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KS3 Science · Year 9Chapter 8 · Rates of reaction and energy in reactions
Year 9 · Chapter 8

Rates of reaction and energy in reactions

By the end of this chapter you will be able to

  • Explain the rate of a reaction in terms of collisions with enough energy
  • Describe how each factor changes the rate, and why
  • Work out a mean rate, and read a rate from a gradient, at basic level
  • Use temperature change and energy diagrams to say whether a reaction is exothermic or endothermic
  • Describe energy stores and transfers in fuels and food, and the conservation of energy

You should already know

  • Factors that change reactions, from Year 8 Science, Chapter 10
  • Energy stores and transfers, from Year 8 Science, Chapter 11

Where this leads

  • Energy, power and efficiency in Chapter 9, which uses the same ideas about stores and transfers

National Curriculum for England, Key Stage 3 science, Year 9. Content: 9C4 collision theory: particles must collide with enough energy; activation energy (basic), 9C4 how concentration, pressure, surface area, temperature and catalysts change the rate (deepens Year 8, Chapter 10), 9C4 rate from the gradient of a graph (basic), 9C4 energy in reactions: exothermic and endothermic; energy stored in chemical bonds (basic); conservation of energy; energy stores in fuels and food.

Exercise P  Do you remember?

1
FWhat is 30 divided by 60? Write the decimal.
2
FWhich gas is needed for combustion of most fuels? Write the word.
3
FWhat is the word for a substance that speeds up a reaction without being used up? Write the word.
4
FHow many seconds are there in one minute? Write the number.
5
FWhat is 20 + 12? Write the number.
8.1

Collisions and activation energy

Year 9
9C4: collision theory: particles must collide with enough energy; activation energy (basic)
Video lesson: Collisions and activation energyComing soon
Story. A pupil crushes a tablet and drops it into water, and it fizzes quickly. Another pupil drops a whole tablet into cold water, and it fizzes slowly. Both mixtures contain the same chemicals, and the teacher asks why the rates differ.

A chemical reaction happens when particles collide. For a collision to work, the particles must collide with enough energy. The minimum energy needed for a successful collision is the activation energy. Most collisions do not have enough energy, so they do not react.

The rate of reaction is how fast the reactants are used up or the products are made. More successful collisions per second means a faster rate.

energyreactantsproductspeakexothermic: energy given out
Energy profile: the peak is the activation energy above the reactants

Key ideas

  • A reaction needs particles to collide with enough energy.
  • Activation energy is the minimum energy for a successful collision.
  • Rate: how fast reactants are used up or products are made.
  • More successful collisions per second means a faster rate.
Worked example 8.1.1
What is the minimum energy needed for a successful collision called? Write the words.

Method. Recall the name.

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

Check. It is shown as the peak on an energy profile. ✓

Worked example 8.1.2
In the energy profile, the reactants have energy 60 and the peak is 90. Find the activation energy. Write the number.

Method. Subtract the reactant energy from the peak.

  1. Subtract. The activation energy is the peak minus the reactants.
    90 - 60
  2. Calculate. So the activation energy is 30.
    90 - 60 = 30
  3. Write the answer.
    30

Check. 60 + 30 = 90. ✓

Worked example 8.1.3
Why does crushing a tablet make the reaction faster? Write the words.

Method. Think about collisions.

  1. Think. Crushing exposes more surface, so more particles can collide.
    more surface
  2. Name it. More collisions happen each second, so the rate is faster.
    more collisions
  3. Write the answer.
    more collisions

Check. Surface area is one of the factors in 8.2. ✓

Watch out

Watch out: a fast collision is not always a successful one. Only collisions with enough energy cause a reaction.

Exam tip

Exam tip. When explaining a change in rate, say “more (or fewer) successful collisions per second”, not just “more collisions”.

Try it (with your teacher, or a grown-up at home). With your teacher, drop one whole effervescent tablet into a beaker of water, and then drop a crushed one of the same mass into another beaker. Time how long each takes to stop fizzing. Use cold water only, and use eye protection.

