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GCSE Chemistry · GCSEChapter 13 · The rate of chemical reactions and catalysts
GCSE · Chapter 13

The rate of chemical reactions and catalysts

By the end of this chapter you will be able to

  • Calculate a mean rate of reaction from data and from a graph
  • Explain the effect of each factor on the rate using collision theory
  • Describe a method to measure the rate of a reaction and record the results
  • Explain how a catalyst speeds up a reaction without being used up
  • [H] Draw a tangent and calculate the instantaneous rate

You should already know

  • Activation energy and energy profiles (Chapter 12)
  • Moles, gas volumes and Mr (Chapters 8 and 9)

Where this leads

  • Reversible reactions and equilibrium (Chapter 14)
  • Haber process and the rate of ammonia production (Chapter 14)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Define the rate of reaction, and calculate a mean rate from data or a graph, Explain how collision theory and activation energy determine the rate, Describe how concentration, surface area, temperature and catalysts change the rate, with reasons, Carry out the magnesium and dilute acid practical to measure the rate, safely, [H] Calculate an instantaneous rate from the gradient of a tangent to a curve.

Exercise P  Do you remember?

1
FWhat is 48 / 60? Write the number as a decimal.
2
FWhat is 40 / 20? Write the number.
3
FWhat is the name for the minimum energy needed to start a reaction? Write the words.
4
FWhat is 1 / 4? Write the number as a decimal.
5
FHow many moles are in 0.12 g of magnesium (Ar 24)? Write the number.
13.1

The rate of reaction and rate graphs

GCSE
Define the rate of reaction, and calculate a mean rate from data or a graph
Video lesson: The rate of reaction and rate graphsComing soon
Context. A pupil collects hydrogen in a gas syringe and records the volume every 10 seconds. “How fast is the reaction going?” she asks. The rate of reaction is how quickly the amount of product forms, or reactant is used up, in a given time.

The rate of reaction is the change in the amount of a reactant or product per unit time. The mean rate over a time interval is:

mean rate = change in quantity / time taken

On a graph of product against time, the rate is the gradient (slope) of the line. A steeper line means a faster rate. The rate is fastest at the start and slows as the reactants are used up.

Gas volume against time01020304050600102030405060time (s)volume (cm3)fast reaction (steep)slow reaction (shallow)
Gas volume against time for a fast and a slow reaction
MeasurementWhat it showsNotes
gas volume (gas syringe)volume of hydrogen against timeread at regular times
mass loss (balance)mass of gas lost against timeloss of gas makes the mass fall
time for magnesium to disappeartime taken; rate = 1 / timeused when the reaction is quick and stops at the end
Ways to measure the rate of a reaction that gives off a gas or loses mass

Key ideas

  • Rate = change in quantity / time.
  • On a graph, the gradient (slope) is the rate.
  • The rate is highest at the start and falls as reactants are used up.
  • Measure the rate with gas volume, mass loss, or the time for a change to occur.
Worked example 13.1.1
In a reaction, 48 cm3 of gas is collected in 60 seconds. What is the mean rate? Write the number in cm3/s.

Method. Divide the volume by the time.

  1. Divide. mean rate = 48 / 60.
    48 / 60
  2. Answer. mean rate = 0.8 cm3/s.
    0.8
  3. Write the answer.
    0.8

Check. 0.8 x 60 = 48 cm3. ✓

Worked example 13.1.2
A flask loses 2.0 g of mass in 40 seconds. What is the mean rate of mass loss, in g/s? Write the number.

Method. Mass lost divided by time.

  1. Divide. rate = 2.0 / 40.
    2.0 / 40
  2. Answer. rate = 0.05 g/s.
    0.05
  3. Write the answer.
    0.05

Check. 0.05 x 40 = 2.0 g. ✓

Worked example 13.1.3
On a graph, the line goes from (0, 0) to (20, 40). What is the rate in cm3/s? Write the number.

Method. Gradient = change in volume / change in time.

