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KS3 Science · Year 8Chapter 10 · Rates of reaction and energy changes
Year 8 · Chapter 10

Rates of reaction and energy changes

By the end of this chapter you will be able to

  • Explain the rate of a reaction in terms of collisions
  • Describe how temperature, concentration, surface area and catalysts change the rate
  • Calculate a rate from the volume of gas produced in a time
  • Say whether a reaction is exothermic or endothermic, and read a reaction profile
  • Describe what a catalyst does, and what it does not do

You should already know

  • Exothermic and endothermic reactions (Year 7 Chapter 9)
  • Word equations and signs of reaction (Year 7 Chapter 9)

Where this leads

  • Energy stores and transfers (Year 8 Chapter 11)

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8C5 factors affecting the rate of reaction: temperature, concentration, surface area and catalysts; measuring the rate from the volume of gas or the loss of mass, 8C6 exothermic and endothermic reactions: energy transfer to and from the surroundings; reaction profile diagrams (basic only).

Exercise P  Do you remember?

1
FWhat is the name of a reaction that gives energy out to the surroundings? Write the word.
2
FWhat is 300 divided by 6? Write the number.
3
FWhat is the name of the reaction in which a metal reacts with an acid to give hydrogen? Write the words.
4
FWhat is 2 times 2 times 5? Write the number.
5
FWhich gas makes up most of the air? Write the word.
10.1

Collisions and the rate of reaction

Year 8
8C5: the rate of reaction, explained by collisions between particles with enough energy
Video lesson: Collisions and the rate of reactionComing soon
Story. Ruby adds a tablet to a glass of water and watches it fizz. “Why does it fizz faster in warm water?” she asks. Her teacher says, “The particles move faster in warm water, so they collide more often, and more collisions have enough energy to react.”

The rate of reaction is how fast reactants are used up, or products are formed. For a reaction to happen, particles must collide with enough energy. A collision with enough energy is a successful collision. The more successful collisions there are each second, the faster the rate.

collideonly collisions with enough energy give a reactiontoo slow
Two particles collide; only a collision with enough energy leads to a reaction.

Key ideas

  • The rate is how fast a reaction happens.
  • Particles must collide with enough energy to react.
  • More successful collisions per second means a faster rate.
  • Anything that increases collisions increases the rate.
Worked example 10.1.1
What must happen for a reaction to occur between two particles? Write the words.

Method. Recall the requirement.

  1. Recall. The particles must collide.
    collide
  2. Name it. The collision must also have enough energy.
    enough energy
  3. Write the answer.
    collide with enough energy

Check. A collision with energy is successful. ✓

Worked example 10.1.2
What is the name for a collision that has enough energy to make a reaction happen? Write the words.

Method. Recall the name.

  1. Recall. A collision with enough energy is a successful collision.
    successful collision
  2. Name it. So the name is successful collision.
    successful collision
  3. Write the answer.
    successful collision

Check. Not every collision is successful. ✓

Worked example 10.1.3
Does a faster rate mean more successful collisions per second? Write yes or no.

Method. Recall the link.

  1. Recall. The rate depends on how many successful collisions happen each second.
    each second
  2. Decide. So a faster rate means more successful collisions; the answer is yes.
    yes
  3. Write the answer.
    yes

Check. Rate and successful collisions go together. ✓

Watch out

Watch out: not every collision leads to a reaction. Only collisions with enough energy are successful.

Exam tip

Exam tip. Explain a rate change with “more collisions” and “more successful collisions”, not just “faster”.

Try it (with your teacher, or a grown-up at home). Look at the collision picture. Explain in your own words why a slow collision does not make a reaction. Do not add any tablet or fizzing chemical to water yourself; your teacher or a grown-up can do this.

