Chemistry questions, chapter 7

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Rate of Reaction

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

Section A: objective questions

13 marks

Choose the best answer for each question.

Question 1

What is meant by the rate of reaction?

The total quantity of product formed
The change in the quantity of reactant or product per unit time
The time taken for a reaction to start
The energy released by a reaction

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Rate of reaction is the change in the quantity of reactant used or product formed per unit time.

For the teacher: Remembering, low difficulty, standard 7.1.2

Textbook p. 221

Question 2

Which of the following is a slow reaction?

Ignition of a match
Combustion of gases
Rusting of iron
Burning of fireworks

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Corrosion of metals, food decay and fermentation are slow. Ignition, combustion of gases and fireworks are fast.

For the teacher: Remembering, low difficulty, standard 7.1.1

Textbook p. 220

Question 3

The reaction between magnesium and hydrochloric acid releases hydrogen gas. Which change can be measured to find the rate of this reaction?

The colour of the acid
The mass of the precipitate
The electrical conductivity of the room
The volume of hydrogen gas collected with time

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Volume of gas collected in a gas syringe or an inverted burette is measured against time.

For the teacher: Understanding, low difficulty, standard 7.1.3

Textbook p. 222

Question 4

60 cm³ of gas is collected in 4 minutes before the reaction stops. What is the overall average rate of reaction?

15 cm³ minute⁻¹
240 cm³ minute⁻¹
0.067 cm³ minute⁻¹
56 cm³ minute⁻¹

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Rate = 60 cm³ ÷ 4 minutes = 15 cm³ minute⁻¹.

For the teacher: Applying, low difficulty, standard 7.1.4

Textbook p. 225

Question 5

Diagram 1 shows the volume of oxygen released from the decomposition of hydrogen peroxide with manganese(IV) oxide. What is the overall average rate of reaction?

Graph of volume of oxygen against time rising to 50 cm3 at 6 minutes — Diagram 1

7.1 cm³ minute⁻¹
50 cm³ minute⁻¹
8.3 cm³ minute⁻¹
6.0 cm³ minute⁻¹

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The volume reaches 50 cm³ at 6.0 minutes and stays there, so the reaction ends at 6.0 minutes. 50 ÷ 6.0 = 8.3 cm³ minute⁻¹. Dividing by 7 minutes (7.1) is the common mistake.

For the teacher: Analysing, high difficulty, standard 7.1.5

Textbook p. 229

Question 6

Why do small marble chips react faster with hydrochloric acid than large ones of the same mass?

They have a larger total surface area
They have a higher activation energy
They have a lower temperature
They produce more gas

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Smaller pieces expose a larger total surface area, so collisions with the acid are more frequent.

For the teacher: Understanding, low difficulty, standard 7.2.1

Textbook p. 230

Question 7

Calcium reacts with 200 cm³ of water at room temperature to give 420 cm³ of hydrogen. The experiment is repeated with the water 10 °C warmer. What is the total volume of hydrogen?

More than 420 cm³
420 cm³
Less than 420 cm³
Not possible to say

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The quantities of reactants are unchanged, so the same volume of gas forms. Only the rate is higher.

For the teacher: Applying, medium difficulty, standard 7.2.1

Textbook p. 237

Question 8

Which statement about a catalyst is correct?

It is used up during the reaction
It increases the quantity of product
It increases the activation energy
Its mass is the same before and after the reaction

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A catalyst is not used up, does not change the quantity of product and lowers the activation energy.

For the teacher: Remembering, low difficulty, standard 7.2.1

Textbook p. 235

Question 9

A reaction takes 80 s at 30 °C. Each rise of 10 °C doubles the rate of reaction. How long will the reaction take at 50 °C?

40 s
160 s
20 s
10 s

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30 °C to 50 °C is two rises of 10 °C, so the rate is 2 × 2 = 4 times higher and the time is 80 ÷ 4 = 20 s.

For the teacher: Applying, medium difficulty, standard 7.2.1

Textbook p. 233

Question 10

The catalytic converter of a car contains platinum. What is its function?

To change pollutant gases in the exhaust into harmless gases
To increase the temperature of the engine
To cool exhaust gases
To remove water from the fuel

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Platinum is a catalyst that converts CO, NOx and unburnt hydrocarbons into CO2, N2 and H2O.

For the teacher: Understanding, low difficulty, standard 7.3.1

Textbook p. 241

Question 11

Which pair of conditions is needed for an effective collision between reactant particles?

Low energy and correct orientation
Energy less than Ea and any orientation
Any energy and any orientation
Energy equal to or more than Ea and correct orientation

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Effective collisions need energy equal to or more than the activation energy and the correct orientation.

For the teacher: Remembering, low difficulty, standard 7.4.1

Textbook p. 243

Question 12

How does a higher temperature increase the rate of reaction?

It lowers the activation energy
More particles have energy equal to or more than the activation energy
It increases the number of particles
It changes the orientation of the particles

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Particles gain kinetic energy, so more reach the activation energy, effective collisions are more frequent and the rate rises. Only a catalyst lowers Ea.

For the teacher: Understanding, medium difficulty, standard 7.4.2

Textbook p. 246

Question 13

Diagram 2 shows an energy profile diagram. Which statement is correct?