  1. Change. The volume changes by 40 and the time by 20.
    40 and 20
  2. Gradient. Rate = 40 / 20 = 2.
    2
  3. Write the answer.
    2

Check. The gradient is 2 cm3/s. ✓

Watch out

Watch out: the mean rate over the whole reaction is not the rate at the start. Use the gradient of the steepest part for the initial rate.

Exam tip

Exam tip. Write the units with every rate: cm3/s, g/s or mol/s.

Exercise 13A  The rate of reaction and rate graphs

1
FWhat is the rate of reaction? Write the words.
2
FWhat is the rate in cm3/s of 30 cm3 of gas made in 30 seconds? Write the number.
3
FWhat is the rate in g/s if 6 g is lost in 3 seconds? Write the number.
4
FA line on a graph goes from (0, 0) to (30, 60). What is its gradient? Write the number.
5
FIn a reaction, 0.8 cm3/s is the mean rate. How much gas is made in 60 s? Write the number in cm3.
6
PSA mass loss experiment gives 0.05 g/s. How long does it take to lose 2.0 g? Write the number in seconds.
7
FWhich part of a rate graph shows the fastest rate: the start or the end? Write the word.
8
PSWhy does the rate of a reaction fall as time goes on? Write one reason.
9
PSA graph has a line that is flat after 50 seconds. What does this show? Write the words.
10
FName one way of measuring the rate of a reaction that makes a gas. Write the words.
11
RExplain why a gas syringe is used to measure the rate of a reaction, in one sentence. Accept any sentence that says it measures the volume of gas made over time.
13.2

Collision theory and the factors that change the rate

GCSE
Explain how collision theory and activation energy determine the rate, Describe how concentration, surface area and temperature change the rate, with reasons
Video lesson: Collision theory and the factors that change the rateComing soon
Context. Two bags of the same mass of marble chips are put into the same acid. One bag holds large lumps and one holds powder. “Why does the powder fizz faster?” asks a pupil. Collision theory explains it: the more particles that meet and collide with enough energy, the faster the reaction.

According to collision theory, a reaction happens when particles collide with enough energy (at least the activation energy) and in the right direction. Any change that increases the number of successful collisions per second increases the rate. The main factors are concentration (or pressure for gases), surface area, temperature and catalysts.

successful collisionunsuccessful collisionenough energy and correct direction: a reactiontoo slow, or wrong direction: no reaction
Collision theory: a successful collision has enough energy and the right direction
FactorEffect on rateWhy (collision theory)
higher concentration (or pressure for gases)fastermore particles per volume: more collisions per second
larger surface area (smaller pieces)fastermore particles exposed on the surface: more collisions
higher temperaturefasterparticles move faster and collide more often, with more energy
catalystfasterlower activation energy: more collisions have enough energy
Factors that change the rate, and why

Key ideas

  • Rate depends on the frequency of successful collisions.
  • Higher concentration or pressure: more particles per volume, so more collisions.
  • Larger surface area: more particles exposed, so more collisions.
  • Higher temperature: particles move faster and have more energy in collisions.
Worked example 13.2.1
Why does a powder react faster than lumps of the same mass? Write the words.

Method. Think about the surface.

  1. Look. A powder has a much larger surface area than lumps.
    larger surface
  2. Conclude. More particles are exposed, so more collisions happen per second.
    more collisions
  3. Write the answer.
    more particles are exposed, so there are more collisions

Check. The same mass gives a greater surface area. ✓

Worked example 13.2.2
Why does a higher temperature increase the rate? Write the words.

Method. Think about the particles.

  1. Look. Particles move faster when they are hotter.
    move faster
  2. Conclude. They collide more often, and more collisions have enough energy.
    more successful collisions
  3. Write the answer.
    particles move faster and collide more often with enough energy

Check. The activation energy is reached more often. ✓

Worked example 13.2.3
Why does a higher concentration increase the rate? Write the words.

Method. Think about the number of particles.

  1. Look. A higher concentration means more particles in the same volume.
    more particles
  2. Conclude. So there are more collisions per second.
    more collisions
  3. Write the answer.
    more particles in the same volume, so more collisions

Check. The same idea applies to pressure of a gas. ✓

Watch out

Watch out: a higher temperature does not speed up the particles so much that they all collide. Explain it with the proportion of particles that have enough energy, and the number of collisions.