Exercise 10A  Collisions and the rate of reaction

1
FWhat is the name for how fast a reaction happens? Write the words.
2
FWhat must particles do to react? Write the word: collide or separate.
3
FWhat is a collision that has enough energy to react called? Write the words.
4
FIn warm water, do the particles move faster or slower? Write the word.
5
FDoes a higher temperature mean more collisions per second? Write yes or no.
6
FDoes a faster rate mean more successful collisions per second? Write yes or no.
7
FIs every collision between particles successful? Write yes or no.
8
FIn a fizzing tablet reaction, does warm water make the fizzing faster or slower? Write the word.
9
PSAli says a reaction is faster when fewer particles collide. Is Ali right? Write yes or no.
10
PSWorking scientifically: a pupil says the reaction is faster because the particles have more energy. Is the pupil describing a collision? Write yes or no.
11
RExplain why a higher temperature increases the rate of reaction. Write one sentence.
12
RChallenge: explain why a rate can be fast even though only a small part of the collisions are successful. Write one sentence.
10.2

Factors that change the rate

Year 8
8C5: temperature, concentration, surface area and catalysts change the rate of reaction
Video lesson: Factors that change the rateComing soon
Story. Omar tests the cross-disappearing reaction at four temperatures, keeping the concentration the same. “The warmer the mixture, the quicker the cross disappears,” he says. “Good. Now check whether the concentration changes the time too,” says his teacher.”

Four factors change the rate of a reaction:

FactorWhy it speeds up the reaction
temperatureparticles move faster, so they collide more often and with more energy
concentrationmore particles in the same volume, so more collisions
surface areamore particles are exposed, so more collisions can happen
catalystprovides a different pathway with a lower activation energy; it is not used up

The table gives the times for a cross to disappear at four temperatures, with the same concentration. This is a teacher demonstration. The time is shorter when the rate is faster.

Temperature (degrees C)Time for the cross to disappear (s)
1080
2040
3020
4010

Key ideas

  • Higher temperature increases the rate of most reactions.
  • Higher concentration gives more particles in the same volume, so more collisions.
  • Greater surface area exposes more particles to collide.
  • A catalyst speeds up a reaction without being used up.
Worked example 10.2.1
Using the table, how many times faster is the reaction at 30 degrees C than at 10 degrees C? Write the number.

Method. Divide the time at 10 degrees by the time at 30 degrees.

  1. Read. The time at 10 degrees C is 80 s; at 30 degrees C it is 20 s.
    80 and 20
  2. Calculate. The ratio is 80 divided by 20.
    80 / 20 = 4
  3. Write the answer.
    4

Check. 20 x 4 = 80. ✓

Worked example 10.2.2
Using the concentration table, what happens to the time when the concentration halves from 1.0 to 0.5? Write the word: doubles or halves.

Method. Compare the two times.

  1. Read. The time goes from 40 s to 80 s.
    40 to 80
  2. Decide. So the time doubles when the concentration halves.
    doubles
  3. Write the answer.
    doubles

Check. Half the concentration means half the rate. ✓

Worked example 10.2.3
Which factor is the reason that powdered chalk reacts faster than a lump of chalk? Write the words.

Method. Think about the particles exposed.

  1. Think. Powder has a larger surface area, so more particles are exposed to collide.
    larger surface area
  2. Name it. So the factor is surface area.
    surface area
  3. Write the answer.
    surface area

Check. More exposed particles mean more collisions. ✓

Watch out

Watch out: doubling the temperature does not double the rate. The change depends on the reaction, and it is not a simple proportion.

Exam tip

Exam tip. Name the factor and say what it does to the collisions: “more particles in the same volume, so more collisions”.

Try it (with your teacher, or a grown-up at home). Look at the temperature table. Describe in one sentence what happens to the time as the temperature rises. Do not handle the chemicals; your teacher runs the demonstration.