Diagram 2

The reaction is exothermic and releases heat
The reaction is exothermic and ΔH is negative
The reaction is endothermic because the products have more energy than the reactants
The reaction is endothermic because the activation energy is high

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Products at a higher energy level than reactants means heat is absorbed: endothermic, ΔH positive.

For the teacher: Analysing, medium difficulty, standard 7.4.3

Textbook p. 248

Section B: structured questions

30 marks

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

Question 14

Chemical reactions occur at different rates.

[1 mark] (a)What is meant by the rate of reaction?

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Change in the quantity of reactant or product per unit time1

Tick each point you got.

[2 marks] (b)Classify each as a fast or a slow reaction: (i) explosion at an oil factory, (ii) fermentation.

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(i) Fast [1]1
(ii) Slow [1]1

Tick each point you got.

[2 marks] (c)Magnesium reacts with hydrochloric acid. State two changes that can be measured to find the rate.

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Volume of hydrogen gas increases [1]1
Mass of magnesium decreases / pH of acid increases / pressure increases [1]1

Tick each point you got.

For the teacher: Understanding, low difficulty, standard 7.1

Textbook p. 221

Question 15

Excess magnesium carbonate is added to sulphuric acid. The volume of carbon dioxide is recorded: 0, 25, 40, 51, 58, 63, 68, 70, 70, 70, 70 cm³ at 0, 1, 2, ... 10 minutes. The reaction stops at the 7th minute.

Graph of volume of carbon dioxide against time levelling at 70 cm3 — Diagram 3

[2 marks] (a)Calculate the average rate of reaction in the 5th minute (4th to 5th minute).

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(63 − 58) ÷ (5 − 4) [1]1
= 5 cm³ minute⁻¹ [1]1

Tick each point you got.

[2 marks] (b)Calculate the overall average rate of reaction.

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70 ÷ 7 [1]1
= 10 cm³ minute⁻¹ [1]1

Tick each point you got.

[1 mark] (c)Why is the rate in the 5th minute lower than the rate in the 1st minute?

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The concentration of the acid / quantity of reactant decreases as the reaction proceeds1

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[1 mark] (d)How is the instantaneous rate at the 2nd minute found from the graph?

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Draw a tangent to the curve at 2 minutes and calculate its gradient (ΔV ÷ Δt)1

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For the teacher: Applying, low difficulty, standard 7.1.5

Textbook p. 227

Question 16

Four experiments use excess dilute sulphuric acid and 40 cm³ of hydrogen gas is collected in each. Experiment I: zinc granules, 1.0 mol dm⁻³ acid, 30 °C, 120 s. Experiment II: zinc powder, 1.0 mol dm⁻³ acid, 30 °C, 40 s. Experiment III: zinc granules, 2.0 mol dm⁻³ acid, 30 °C, 60 s. Experiment IV: zinc granules, 1.0 mol dm⁻³ acid, 40 °C, 60 s.

[1 mark] (a)Name the manipulated variable when comparing experiments I and II.

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Size of the zinc (granules or powder)1

Tick each point you got.

[2 marks] (b)Calculate the average rate of reaction in experiment II.

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40 ÷ 40 [1]1
= 1.0 cm³ s⁻¹ [1]1

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[3 marks] (c)Explain, using the collision theory, why experiment II is faster than experiment I.

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Zinc powder has a larger total surface area [1]1
More particles exposed to collision, so frequency of collision increases [1]1
Frequency of effective collisions increases, so rate increases [1]1

Tick each point you got.

[2 marks] (d)Explain why experiment IV is faster than experiment I.

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Higher temperature, so particles have more kinetic energy and more reach the activation energy [1]1
Frequency of effective collisions increases [1]1

Tick each point you got.

For the teacher: Analysing, medium difficulty, standard 7.2.1

Textbook p. 232

Question 17

Diagram 4 shows an energy profile diagram of a reaction with and without a catalyst. The solid curve is the reaction without a catalyst and the dashed curve is the reaction with a catalyst.

Diagram 4

[1 mark] (a)What is meant by activation energy?

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The minimum energy that the reactant particles must have to react (to form products)1

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[2 marks] (b)Which curve is for the reaction with a catalyst? Explain how it increases the rate of reaction.

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Dashed curve [1]1
Provides an alternative pathway with lower activation energy, so more particles can reach it and effective collisions are more frequent [1]1

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[2 marks] (c)State whether the reaction is exothermic or endothermic. Give a reason.

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Exothermic [1]1
Products have lower energy than reactants / heat released [1]1

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For the teacher: Understanding, low difficulty, standard 7.4.3

Textbook p. 247

Question 18

A cook wants to prepare a stew with tough meat in a short time.

[3 marks] (a)State three ways to cook the meat faster and the factor involved in each.

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Cut the meat into small pieces: size / surface area [1]1
Cook at a higher temperature (for example in a pressure cooker or in boiling oil): temperature [1]1
Add a catalyst such as the enzyme in papaya (meat tenderiser): catalyst [1]1

Tick each point you got.

[2 marks] (b)Use the collision theory to explain one way.

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Smaller pieces give a larger total surface area [1]1
More frequent effective collisions, so a higher rate [1]1

Tick each point you got.

[1 mark] (c)Why is leftover stew kept in a refrigerator?

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Low temperature slows the reactions of the bacteria that spoil food1

Tick each point you got.

For the teacher: Evaluating, high difficulty, standard 7.3.1

Textbook p. 240

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