Exam tip

Exam tip. When you explain a rate change, use the words “collisions”, “energy” and “successful”.

Exercise 13B  Collision theory and the factors that change the rate

1
FAccording to collision theory, what must particles do to react? Write the words.
2
FWhich factor increases the number of particles per unit volume in a solution? Write the word.
3
FDoes a larger surface area increase or decrease the rate? Write the word.
4
FDoes a higher temperature increase or decrease the rate? Write the word.
5
FWhich factor is changed by using a gas at a higher pressure? Write the word.
6
FWhat is the effect of cutting a solid into smaller pieces on the rate? Write the word.
7
RExplain why a higher temperature increases the rate of reaction, in one sentence. Accept any sentence that says particles collide more often and more collisions have enough energy.
8
RExplain why doubling the concentration of an acid increases the rate of reaction with a metal, in one sentence. Accept any sentence that says there are more acid particles per volume, so more collisions occur.
9
RExplain why a lump of marble reacts more slowly than powder of the same mass, in one sentence. Accept any sentence that says the lump has a smaller surface area, so fewer particles collide with the acid.
10
PSA reaction at 20 degrees C has a rate of 1.0 units. Suppose the rate doubles when the temperature rises by 10 degrees C. What is the new rate? Write the number.
11
PSTwo reactions have the same mass of magnesium, but one uses a powder and the other uses a ribbon. Which reacts faster? Write the word, powder or ribbon.
13.3

Required practical: the rate of magnesium and dilute acid

GCSE
Carry out the magnesium and dilute acid practical to measure the rate, safely
Video lesson: Required practical: the rate of magnesium and dilute acidComing soon
Context. A class adds magnesium ribbon to dilute hydrochloric acid and measures how quickly the gas syringe fills. “How do we make it a fair test?” asks the teacher. Keep the mass of magnesium, the volume and concentration of acid, and the temperature the same; change only the variable being tested.

In the required practical, magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen: Mg + 2HCl -> MgCl2 + H2. The rate is found by measuring the volume of hydrogen collected at regular times, or the time taken for the magnesium to disappear. The variable can be the concentration of acid, the size of the magnesium pieces (surface area) or the temperature.

Try it (with your teacher). Required practical: the rate of reaction of magnesium with dilute hydrochloric acid. Use dilute acid, small pieces of magnesium ribbon and goggles. Record the gas volume every 10 or 20 seconds for each condition, and repeat each test. Keep the acid volume, the magnesium mass and the other factors the same. Do not heat the acid above the temperature your teacher sets. Clean up the magnesium and acid at the end, and wash your hands.

MeasurementWhat it showsNotes
gas volume (gas syringe)volume of hydrogen against timeread at regular times
mass loss (balance)mass of gas lost against timeloss of gas makes the mass fall
time for magnesium to disappeartime taken; rate = 1 / timeused when the reaction is quick and stops at the end
How to measure the rate in this practical

Key ideas

  • Variables: change one (for example the acid concentration); keep the others the same.
  • Repeat each test to check the results and find a mean.
  • Record time and gas volume; plot a graph of volume against time.
  • Rate = gradient of the graph, or 1 / time for magnesium to disappear.
Worked example 13.3.1
In the practical, 40 cm3 of gas is collected in 20 seconds. What is the mean rate? Write the number in cm3/s.

Method. Divide the volume by the time.

  1. Divide. rate = 40 / 20.
    40 / 20
  2. Answer. rate = 2 cm3/s.
    2
  3. Write the answer.
    2

Check. 2 x 20 = 40 cm3. ✓

Worked example 13.3.2
In the practical, magnesium takes 50 seconds to disappear. What is the rate, using 1 / time, in s-1? Write the number to 2 decimal places.

Method. Rate = 1 / time.

  1. Divide. rate = 1 / 50.
    1 / 50
  2. Answer. rate = 0.02 s-1.
    0.02
  3. Write the answer.
    0.02

Check. 0.02 x 50 = 1. ✓

Worked example 13.3.3
Name the independent variable in the practical that tests the effect of acid concentration. Write the words.