Exercise 8A  Collisions and activation energy

1
FWhat must particles do for a reaction to happen? Write the words.
2
FWhat is the name of the minimum energy needed for a successful collision? Write the words.
3
FWhat is the name for how fast the reactants are used up? Write the word.
4
FDoes a higher rate mean more successful collisions per second? Write yes or no.
5
FOn an energy profile, what does the peak show? Write the words.
6
PSIn an energy profile, the reactants have energy 20 and the peak is 85. Find the activation energy. Write the number.
7
PSA reaction has an activation energy of 30 and the reactants have energy 60. What is the energy at the peak? Write the number.
8
PSWhy does a powder react faster than a lump of the same substance? Write the words.
9
PSWhich has more particles with enough energy at a higher temperature? Write the word.
10
RExplain why most collisions in a reaction do not lead to a reaction. Write one sentence. Accept any sentence saying most collisions do not have enough energy to reach the activation energy.
11
RChallenge: explain why the crushed tablet reacts faster, using collisions and activation energy. Accept any sentence saying more particles collide per second, and more of these collisions have energy above the activation energy.
8.2

Factors that change the rate

Year 9
9C4: how concentration, pressure, surface area, temperature and catalysts change the rate (deepens Year 8, Chapter 10)
Video lesson: Factors that change the rateComing soon
Story. A school chemistry technician tests the same reaction in four ways: a stronger acid, a warmer mixture, a powdered solid and a catalyst. The class predicts which change will make the reaction fastest, and explains why.

Five factors change the rate of a reaction in a way that can be explained with collisions. A higher concentration (or a higher pressure for a gas) means more particles in the same space, so more collisions. A higher temperature makes particles move faster, so more collisions have enough energy. A greater surface area, such as a powder, gives more collisions. A catalyst gives another route with a lower activation energy, so more collisions succeed. A catalyst is not used up.

FactorWhy it changes the rate
concentration (or pressure for gases)more particles in the same space, so more collisions per second
temperatureparticles move faster, so collide more often and with more energy
surface areaa powder has more exposed surface than a lump, so more collisions happen
catalystprovides another route with a lower activation energy, so the rate increases
Factors that change the rate of a reaction, and why

Key ideas

  • Higher concentration (or pressure) means more collisions.
  • Higher temperature means particles collide with more energy.
  • Greater surface area (a powder) means more collisions.
  • A catalyst lowers the activation energy and is not used up.
Worked example 8.2.1
Name a factor that makes a reaction faster by giving more collisions per second, but not by changing the energy of collisions. Write the word.

Method. Think about what changes only the number of collisions.

  1. Think. Concentration increases the number of particles, so more collisions happen.
    more particles
  2. Name it. So concentration is the factor.
    concentration
  3. Write the answer.
    concentration

Check. Temperature changes the energy as well. ✓

Worked example 8.2.2
Why does a catalyst make a reaction faster? Write the words.

Method. Think about the activation energy.

  1. Think. A catalyst gives a different route with a lower activation energy.
    lower activation energy
  2. Name it. So more collisions have enough energy, and the rate increases.
    lower activation energy
  3. Write the answer.
    lower activation energy

Check. The catalyst is not used up. ✓

Worked example 8.2.3
Which will react faster: a lump or a powder of the same mass? Write the word.

Method. Compare the surface area.

  1. Compare. A powder has much more surface area than a lump.
    more surface area
  2. Decide. So the powder reacts faster.
    powder
  3. Write the answer.
    powder

Check. More surface gives more collisions. ✓

Watch out

Watch out: a catalyst does not change the energy of the reactants or products, and it is not used up in the reaction.

Exam tip

Exam tip. For temperature, explain both the faster movement and the greater number of collisions with enough energy.

Try it (with your teacher, or a grown-up at home). With your teacher, compare the speed of a reaction with a catalyst and without one (for example, hydrogen peroxide with and without a small amount of manganese dioxide). Use eye protection, and do not taste or touch the chemicals. Record which is faster and say why.