Exercise 10B  Factors that change the rate

1
FName one factor that increases the rate of reaction, other than temperature. Accept any one: concentration, surface area, or a catalyst.
2
FUsing the temperature table, what is the time at 20 degrees C? Write the number of seconds.
3
FUsing the temperature table, at which temperature is the time shortest? Write the number of degrees C.
4
FUsing the concentration table, what is the time at a concentration of 0.25 mol/dm3? Write the number of seconds.
5
FUsing the concentration table, does a lower concentration give a faster or slower reaction? Write the word.
6
FDoes powdered calcium carbonate react faster than a lump of the same mass? Write yes or no.
7
FWhich factor increases the number of particles in the same volume of solution? Write the word.
8
PSUsing the temperature table, how many times faster is the reaction at 40 degrees C than at 20 degrees C? Write the number.
9
PSRuby says doubling the temperature always doubles the rate. Is Ruby right? Write yes or no.
10
PSWorking scientifically: name one control variable in the temperature test. Accept any one: the concentration, the volume of acid, or the cross size.
11
RExplain why a powder reacts faster than a lump. Write one sentence.
12
RChallenge: explain why the rate at a higher concentration is faster, using the idea of collisions. Write one sentence.
10.3

Measuring the rate

Year 8
8C5: measuring the rate of reaction from the volume of gas produced, or the loss of mass, over time
Video lesson: Measuring the rateComing soon
Story. In a class experiment, a gas syringe collects the gas made when marble chips react with acid. “How do we know how fast it is?” asks Omar. His teacher says, “Measure the volume of gas at regular times. The rate is the change in volume divided by the time taken.”

The rate of a reaction that produces a gas can be measured by the volume of gas collected in a set time. The rate = change in volume / time. The steeper the graph of volume against time, the faster the rate. The rate falls as the reactants are used up, so the graph becomes flatter.

The rate can also be measured by the loss of mass, when a gas escapes from the flask on a balance.

The table gives the gas volume collected over time. This is example data from a class experiment.

Time (s)Gas volume (cm3)
00
2020
4030
6035
8038
Volume of gas collected over time (example data)020406080010203040Time (s)Gas volume (cm3)
Volume of gas against time: steep at the start, then levelling off as the reaction slows.

Key ideas

  • Rate = change in volume (or mass) divided by time.
  • A steeper graph means a faster rate.
  • The rate falls over time as reactants are used up.
  • Loss of mass shows the gas that escapes.
Worked example 10.3.1
Using the table, what is the rate of gas production in the first 20 seconds? Write the number, in cm3 per second.

Method. Divide the volume change by the time.

  1. Read. The volume rises from 0 to 20 cm3 in 20 seconds.
    20 - 0
  2. Calculate. The rate is 20 divided by 20.
    20 / 20 = 1
  3. Write the answer.
    1 cm3/s

Check. 1 x 20 = 20. ✓

Worked example 10.3.2
Using the table, what is the rate between 40 and 60 seconds? Write the number, in cm3 per second.

Method. Divide the volume change by the time.

  1. Read. The volume goes from 30 to 35 cm3 between 40 and 60 seconds.
    35 - 30 = 5
  2. Calculate. The rate is 5 divided by 20.
    5 / 20 = 0.25
  3. Write the answer.
    0.25 cm3/s

Check. 0.25 x 20 = 5. ✓

Worked example 10.3.3
Is the rate faster at the start or at the end of the reaction? Write the word.

Method. Compare the two rates.

  1. Compare. The first rate is 1 cm3/s, and the later rate is 0.25 cm3/s.
    1 and 0.25
  2. Decide. So the rate is faster at the start.
    start
  3. Write the answer.
    start

Check. The graph is steeper at the start. ✓

Watch out

Watch out: the rate changes over the reaction. Give the time at which you measure, and do not use one overall rate for the whole experiment unless you say so.

Exam tip

Exam tip. Write the rate with its unit: cm3 per second, or g per second for a loss of mass.

Try it (with your teacher, or a grown-up at home). Look at the volume and time table, and draw a rough curve through the points on squared paper. Say at which time the curve is steepest. Do not carry out a gas experiment yourself unless your teacher sets it up.

