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GCSE Chemistry · GCSEChapter 12 · Energy changes, bond energies and cells
GCSE · Chapter 12

Energy changes, bond energies and cells

By the end of this chapter you will be able to

  • Classify a reaction as exothermic or endothermic from data and examples
  • Calculate the energy transferred in a temperature-change experiment using Q = m c change in temperature
  • Interpret an energy level diagram and identify the activation energy
  • [H] Calculate the energy change of a reaction using bond energies
  • Explain the reactions in a cell and the advantages of a fuel cell

You should already know

  • Ions and electrons (Chapter 4)
  • Reactivity series (Chapter 3)
  • Moles and Mr (Chapters 7 and 8)

Where this leads

  • Rates of reaction and catalysts (Chapter 13)
  • Electrolysis and cells (Chapter 11)

GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Classify reactions as exothermic or endothermic from temperature change, Carry out the temperature-change practical with dilute solutions, safely, and calculate the energy change, Draw and interpret energy level diagrams, including activation energy, [H] Calculate the overall energy change of a reaction from bond energies, Explain how a cell produces electricity, and describe a hydrogen fuel cell.

Exercise P  Do you remember?

1
FWhat is 50 x 4.2? Write the number.
2
FWhat is 210 x 6? Write the number.
3
FWhich metal is more reactive: zinc or copper? Write the name.
4
FWhat is the name of the gas formed when hydrogen burns in oxygen? Write the name of the product.
5
FWhat is 1260 / 1000? Write the number as a decimal.
12.1

Exothermic and endothermic reactions

GCSE
Classify reactions as exothermic or endothermic from temperature change
Video lesson: Exothermic and endothermic reactionsComing soon
Context. A pupil mixes two solutions and the beaker feels warm. Another mixes solid ammonium nitrate with water and the beaker feels cold. “Both are reactions, so why is one warm and one cold?” asks the pupil. The answer is whether energy is released or taken in.

An exothermic reaction transfers 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 displacement reactions are exothermic. Thermal decomposition and photosynthesis are endothermic.

ReactionTypeExample
combustion, neutralisation, displacementexothermictemperature rises
thermal decomposition, photosynthesisendothermictemperature falls
dissolving ammonium nitrateendothermica cold pack
Some exothermic and endothermic reactions

Key ideas

  • Exothermic: energy is released; temperature rises.
  • Endothermic: energy is taken in; temperature falls.
  • Combustion, neutralisation and displacement are exothermic.
  • Thermal decomposition and photosynthesis are endothermic.
Worked example 12.1.1
A reaction makes a beaker warmer. Is it exothermic or endothermic? Write the word.

Method. A rise in temperature shows energy given out.

  1. Look. The temperature of the surroundings rises.
    rises
  2. Decide. Energy is released, so the reaction is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. Energy goes from the reaction to the surroundings. ✓

Worked example 12.1.2
Is the thermal decomposition of calcium carbonate exothermic or endothermic? Write the word.

Method. Decomposition needs heat to break the bonds.

  1. Recall. Thermal decomposition needs heat energy to break down the compound.
    needs heat
  2. Decide. So it is endothermic.
    endothermic
  3. Write the answer.
    endothermic

Check. Calcium carbonate needs strong heating to decompose. ✓

Worked example 12.1.3
A reaction in a cup changes the temperature of the solution from 21 degrees C to 17 degrees C. Is it exothermic or endothermic? Write the word.

Method. Find the direction of the change.

  1. Change. The temperature falls by 4 degrees C.
    -4
  2. Decide. A fall means energy is taken in from the surroundings.
    taken in
  3. Answer. So the reaction is endothermic.
    endothermic
  4. Write the answer.
    endothermic

Check. 21 - 4 = 17. ✓

Watch out

Watch out: a temperature rise means exothermic, and a temperature fall means endothermic. Do not use the words “hot” and “cold” for the reaction itself; describe the change in temperature.

Exam tip

Exam tip. Write “energy is released to the surroundings, so the temperature rises”, not just “it gets hot”.