Exercise 8B  Factors that change the rate

1
FWhich factor makes particles move faster and collide with more energy? Write the word.
2
FWhat is a substance that speeds up a reaction and is not used up? Write the word.
3
FWhich factor is changed by using a powder instead of a lump? Write the words.
4
FWhat happens to the rate when the concentration of an acid is increased? Write the word: increases or decreases.
5
FWhat happens to the rate of a gas reaction when the pressure is increased? Write the word: increases or decreases.
6
FDoes a catalyst change the energy of the reactants? Write yes or no.
7
PSWhich will react faster: a 2 mol/dm3 acid or a 1 mol/dm3 acid, at the same temperature? Write the number.
8
PSA mixture is heated from cold to warm. Does the rate increase or decrease? Write the word.
9
PSName two factors from this section that increase the rate by giving more collisions per second. Accept any two of: concentration, pressure, surface area.
10
RExplain why adding a catalyst makes the reaction faster. Write one sentence. Accept any sentence saying the catalyst lowers the activation energy, so more collisions have enough energy.
11
RChallenge: a pupil says a higher temperature only makes particles bigger. Write one sentence to correct him. Accept any sentence saying higher temperature makes particles move faster, so they collide more often and with more energy.
8.3

Measuring the rate from a graph

Year 9
9C4: rate from the gradient of a graph (basic)
Video lesson: Measuring the rate from a graphComing soon
Story. A pupil collects gas from a reaction every 20 seconds. She plots volume against time, and two lines appear: one for a cold mixture and one for a warm one. She asks how to compare the two rates from the graph.

The mean rate of a reaction is the amount of product made divided by the time taken. The steeper a line on a volume-time graph, the faster the rate. The gradient of the line is the change in volume divided by the change in time, so it gives the rate at that point.

Gas collected against time010203040506005101520253035time (s)volume (cm3)warm: steepercold: less steep
Gas volume against time for a cold and a warm experiment
Time (s)Cold: total gas (cm3)Warm: total gas (cm3)
000
2010-
4020-
6030-
The gas collected in each experiment (a dash means no reading at that time)

Key ideas

  • Mean rate = amount of product / time taken.
  • Gradient = change in volume / change in time.
  • A steeper line means a faster rate.
  • The warm experiment is steeper than the cold one.
Worked example 8.3.1
In the cold experiment, 30 cm3 of gas is made in 60 seconds. Find the mean rate, in cm3 per second. Write the number.

Method. Divide the volume by the time.

  1. Divide. Mean rate = 30 / 60.
    30 / 60
  2. Calculate. So the mean rate is 0.5 cm3 per second.
    0.5
  3. Write the answer.
    0.5

Check. 0.5 x 60 = 30 cm3. ✓

Worked example 8.3.2
In the warm experiment, 30 cm3 is made in 30 seconds. Find the mean rate. Write the number.

Method. Divide the volume by the time.

  1. Divide. Mean rate = 30 / 30.
    30 / 30
  2. Calculate. So the mean rate is 1 cm3 per second.
    1
  3. Write the answer.
    1

Check. 1 x 30 = 30 cm3. ✓

Worked example 8.3.3
On a graph, a line goes from (20 s, 10 cm3) to (40 s, 20 cm3). Find the gradient. Write the number.

Method. Gradient is the change in volume over the change in time.

  1. Change in volume. 20 - 10 = 10 cm3.
    20 - 10 = 10
  2. Change in time. 40 - 20 = 20 s.
    40 - 20 = 20
  3. Divide. Gradient = 10 / 20.
    10 / 20 = 0.5
  4. Write the answer.
    0.5

Check. The gradient matches the cold rate of 0.5 cm3/s. ✓

Watch out

Watch out: read the gradient from two points on the line, not from the axes. Use points you can read exactly.

Exam tip

Exam tip. Show the change in volume and the change in time separately, then divide. Include the units.

Try it (with your teacher, or a grown-up at home). With your teacher, draw a volume-time graph for a simple reaction that gives off gas (for example, a small amount of sodium carbonate and dilute acid, with eye protection). Draw the tangent at the start, and find the gradient. Do not use strong acids.