Exercise 10C  Measuring the rate

1
FWhat is the name of the volume of gas made in a reaction measured against time? Write the words.
2
FWhat is the formula for the rate of a reaction that produces gas? Write the word: change in volume divided by time.
3
FUsing the table, what is the volume of gas at 60 seconds? Write the number of cm3.
4
FUsing the table, what is the volume of gas at 40 seconds? Write the number of cm3.
5
FUsing the table, what is the change in volume between 0 and 20 seconds? Write the number of cm3.
6
FUsing the table, what is the rate between 0 and 20 seconds? Write the number, in cm3 per second.
7
FUsing the table, what is the rate between 60 and 80 seconds? Write the number, in cm3 per second.
8
FIs the rate between 60 and 80 seconds less than 1 cm3 per second? Write yes or no.
9
FWhen the rate is measured by loss of mass, what is it that escapes? Write the word.
10
FIs the rate of reaction faster at the start of the reaction than at the end? Write yes or no.
11
PSAli says the rate stays the same throughout the reaction. Is Ali right? Write yes or no.
12
PSWorking scientifically: a pupil wants to measure the rate for 30 seconds only. What is the time that must be used to find a rate? Write the number of seconds.
13
RExplain why the rate falls over the course of a reaction. Write one sentence.
14
RChallenge: a pupil measures 30 cm3 of gas in 60 seconds. What is the average rate, in cm3 per second? Write the number.
10.4

Catalysts

Year 8
8C5: a catalyst speeds up a reaction, is not used up, and provides a pathway with a lower activation energy
Video lesson: CatalystsComing soon
Story. Sam sees a teacher add a small amount of black powder to a solution of hydrogen peroxide. The gas comes off much faster. “Did the powder get used up?” he asks. His teacher says, “No. It is a catalyst. It speeds the reaction up, and you could use it again.”

A catalyst speeds up a reaction without being used up in it. A catalyst provides a different pathway for the reaction, with a lower activation energy: the minimum energy needed for a collision to be successful.

Enzymes are biological catalysts, such as amylase in your mouth. Catalysts do not change the products of a reaction, and they do not change the energy released or taken in overall.

Key ideas

  • A catalyst speeds up a reaction and is not used up.
  • A catalyst lowers the activation energy.
  • A catalyst does not change the products.
  • Enzymes are biological catalysts.
Worked example 10.4.1
What is the name for the minimum energy needed for a collision to be successful? Write the words.

Method. Recall the name.

  1. Recall. The minimum energy for a successful collision is the activation energy.
    activation energy
  2. Name it. So the answer is activation energy.
    activation energy
  3. Write the answer.
    activation energy

Check. A catalyst lowers this energy. ✓

Worked example 10.4.2
Is a catalyst used up in the reaction? Write yes or no.

Method. Recall the definition.

  1. Recall. A catalyst speeds up the reaction without being used up.
    not used up
  2. Decide. So the answer is no.
    no
  3. Write the answer.
    no

Check. It can be used again. ✓

Worked example 10.4.3
Name a biological catalyst found in saliva. Write the word.

Method. Recall the example.

  1. Recall. Amylase is an enzyme in saliva.
    amylase
  2. Name it. So the biological catalyst is amylase.
    amylase
  3. Write the answer.
    amylase

Check. Enzymes are biological catalysts. ✓

Watch out

Watch out: a catalyst does not make a reaction produce more product. It only speeds up how fast the same products form.

Exam tip

Exam tip. Describe a catalyst as “speeds up the reaction and is not used up”.

Try it (with your teacher, or a grown-up at home). Name one catalyst that you have met in this book, and say what it speeds up. Do not handle any catalyst chemical; describe it from the text.