Exercise 12A  Exothermic and endothermic reactions

1
FIs a reaction exothermic if its temperature rises? Write yes or no.
2
FIs a reaction endothermic if its temperature falls? Write yes or no.
3
FIs combustion exothermic or endothermic? Write the word.
4
FIs neutralisation exothermic or endothermic? Write the word.
5
FIs photosynthesis exothermic or endothermic? Write the word.
6
FIs the dissolving of ammonium nitrate in water exothermic or endothermic? Write the word.
7
FName one type of reaction that is always exothermic. Write the name. Accept combustion or neutralisation.
8
FName one type of reaction that is endothermic. Write the name. Accept thermal decomposition or photosynthesis.
9
PSA reaction causes the temperature of the surroundings to fall from 20 degrees C to 17 degrees C. Is it exothermic or endothermic? Write the word.
10
PSA reaction causes the temperature to rise from 18 degrees C to 25 degrees C. How large is the temperature change? Write the number of degrees.
11
RExplain why a reaction that releases energy feels warm, in one sentence. Accept any sentence that says the energy is transferred to the surroundings, raising their temperature.
12
RExplain why calcium carbonate needs to be heated strongly to decompose, in one sentence. Accept any sentence that says heat energy is needed to break the bonds in the compound.
12.2

Required practical: measuring a temperature change

GCSE
Carry out the temperature-change practical with dilute solutions, safely, and calculate the energy change
Video lesson: Required practical: measuring a temperature changeComing soon
Context. A class mixes 25 cm3 of dilute acid with 25 cm3 of dilute alkali in a polystyrene cup, and records the temperature every half minute. “How much energy did the reaction give out?” asks the teacher. The temperature rise and the mass of solution give the answer.

In the required practical, the temperature is measured before and after a reaction in a polystyrene cup with a lid. The highest temperature reached gives the temperature change. The energy transferred to the solution is found using Q = m x c x change in temperature, where m is the mass of solution in grams and c is 4.2 J/g/degrees C for dilute aqueous solutions. The answer is in joules; divide by 1000 to get kilojoules.

thermometerdilute acid + alkalipolystyrene cup with a lidacid 25 cm3alkali 25 cm3
The apparatus for measuring a temperature change in a dilute neutralisation (pupils wear goggles)

Try it (with your teacher). Required practical: temperature change in a neutralisation. Use dilute acid and dilute alkali only. Wear goggles. Measure the temperature of the acid, add the alkali, stir, and record the highest temperature. Keep the volumes the same for each test, so that the test is fair. Clean up spills at once, and wash your hands at the end.

Key ideas

  • Temperature change: final temperature minus starting temperature.
  • Energy transferred Q (J) = mass of solution (g) x 4.2 x change in temperature (degrees C).
  • Mass of solution: about 1 g per cm3 for dilute solutions.
  • Divide the energy in joules by 1000 to give kilojoules.
Worked example 12.2.1
A reaction in 50 g of solution rises by 6.0 degrees C. Calculate the energy transferred, in J. Write the number.

Method. Use Q = m x c x change in temperature.

  1. Multiply. Q = 50 x 4.2 x 6.0.
    50 x 4.2 x 6.0
  2. Answer. Q = 1260 J.
    1260
  3. Write the answer.
    1260

Check. 1260 J = 1.26 kJ. ✓

Worked example 12.2.2
In the same reaction, what is the energy transferred in kJ? Write the number.

Method. Divide the joules by 1000.

  1. Joules. The energy from the previous example is 1260 J.
    1260 J
  2. Convert. 1260 J / 1000 = 1.26 kJ.
    1.26
  3. Write the answer.
    1.26

Check. 1.26 x 1000 = 1260 J. ✓

Worked example 12.2.3
A thermometer reads 19.5 degrees C at the start and 24.5 degrees C at the highest point. What is the temperature change? Write the number.

Method. Subtract the start from the highest.

  1. Read. The start is 19.5 degrees C and the highest is 24.5 degrees C.
    19.5 and 24.5
  2. Subtract. 24.5 - 19.5 = 5.0.
    5.0
  3. Write the answer.
    5

Check. 19.5 + 5.0 = 24.5. ✓

Watch out

Watch out: use the highest temperature reached, not the temperature after several minutes. The reaction may cool the solution after it has finished heating.

Exam tip

Exam tip. In the practical, keep the volumes and concentrations the same for each test, and say which variable you changed.