Method. The independent variable is the one that is changed.

  1. Look. The concentration of acid is changed on purpose.
    changed
  2. Decide. So acid concentration is the independent variable.
    acid concentration
  3. Write the answer.
    the concentration of the acid

Check. The rate is the dependent variable. ✓

Watch out

Watch out: a fair test changes one thing only. If the magnesium pieces are different sizes, the surface area changes too, and the result is not fair.

Exam tip

Exam tip. Name the independent, dependent and control variables, and say how you keep each control variable the same.

Exercise 13C  Required practical: the rate of magnesium and dilute acid

1
FWhat is the chemical equation for magnesium reacting with hydrochloric acid, using the products? Write the products.
2
FWhat is the independent variable if you test the effect of temperature? Write the words.
3
FName one variable that must be kept the same, to keep the test fair. Write the words.
4
FWhich protective equipment must be worn in the practical? Write the name.
5
FIn the practical, what gas is collected in the gas syringe? Write the name.
6
FIn the practical, 60 cm3 of gas is collected in 30 seconds. What is the mean rate in cm3/s? Write the number.
7
PSIn the practical, 0.12 g of magnesium is used. How many moles of Mg is this? Write the number. (Ar of Mg is 24.)
8
RWhy are repeat tests carried out in the practical? Write one reason.
9
RA pupil uses ribbon in one test and powder in another. Explain why this is not a fair test, in one sentence. Accept any sentence that says the surface area is different, so the surface area variable is changed as well.
10
RExplain why the gas syringe should be read at regular intervals, in one sentence. Accept any sentence that says regular readings show how the rate changes over time.
13.4

Catalysts

GCSE
Explain how a catalyst speeds up a reaction without being used up
Video lesson: CatalystsComing soon
Context. A pupil adds a little manganese dioxide to hydrogen peroxide and the bubbling is faster. “Is the manganese dioxide used up?” she asks. A catalyst speeds up a reaction, but it is not used up, and it can be recovered and used again.

A catalyst is a substance that speeds up a reaction without being used up. It provides an alternative pathway with a lower activation energy, so more particles have enough energy to react. It does not change the energy of the reactants or products, or the energy change. Enzymes are biological catalysts; they work best at a particular temperature and pH.

Catalyst lowers the activation energywithout catalystwith catalyst (lower peak)reactantsproducts
A catalyst gives a lower peak: the activation energy is reduced

Key ideas

  • A catalyst speeds up a reaction and is not used up.
  • It lowers the activation energy.
  • It does not change the energy change of the reaction.
  • Enzymes are biological catalysts, for example in digestion.
Worked example 13.4.1
What does a catalyst do to the activation energy? Write the word.

Method. Recall the effect.

  1. Recall. A catalyst provides a pathway with a lower activation energy.
    lower
  2. Effect. So more particles have enough energy to react.
    more successful
  3. Write the answer.
    lowers

Check. The energy change is unchanged. ✓

Worked example 13.4.2
Name a biological catalyst. Write the name.

Method. Recall the name.

  1. Recall. Enzymes are biological catalysts.
    enzyme
  2. Check. Enzymes are found in living cells, for example in digestion.
    living cells
  3. Write the answer.
    enzyme

Check. Enzymes are proteins. ✓

Worked example 13.4.3
Why is a catalyst not used up in a reaction? Write one reason.

Method. Think about what a catalyst does.

  1. Look. A catalyst takes part in the reaction, but is regenerated at the end.
    regenerated
  2. Conclude. So it is still there in the same amount at the end.
    same amount
  3. Write the answer.
    it is regenerated and is still there at the end

Check. Catalysts are recovered and reused. ✓

Watch out

Watch out: a catalyst does not change the energy change of a reaction, and it does not change the amount of product made. It only changes how fast the product forms.

Exam tip

Exam tip. Explain a catalyst with two ideas: a lower activation energy, and not used up.