Exercise 8C  Measuring the rate from a graph

1
FWhat is the name of the rate found by dividing the amount of product by the time taken? Write the words.
2
FWhat does a steeper line on a volume-time graph show? Write the words.
3
FIn the cold experiment, what is the total volume after 60 seconds? Write the number, in cm3.
4
FIn the warm experiment, what is the volume after 15 seconds? Write the number, in cm3.
5
PSFind the mean rate of the cold experiment, in cm3 per second. Write the number.
6
PSFind the mean rate of the warm experiment, in cm3 per second. Write the number.
7
PSHow many times faster is the warm experiment than the cold one? Write the number.
8
PSA line goes from (0 s, 0 cm3) to (30 s, 15 cm3). What is its gradient? Write the number.
9
PSA reaction makes 40 cm3 of gas in 80 seconds. What is the mean rate, in cm3 per second? Write the number.
10
RWhy is the gradient of the warm line steeper than the cold one? Write one sentence. Accept any sentence saying the warm mixture makes gas faster, so more gas is made each second.
11
RChallenge: a graph of gas volume levels off after 40 seconds. Explain what this shows about the rate. Accept any sentence saying the rate has dropped to zero because the reactant is used up.
8.4

Exothermic and endothermic reactions

Year 9
9C4: energy in reactions: exothermic and endothermic reactions (qualitative and basic)
Video lesson: Exothermic and endothermic reactionsComing soon
Story. A pupil mixes two solutions in a cup and notices the temperature rises. In another mix, the cup becomes colder. The teacher says the first reaction gives out energy, and the second takes in energy from its surroundings.

An exothermic reaction gives out energy to the surroundings, so the temperature rises. An endothermic reaction takes in energy from the surroundings, so the temperature falls. Combustion, neutralisation and many oxidations are exothermic. Some dissolving and thermal decompositions are endothermic.

ReactionStart (degrees C)End (degrees C)Change (degrees C)
Exothermic2032+12
Endothermic2014-6
Temperature change in an exothermic and an endothermic reaction
energyreactantsproductspeakendothermic: energy taken in
Energy profile of an endothermic reaction: products have more energy than reactants

Key ideas

  • Exothermic: energy given out; temperature rises.
  • Endothermic: energy taken in; temperature falls.
  • On an energy profile, exothermic products are lower than reactants.
  • Combustion is exothermic.
Worked example 8.4.1
In the exothermic reaction, the temperature rises from 20 degrees C to 32 degrees C. What is the rise? Write the number.

Method. Subtract the start from the end.

  1. Subtract. The rise is 32 - 20.
    32 - 20
  2. Calculate. So the temperature rises by 12 degrees C.
    32 - 20 = 12
  3. Write the answer.
    12

Check. 20 + 12 = 32. ✓

Worked example 8.4.2
An endothermic reaction takes the temperature from 20 degrees C down to 14 degrees C. What is the fall? Write the number.

Method. Subtract the end from the start.

  1. Subtract. The fall is 20 - 14.
    20 - 14
  2. Calculate. So the temperature falls by 6 degrees C.
    20 - 14 = 6
  3. Write the answer.
    6

Check. 14 + 6 = 20. ✓

Worked example 8.4.3
Is combustion of a fuel exothermic or endothermic? Write the word.

Method. Recall what combustion gives out.

  1. Recall. Burning a fuel gives out heat and light.
    gives out heat
  2. Name it. So combustion is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. Combustion is an exothermic reaction. ✓

Watch out

Watch out: an exothermic reaction does not mean there is no activation energy. Energy must still be put in to start it.

Exam tip

Exam tip. Link the word to the temperature change: exo = out = warmer; endo = in = cooler.

Try it (with your teacher, or a grown-up at home). With your teacher, mix a small amount of vinegar and sodium hydrogencarbonate in a plastic cup, and measure the temperature before and after with a thermometer. Say whether the reaction is exothermic or endothermic, and how you know.

Exercise 8D  Exothermic and endothermic reactions

1
FIs an exothermic reaction one that gives out or takes in energy? Write the words.
2
FIs an endothermic reaction one that gives out or takes in energy? Write the words.
3
FIn an exothermic reaction, does the temperature rise or fall? Write the word.
4
FIs burning a fuel exothermic or endothermic? Write the word.
5
PSIn an endothermic reaction, the temperature falls from 25 degrees C to 19 degrees C. What is the fall? Write the number.
6
PSIn an exothermic reaction, the temperature rises from 18 degrees C to 30 degrees C. What is the rise? Write the number.
7
PSOn an energy profile, are the products higher or lower than the reactants in an exothermic reaction? Write the word.
8
PSOn an energy profile, are the products higher or lower than the reactants in an endothermic reaction? Write the word.
9
PSIs the reaction of a hand warmer that gets warm when activated exothermic or endothermic? Write the word.
10
RExplain why a cold pack gets cold when activated. Write one sentence. Accept any sentence saying it is an endothermic reaction that takes in energy from its surroundings.
11
RChallenge: explain why an exothermic reaction can still need heating to start. Accept any sentence saying the activation energy must be supplied first, and then the reaction gives out more energy than was put in.
8.5

