroundings
P is an exothermic reaction and its temperature rises by 13 °C
P is an endothermic reaction and Q is an exothermic reaction
Both reactions release heat but P releases more

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P rises from 30 °C to 43 °C (a rise of 13 °C), so heat is released: exothermic. Q falls from 31 °C to 22 °C, so Q absorbs heat: endothermic.

For the teacher: Analysing, medium difficulty, standard 5.1.3

Textbook p. 151

Question 8

Which pair contains only exothermic reactions?

Photosynthesis and baking a cake
Respiration and photosynthesis
Neutralisation and baking a cake
Burning of petrol and respiration

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Burning of petrol and respiration both release heat. Photosynthesis and baking a cake absorb heat.

For the teacher: Understanding, medium difficulty, standard 5.1.4

Textbook p. 154

Question 9

Why does the body temperature of a boy increase when he runs vigorously?

The rate of respiration, an exothermic reaction, increases and more heat is released
The rate of respiration, an endothermic reaction, increases and more heat is absorbed
Photosynthesis takes place faster in his body
The surroundings give heat to his body through an endothermic reaction

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Respiration is exothermic. A faster rate of respiration during exercise releases more heat, so body temperature rises.

For the teacher: Applying, medium difficulty, standard 5.1.4

Textbook p. 154

Question 10

In Experiment 5.1, 50 mL of water is used each time to dissolve different solids. What is the manipulated variable?

Volume of water
Type of chemical substance
Final temperature reading
Size of the polystyrene cup

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The type of chemical substance is changed on purpose. The final temperature reading is the responding variable and the volume of water is a constant variable.

For the teacher: Analysing, medium difficulty, standard 5.1.3

Textbook p. 150

Question 11

A student wraps the polystyrene cup with cotton wool and covers it with a lid. What is the purpose?

To make the reaction faster
To increase the volume of the reaction mixture
To reduce heat transfer to or from the surroundings so that the reading is more accurate
To stop the thermometer from touching the mixture

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Cotton wool and the lid are heat insulators. They reduce heat loss or gain, so the maximum or minimum temperature is closer to the true value.

For the teacher: Understanding, high difficulty, standard 5.1.3

Textbook p. 151

Question 12

Which action helps most to reduce the effect of global warming caused by exothermic reactions?

Burning more fossil fuels for electricity
Burning waste in open fires
Using more instant hot packs
Replanting trees to increase photosynthesis

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Burning fuels is exothermic and releases heat. Trees carry out photosynthesis, an endothermic reaction that absorbs heat from the surroundings, so the surroundings become cooler.

For the teacher: Evaluating, high difficulty, standard 5.1.4

Textbook p. 154

Section B: structured questions

31 marks

Answer all questions. Write your answers in the spaces provided.

Question 13

Choose the type of reaction for each process.

[1 mark] (a)Burning of petrol

AExothermicBEndothermic

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Exothermic

Combustion releases heat.

[1 mark] (b)Photosynthesis

AExothermicBEndothermic

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Endothermic

Heat is absorbed from the surroundings.

[1 mark] (c)Making bread (baking)

AExothermicBEndothermic

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Endothermic

Heat must be supplied and absorbed.

[1 mark] (d)Rusting of iron

AExothermicBEndothermic

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Exothermic

Rusting releases heat (slowly).

For the teacher: Understanding, low difficulty, standard 5.1.4

Textbook p. 155

Question 14

Diagram 2 shows the temperature of two reactions, R and S, recorded every minute for 5 minutes.

Graph of reaction R: temperature rises from 26 to 41 degrees Celsius and then stays constant — Reaction R

Graph of reaction S: temperature falls from 33 to 23 degrees Celsius and then stays constant — Reaction R

[1 mark] (a)Which reaction is endothermic?

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Tick each point you got.

[1 mark] (b)Calculate the rise in temperature of reaction R.

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41 − 26 = 15 °C1

Tick each point you got.

[1 mark] (c)Calculate the drop in temperature of reaction S.

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33 − 23 = 10 °C1

Tick each point you got.

[2 marks] (d)The temperature of R stays at 41 °C from the 4th minute onwards. Explain why.

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The reaction products and the thermometer have reached thermal equilibrium1
The net rate of heat transfer between them is zero1

Tick each point you got.