Exercise 13D  Catalysts

1
FIs a catalyst used up in a reaction? Write yes or no.
2
FWhat does a catalyst do to the activation energy? Write the word.
3
FDoes a catalyst change the energy change of a reaction? Write yes or no.
4
FWhat are biological catalysts called? Write the word.
5
FName the catalyst used in the Haber process. Write the name. (Hint: it is a metal.)
6
PSDoes a catalyst increase the amount of product made at equilibrium? Write yes or no.
7
RExplain why a catalyst makes a reaction faster, using the words activation energy, in one sentence. Accept any sentence that says a lower activation energy means more particles have enough energy to react.
8
RExplain why a catalyst is not used up, in one sentence. Accept any sentence that says the catalyst takes part but is regenerated at the end.
9
RExplain why enzymes work best at a particular temperature, in one sentence. Accept any sentence that says high temperatures change the shape of the enzyme, so it stops working.
13.5

Instantaneous rate from a tangent [H]

GCSE
[H] Calculate an instantaneous rate from the gradient of a tangent to a curve
Video lesson: Instantaneous rate from a tangent [H]Coming soon
Context. On a curve of gas volume against time, the line is steep at the start and then flattens. “What is the rate at exactly 10 seconds?” asks a pupil. The mean rate over a long time does not answer this; a tangent to the curve gives the rate at one moment.

[H] Higher tier only. The instantaneous rate at a point is the gradient of the tangent to the curve at that point. Draw a straight line that just touches the curve at the point, then find its gradient = change in y / change in x, using two points on the tangent.

Key ideas

  • The rate at a moment is the gradient of the tangent at that point.
  • Use two points well apart on the tangent to find the gradient.
  • The initial rate is the gradient at time zero.
  • The rate falls as the curve flattens.
Worked example 13.5.1
[H] A tangent passes through (5, 20) and (15, 50). What is the gradient, and so the rate? Write the number.

Method. Gradient = change in y / change in x.

  1. Change in y. 50 - 20 = 30.
    30
  2. Change in x. 15 - 5 = 10.
    10
  3. Gradient. 30 / 10 = 3 units per second.
    3
  4. Write the answer.
    3

Check. The instantaneous rate is 3 cm3/s at 10 s. ✓

Worked example 13.5.2
[H] A tangent passes through (0, 0) and (10, 40). What is the gradient, in cm3/s? Write the number.

Method. Find the change in y and the change in x.

  1. Change. 40 - 0 = 40; 10 - 0 = 10.
    40 / 10
  2. Gradient. 40 / 10 = 4 cm3/s.
    4
  3. Write the answer.
    4

Check. The initial rate is 4 cm3/s. ✓

Worked example 13.5.3
[H] A tangent passes through (20, 30) and (40, 40). What is its gradient? Write the number.

Method. Use the two points.

  1. Change. y changes by 10; x changes by 20.
    10 and 20
  2. Gradient. 10 / 20 = 0.5.
    0.5
  3. Write the answer.
    0.5

Check. The rate has fallen to 0.5 cm3/s at 30 s. ✓

Watch out

Watch out: do not use two points on the curve for an instantaneous rate; use the tangent. Two points on the curve give a mean rate over that interval.

Exam tip

Exam tip. Draw the tangent with a ruler, and make the two points on it far apart to reduce the error.

Exercise 13E  Instantaneous rate from a tangent [H]

1
F[H] What is the rate at a point on a curve called, if the tangent is used to find it? Write the words.
2
F[H] What is the gradient of a line from (0, 0) to (4, 12)? Write the number.
3
F[H] A tangent passes through (10, 10) and (20, 30). What is the gradient? Write the number.
4
F[H] On a curve, the tangent at 0 s has gradient 4 cm3/s. What is the initial rate? Write the number.
5
R[H] Explain why the rate of a reaction falls as the curve flattens, in one sentence. Accept any sentence that says the reactants are used up, so fewer collisions occur.
6
R[H] Why should a tangent be drawn, rather than using two points on the curve, to find an instantaneous rate? Write one sentence. Accept any sentence that says two points on a curve give a mean rate over the interval.