Energy stores, fuels, food and conservation

Year 9
9C4: energy stores in fuels and food; the conservation of energy in chemical reactions (basic)
Video lesson: Energy stores, fuels, food and conservationComing soon
Story. A camping stove burns gas to heat a pan of water. A pupil asks where the energy goes, and whether any energy is lost. The teacher explains that the energy is stored in the gas, transferred to the water, and spread out to the surroundings.

Fuels and food store energy in their chemical store. When a fuel burns, the chemical store falls and the energy is transferred to the surroundings as heat and light. Bonds in the fuel are broken, and new bonds are made, which changes the energy stored.

The law of conservation of energy says that energy is never created or destroyed. The total energy before a reaction equals the total energy after it. Energy is only transferred or stored in another store.

Energy storeExample
chemical store (fuel)petrol, wood, food
thermal storethe warmth of a hot object
kinetic storea moving object
gravitational potential storean object raised above the ground
Energy stores and examples

Key ideas

  • Chemical stores hold energy in fuels and food.
  • Burning a fuel transfers energy from the chemical store to the surroundings.
  • Energy is conserved: the total is the same before and after.
  • Energy is wasted as heat to the surroundings.
Worked example 8.5.1
Name the energy store of a fuel such as petrol. Write the words.

Method. Recall the store.

  1. Recall. Fuels store energy chemically.
    store energy chemically
  2. Name it. So the energy store is the chemical store.
    chemical store
  3. Write the answer.
    chemical store

Check. Chemical stores are in fuels and food. ✓

Worked example 8.5.2
When a fuel burns, where does the energy go? Write the words.

Method. Think about the transfers.

  1. Think. The energy goes to the surroundings as heat and light.
    heat and light
  2. Name it. So it is transferred to the surroundings.
    to the surroundings
  3. Write the answer.
    to the surroundings

Check. The energy is not destroyed. ✓

Worked example 8.5.3
A chemical reaction has 100 J of energy in its reactants. How much energy is in the products, if none is lost? Write the number.

Method. Use the conservation of energy.

  1. Think. Energy is conserved, so none is destroyed.
    conserved
  2. Calculate. So the products have 100 J as well.
    100 J
  3. Write the answer.
    100

Check. The total is the same before and after. ✓

Watch out

Watch out: energy is not used up. It is transferred to other stores, mostly the thermal store of the surroundings.

Exam tip

Exam tip. When describing an energy transfer, name the store that falls and the store that rises.

Try it (with your teacher, or a grown-up at home). With your teacher, look at a food label and a fuel label, and compare the energy values per 100 g. Say which store the energy is in, and where it goes when used.

Exercise 8E  Energy stores, fuels, food and conservation

1
FWhich energy store holds energy in fuels and food? Write the words.
2
FWhat does the law of conservation of energy say about energy in a reaction? Write the words.
3
FWhen a fuel burns, what does the energy become in the surroundings? Write the word.
4
FIs energy created or destroyed in a reaction? Write yes or no.
5
FWhat is the energy store of a moving object called? Write the words.
6
PSA reaction starts with 200 J of energy in its reactants. How much energy is in the products, if none is lost? Write the number.
7
PSIn a reaction, 150 J of energy is stored and 40 J is given to the surroundings as heat. How much energy is left in the chemical store? Write the number.
8
PSIn a lamp, 100 J of electrical energy becomes 20 J of light and 80 J of heat. Is energy conserved? Write yes or no.
9
RWhy does a camping stove warm a pan less than the energy in the gas would suggest? Write one sentence. Accept any sentence saying some energy is transferred to the surroundings and is not used to heat the pan.
10
RChallenge: explain why food is an energy store, using the chemical store. Accept any sentence saying the bonds in food hold chemical energy, which is released when the food is digested or burned.