Exercise 10D  Catalysts

1
FWhat is a substance that speeds up a reaction and is not used up? Write the word.
2
FWhat is the minimum energy needed for a collision to be successful? Write the words.
3
FDoes a catalyst raise or lower the activation energy? Write the word.
4
FIs a catalyst used up in the reaction? Write yes or no.
5
FName the biological catalyst in saliva. Write the word.
6
FWhat type of catalyst is an enzyme? Write the words.
7
FDoes a catalyst change the products of a reaction? Write yes or no.
8
FDoes a catalyst make more product in total? Write yes or no.
9
PSIn the hydrogen peroxide reaction, the black powder is a catalyst. Is the powder used up? Write yes or no.
10
PSAli says a catalyst makes the products have more energy. Is Ali right? Write yes or no.
11
PSWorking scientifically: name one thing that a fair test with a catalyst must keep the same. Accept any one: the amount of substance, or the temperature.
12
RExplain why a catalyst speeds up a reaction. Write one sentence.
13
RChallenge: explain why a catalyst is not used up, although it takes part in the reaction. Write one sentence.
10.5

Energy changes and reaction profiles

Year 8
8C6: exothermic and endothermic reactions, energy transfer to and from the surroundings, and reaction profile diagrams (basic)
Video lesson: Energy changes and reaction profilesComing soon
Story. Ruby uses a hand warmer on a cold walk, and it becomes hot. Later, a cold pack gets cold after it is activated. “Both are reactions,” she says. “Yes,” says her teacher. “One gives energy out and the other takes it in. A reaction profile shows which.”

An exothermic reaction gives energy out to the surroundings, so the temperature rises. An endothermic reaction takes energy in, so the temperature falls.

A reaction profile shows the energy of the reactants and the products. In an exothermic reaction the products have less energy than the reactants. In an endothermic reaction the products have more. The hump is the activation energy.

exothermicreactantsproductsactivation energyenergy outendothermicreactantsproductsenergy taken inDashed line: the path of the reaction, with its activation energy hump.
Reaction profiles: exothermic (products lower) and endothermic (products higher), with the activation energy hump.

Key ideas

  • Exothermic: energy given out; products lower than reactants.
  • Endothermic: energy taken in; products higher than reactants.
  • The hump on a profile is the activation energy.
  • A catalyst lowers the hump; it does not change the overall energy change.
Worked example 10.5.1
In a reaction profile, the products are lower than the reactants. Is the reaction exothermic or endothermic? Write the word.

Method. Recall the profile.

  1. Recall. If the products have less energy, energy is given out.
    given out
  2. Decide. So the reaction is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. Hand warmers are exothermic. ✓

Worked example 10.5.2
A reaction changes the temperature from 25 degrees C to 19 degrees C. Is it exothermic or endothermic? Write the word.

Method. Look at the direction of the change.

  1. Look. The temperature falls by 6 degrees.
    falls
  2. Decide. A fall means energy is taken in, so the reaction is endothermic.
    endothermic
  3. Write the answer.
    endothermic

Check. Cold packs are endothermic. ✓

Worked example 10.5.3
What is the name of the hump on a reaction profile? Write the words.

Method. Recall the name.

  1. Recall. The hump is the energy needed to start the reaction.
    start the reaction
  2. Name it. That is the activation energy.
    activation energy
  3. Write the answer.
    activation energy

Check. A catalyst lowers this hump. ✓

Watch out

Watch out: exothermic does not mean the reaction is hot at the start. It means energy is given out, so the products have less energy than the reactants.

Exam tip

Exam tip. On a profile, check the products: lower than reactants is exothermic; higher is endothermic.

Try it (with your teacher, or a grown-up at home). Look at the reaction profile picture. Say which curve is exothermic, and which is endothermic. Explain in one sentence what the dashed line shows. Do not handle any hot pack or cold pack.