For the teacher: Applying, medium difficulty, standard 5.1.2

Textbook p. 151

Question 15

Diagram 3 shows how 25 mL of sodium hydroxide solution is poured into 25 mL of hydrochloric acid in a polystyrene cup. The initial temperature of the acid is 29 °C and the highest temperature reached is 36 °C. The experiment is repeated with sodium hydrogen carbonate powder in place of sodium hydroxide solution; the temperature falls to 24 °C from the same initial temperature of 29 °C.

Sodium hydroxide solution poured into hydrochloric acid in a polystyrene cup with a thermometer — Diagram 3 (textbook Figure 5.2, p. 150)

[1 mark] (a)State the manipulated variable.

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Type of chemical substance (sodium hydroxide solution or sodium hydrogen carbonate)1

Tick each point you got.

[2 marks] (b)State the type of reaction between hydrochloric acid and sodium hydroxide solution. Give a reason.

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Exothermic1
The temperature rises from 29 °C to 36 °C (heat is released)1

Tick each point you got.

[2 marks] (c)Calculate the change in temperature when sodium hydrogen carbonate is used and state the type of reaction.

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29 − 24 = 5 °C drop1
Endothermic1

Tick each point you got.

[2 marks] (d)Suggest one way to make the maximum or minimum temperature reading more accurate and explain it.

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Wrap the cup with cotton wool or felt / cover it with a lid1
Heat insulator that reduces heat transfer to or from the surroundings1

Tick each point you got.

For the teacher: Analysing, medium difficulty, standard 5.1.3

Textbook p. 150

Question 16

A student heats calcium carbonate strongly in a boiling tube and passes the gas produced into limewater. The limewater turns cloudy, and the student notices that the calcium carbonate must be heated all the time.

[2 marks] (a)Is the heating of calcium carbonate an exothermic or an endothermic reaction? Explain your answer.

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Endothermic1
Heat is absorbed from the surroundings (heat must be supplied continuously) to decompose calcium carbonate1

Tick each point you got.

[1 mark] (b)Name the gas that turns limewater cloudy.

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Carbon dioxide1

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[2 marks] (c)In a thermite reaction, iron(II) oxide, aluminium powder and magnesium tape are heated and iron is melted. Is this reaction exothermic or endothermic? Describe one use of the molten iron.

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Exothermic, because a large amount of heat is released that melts the iron1
Repairing / joining broken railway tracks1

Tick each point you got.

For the teacher: Applying, medium difficulty, standard 5.1.4

Textbook p. 156

Question 17

A first-aid team has two thin small plastic bags of water, two large thick plastic bags, calcium chloride powder, ammonium nitrate powder and a toothpick. They want to make an instant hot pack for muscle cramp and an instant cold pack for a swollen wound.

[2 marks] (a)Which powder should be used in the instant hot pack? Explain.

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Calcium chloride1
Dissolving it in water is an exothermic reaction that releases heat1

Tick each point you got.

[2 marks] (b)Describe how the instant cold pack is made to work.

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Any 2 points

Put the small bag of water and ammonium nitrate powder together in the large bag1
Prick / press the small bag with the toothpick so the water mixes with the powder1
Dissolving ammonium nitrate is endothermic, so heat is absorbed and the pack becomes cold1

Tick each point you got.

[1 mark] (c)State one use of the instant hot pack in the body.

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Any 1 points

Relieves muscle cramp1
Increases the lumen of blood capillaries so blood circulates faster1

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[1 mark] (d)Why is a large thick plastic bag used for the outer layer?

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It is strong, so it holds the chemicals and does not tear or leak when the small bag is pressed or pricked1

Tick each point you got.

For the teacher: Creating, high difficulty, standard 5.1.5

Textbook p. 158

Question 18

Global warming is a phenomenon linked to exothermic reactions.

[1 mark] (a)Name one exothermic reaction that contributes to global warming.

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Burning of fossil fuels (petrol, coal, natural gas)1

Tick each point you got.

[1 mark] (b)State one action that can reduce this phenomenon.

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Any 1 points

Reduce the burning of fossil fuels1
Replant trees1

Tick each point you got.

[2 marks] (c)Explain how replanting trees reduces the effect of global warming.

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Replanting increases the rate of photosynthesis1
Photosynthesis is endothermic, so more heat is absorbed from the surroundings and the temperature falls1

Tick each point you got.

For the teacher: Evaluating, high difficulty, standard 5.1.4

Textbook p. 154

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