Chapter summary

SkillMethod
Collision theoryParticles must collide with at least the activation energy to react; the rate is the rate of successful collisions.
FactorsConcentration, pressure, surface area and temperature increase the rate; a catalyst lowers the activation energy.
RateMean rate = amount of product / time; the gradient of a volume-time graph gives the rate.
ExothermicGives out energy; the temperature rises; the products have less energy than the reactants.
EndothermicTakes in energy; the temperature falls; the products have more energy than the reactants.
Energy storesFuels and food store energy chemically; energy is conserved and transferred to the surroundings.

Can you…?

  • I can explain the rate of a reaction in terms of collisions with enough energy
  • I can describe how concentration, pressure, surface area, temperature and catalysts change the rate
  • I can work out a mean rate, and read a rate from a gradient, at basic level
  • I can say whether a reaction is exothermic or endothermic from its temperature change
  • I can describe energy stores in fuels and food, and the conservation of energy

Key terms

collision theory
the idea that a reaction happens when particles collide with enough energy
activation energy
the minimum energy needed for a successful collision
rate of reaction
how fast the reactants are used up or the products are made
catalyst
a substance that speeds up a reaction and is not used up
gradient
the change in one quantity divided by the change in another, on a graph
exothermic
a reaction that gives out energy to the surroundings
endothermic
a reaction that takes in energy from the surroundings
chemical store
energy stored in the bonds of a substance, such as a fuel or food
conservation of energy
the law that energy is never created or destroyed, only transferred
energy profile
a diagram showing the energy of the reactants and products, and the activation energy

Exercise 8Z  Check-up

1
What is the minimum energy needed for a successful collision called? Write the words.
2
Name a factor that increases the rate by making particles move faster. Write the word.
3
What is a substance that speeds up a reaction and is not used up? Write the word.
4
Is a reaction that gives out energy exothermic or endothermic? Write the word.
5
Write the mean rate of a reaction that makes 60 cm3 of gas in 30 seconds, in cm3 per second. Write the number.
6
An exothermic reaction changes the temperature from 20 degrees C to 26 degrees C. What is the rise? Write the number.
7
Name the energy store of a fuel such as wood. Write the words.
8
In a gradient question, a line goes from (0 s, 0 cm3) to (40 s, 20 cm3). What is the gradient? Write the number.
9
Is energy created or destroyed in a chemical reaction? Write yes or no.
10
Explain why a higher concentration makes a reaction faster. Write one sentence. Accept any sentence saying there are more particles in the same volume, so more collisions happen each second.

Total 21 marks. Allow about 21 minutes.

Exercise 8X  Practice test questions

1
FName the factor that gives a faster reaction when a solid is powdered. Write the words.[1 mark]
2
FIs a reaction that takes in energy from its surroundings exothermic or endothermic? Write the word.[1 mark]
3
PSA gas syringe collects 30 cm3 of gas in 60 seconds. What is the mean rate, in cm3 per second? Write the number.[2 marks]
4
RExplain why a catalyst speeds up a reaction, in one sentence. Accept any sentence saying it lowers the activation energy, so more collisions have enough energy.[2 marks]
5
RIn an exothermic reaction, the temperature rises from 18 degrees C to 30 degrees C. Explain what this shows about the energy. Write one sentence. Accept any sentence saying energy is given out to the surroundings.[2 marks]
6
RDescribe how the rate changes if the temperature of a reaction is increased. Write two sentences. Accept any answer saying particles move faster, and collide more often with more energy.[3 marks]
7
PSA reaction makes 40 cm3 of gas in 20 seconds in one experiment, and 40 cm3 in 80 seconds in another. Which is faster, and by what factor? Write the number of times faster.[3 marks]
8
PSA reaction changes the temperature from 25 degrees C to 19 degrees C. Is it exothermic or endothermic, and how much does the temperature change? Write the word, and the number of degrees.[2 marks]
9
RExplain how energy is conserved when a fuel burns, using the terms chemical store and surroundings. Write two sentences. Accept any answer saying the chemical store falls, and the same energy is transferred to the surroundings.[3 marks]
10
FDescribe two factors that increase the rate of a reaction by giving more collisions per second. Accept any two of concentration, pressure for a gas, and surface area.[2 marks]
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