Exercise 10E  Energy changes and reaction profiles

1
FWhat is a reaction that gives energy out called? Write the word.
2
FWhat is a reaction that takes energy in called? Write the word.
3
FOn a reaction profile of an exothermic reaction, are the products higher or lower than the reactants? Write the word.
4
FOn a reaction profile of an endothermic reaction, are the products higher or lower than the reactants? Write the word.
5
FWhat is the name of the hump on a reaction profile? Write the words.
6
FDoes a catalyst change the overall energy change of a reaction? Write yes or no.
7
PSA reaction changes the temperature from 15 degrees C to 22 degrees C. Is it exothermic or endothermic? Write the word.
8
PSA reaction changes the temperature from 25 degrees C to 19 degrees C. What is the change in temperature? Write the number of degrees.
9
PSAli says that an exothermic reaction has products higher than the reactants. Is Ali right? Write yes or no.
10
PSWorking scientifically: a pupil draws a profile with products at the same level as the reactants. What does this suggest about the energy change? Write the word: none.
11
RExplain why a cold pack gets colder. Write one sentence.
12
RChallenge: explain why the activation energy must be supplied, even in an exothermic reaction. Write one sentence.

Chapter summary

SkillMethod
RateHow fast a reaction happens; rate = change in volume (or mass) divided by time.
CollisionsParticles must collide with enough energy; successful collisions give a reaction.
FactorsTemperature, concentration and surface area increase collisions; catalysts lower activation energy.
CatalystSpeeds up a reaction; not used up; does not change the products.
Energy changesExothermic: energy out, products lower. Endothermic: energy in, products higher.

Can you…?

  • I can explain the rate of a reaction in terms of collisions
  • I can describe how temperature, concentration, surface area and catalysts change the rate
  • I can calculate a rate from the volume of gas produced in a time
  • I can say whether a reaction is exothermic or endothermic, and read a reaction profile
  • I can describe what a catalyst does, and what it does not do

Key terms

rate of reaction
how fast a reaction happens
successful collision
a collision between particles with enough energy to react
catalyst
a substance that speeds up a reaction and is not used up
activation energy
the minimum energy needed for a successful collision
reaction profile
a diagram showing the energy of the reactants and products
exothermic
a reaction that gives energy out, so the temperature rises
endothermic
a reaction that takes energy in, so the temperature falls
concentration
the amount of particles in a given volume

Exercise 10Z  Check-up

1
What is the name of a substance that speeds up a reaction and is not used up? Write the word.
2
What must particles do, and with what, to react? Write the words.
3
Using the temperature table, at which temperature is the time shortest? Write the number of degrees C.
4
Using the gas table, what is the volume at 80 seconds? Write the number of cm3.
5
Using the gas table, what is the rate in the first 20 seconds, in cm3 per second? Write the number.
6
Which factor is the reason a powder reacts faster than a lump? Write the words.
7
Does a catalyst raise or lower the activation energy? Write the word.
8
In a reaction profile, the products are higher than the reactants. Is it exothermic or endothermic? Write the word.
9
Name the hump on a reaction profile. Write the words.
10
Explain why a higher temperature increases the rate. Write one sentence.

Total 15 marks. Allow about 15 minutes.

Exercise 10X  Practice test questions

1
RWhat is a successful collision? Write one sentence.[2 marks]
2
FName one factor that increases the rate of a reaction by increasing the number of particles in the same volume. Write the word.[1 mark]
3
PSUsing the gas table, what is the rate between 40 and 60 seconds, in cm3 per second? Write the number.[2 marks]
4
FWhat is the name of the minimum energy needed for a successful collision? Write the words.[1 mark]
5
RDescribe what a catalyst does, in one sentence. Write one sentence.[2 marks]
6
FIn an exothermic reaction, does the temperature of the surroundings rise or fall? Write the word.[1 mark]
7
PSUsing the temperature table, how many times faster is the reaction at 30 degrees C than at 10 degrees C? Write the number.[2 marks]
8
FName the energy profile feature that a catalyst reduces. Write the words.[1 mark]
9
RExplain why a fair test of concentration keeps the temperature the same. Write one sentence.[2 marks]
10
PSA reaction changes the temperature from 20 degrees C to 26 degrees C. Is it exothermic or endothermic? Write the word.[1 mark]
← Chapter 9: Acids, alkalis and pHChapter 11: Energy stores, transfers and heating →

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