Chapter summary

SkillMethod
Rate of reactionChange in quantity / time; the gradient of a graph.
Collision theoryParticles must collide with enough energy and the right direction.
FactorsConcentration or pressure, surface area and temperature increase the rate.
CatalystSpeeds up a reaction, lowers the activation energy, not used up.
PracticalMagnesium and dilute acid: measure gas volume or time; control other variables.
[H] Instantaneous rateThe gradient of the tangent to the curve at that point.

Can you…?

  • I can define the rate of reaction and calculate a mean rate from data and graphs
  • I can explain the effect of each factor on the rate using collision theory
  • I can describe the magnesium and dilute acid practical, safely, with fair testing
  • I can explain how a catalyst speeds up a reaction, and name enzymes as catalysts
  • [H] I can calculate an instantaneous rate from a tangent

Key terms

rate of reaction
the change in the amount of a reactant or product per unit time
collision theory
reactions occur when particles collide with enough energy and in the right direction
catalyst
a substance that speeds up a reaction without being used up
enzyme
a biological catalyst made of protein
tangent
[H] a straight line that touches a curve at one point, used to find an instantaneous rate
instantaneous rate
[H] the rate at one moment, found from the gradient of the tangent

Exercise 13Z  Check-up

1
What is the mean rate of gas production, in cm3/s, if 60 cm3 is made in 40 seconds? Write the number.
2
Which factor increases the rate by increasing the number of particles per volume? Write the word.
3
What is the name of the substance that speeds up a reaction without being used up? Write the word.
4
What happens to the rate when the temperature is increased? Write the word.
5
Which powder reacts faster with acid: a lump or a powder of the same mass? Write the word.
6
Name the product of the reaction of magnesium with hydrochloric acid, other than magnesium chloride. Write the name.
7
[H] A tangent passes through (0, 0) and (5, 15). What is the instantaneous rate at the start? Write the number.
8
Explain why a catalyst increases the rate of reaction, in one sentence. Accept any sentence that says it lowers the activation energy so more collisions are successful.
9
Explain why the rate of reaction is faster at a higher concentration, in one sentence. Accept any sentence that says there are more particles per volume, so more collisions happen.

Total 23 marks. Allow about 23 minutes.

Exercise 13X  Exam-style questions

1
FIn a reaction, 30 cm3 of gas is collected in 15 seconds. Calculate the mean rate, in cm3/s. Write the number.[2 marks]
2
RExplain how increasing the temperature increases the rate of reaction, in two sentences. Accept any answer that says particles move faster and collide more often, with more collisions having enough energy.[3 marks]
3
RDescribe what a catalyst does, in one or two sentences. Accept any answer that says it speeds up a reaction by lowering the activation energy, and is not used up.[2 marks]
4
FIn the magnesium and acid practical, a pupil wants to test the effect of temperature. Name the independent variable and one control variable. Write two answers.[2 marks]
5
RExplain why the rate of a reaction is faster for a powder than for a lump of the same mass, in one or two sentences. Accept any answer that says the powder has a larger surface area, so more collisions occur.[3 marks]
6
PSOn a graph of volume against time, the line from (0, 0) to (10, 25) is drawn. What is the rate between 0 and 10 seconds, in cm3/s? Write the number.[2 marks]
7
PS[H] A tangent to a curve passes through (10, 20) and (20, 60). Calculate the instantaneous rate at 15 seconds, in cm3/s. Write the number.[2 marks]
8
RExplain why a catalyst is not included in the equation for the reaction, in one sentence. Accept any sentence that says it is not used up, so it appears on neither side.[2 marks]
9
RExplain why a small rise in temperature can cause a large increase in the rate of reaction, in one sentence. Accept any sentence that says more particles have the energy to react, so the proportion of successful collisions increases.[3 marks]
10
FGive two factors that change the rate of a reaction in a gas, and say how each one changes it. Accept pressure (higher means faster) and temperature (higher means faster).[2 marks]
← Chapter 12: Energy changes, bond energies and cellsChapter 14: Reversible reactions, equilibrium and the Haber process →

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