Exercise 12B  Required practical: measuring a temperature change

1
FWhat is the unit of energy transferred in the calculation Q = m c change in temperature? Write the unit.
2
FWhat value of c is used for dilute aqueous solutions? Write the number in J/g/degrees C.
3
FWhat is the mass of 50 cm3 of dilute solution, taking 1 cm3 as about 1 g? Write the number in g.
4
FCalculate the energy transferred when 50 g of solution rises by 2.0 degrees C. Write the number in J.
5
PSCalculate the energy transferred when 100 g of solution rises by 3.0 degrees C. Write the number in J.
6
PSIn a reaction, 50 g of solution rises by 6.0 degrees C. What is the energy in kJ? Write the number.
7
PSA solution of mass 80 g has a temperature change of 5.0 degrees C. Calculate the energy transferred in J. Write the number.
8
RWhy must the temperature be recorded at the highest point, in one sentence? Accept any sentence that says the highest point shows the maximum energy given out.
9
RExplain why a polystyrene cup with a lid is used, in one sentence. Accept any sentence that says it reduces heat loss to the surroundings.
10
RExplain why the volumes of the acid and alkali are kept the same in each test, in one sentence. Accept any sentence that says it keeps the test fair.
12.3

Energy level diagrams and activation energy

GCSE
Draw and interpret energy level diagrams, including activation energy
Video lesson: Energy level diagrams and activation energyComing soon
Context. A chemist draws an energy diagram for a reaction and marks a hump at the top. “What does the hump show?” asks a pupil. The hump is the activation energy: the minimum energy needed for the reaction to start.

An energy level diagram shows the energy of the reactants and products. If the products are lower than the reactants, the reaction is exothermic; if they are higher, it is endothermic. The activation energy is the minimum energy needed for a reaction to start; it is shown as the peak between the reactants and the products.

Exothermic reaction: energy is releasedreactantsproductsenergyactivation energyenergy released
Energy level diagram for an exothermic reaction
Endothermic reaction: energy is taken inreactantsproductsenergyactivation energyenergy taken in
Energy level diagram for an endothermic reaction

Key ideas

  • Products lower than reactants: exothermic.
  • Products higher than reactants: endothermic.
  • Activation energy: the minimum energy needed for reactions to start.
  • The energy change is the difference between the reactants and the products.
Worked example 12.3.1
On an energy diagram, the products are lower than the reactants. Is the reaction exothermic or endothermic? Write the word.

Method. Compare the two levels.

  1. Compare. Products lower means energy has been released.
    released
  2. Decide. So the reaction is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. Energy is given out to the surroundings. ✓

Worked example 12.3.2
What is the activation energy? Write the words.

Method. Find the peak on the diagram.

  1. Look. The activation energy is the height of the peak above the reactants.
    peak above reactants
  2. Define. It is the minimum energy needed for the reaction to start.
    minimum energy
  3. Write the answer.
    the minimum energy needed for a reaction to start

Check. It is needed even in exothermic reactions. ✓

Worked example 12.3.3
A catalyst is added to a reaction. How does the activation energy change? Write the word.

Method. A catalyst gives an alternative pathway.

  1. Look. A catalyst provides a pathway with a lower peak.
    lower peak
  2. Decide. So the activation energy is lowered.
    lowered
  3. Write the answer.
    lowered

Check. The energy of the reactants and products is unchanged. ✓

Watch out

Watch out: a catalyst lowers the activation energy, but it does not change the energy of the products or the reactants, so it does not change the energy change of the reaction.

Exam tip

Exam tip. Label the reactants, the products, the energy change (between the levels) and the activation energy (from the reactants to the peak).

Exercise 12C  Energy level diagrams and activation energy

1
FWhat is the activation energy? Write the words.
2
FOn an energy diagram, which is higher for an exothermic reaction: the reactants or the products? Write the word.
3
FOn an energy diagram, which is higher for an endothermic reaction: the reactants or the products? Write the word.
4
FWhat effect does a catalyst have on the activation energy? Write the word.
5
FWhat effect does a catalyst have on the energy change of a reaction? Write the word. Accept no effect.
6
PSWhat is the energy change of a reaction if the reactants are at 200 kJ and the products are at 150 kJ? Write the number in kJ, with the sign.
7
PSWhat is the energy change of a reaction if the reactants are at 100 kJ and the products are at 180 kJ? Write the number in kJ, with the sign.
8
PSAn energy diagram shows the reactants at 60 kJ, the peak at 110 kJ and the products at 20 kJ. What is the activation energy? Write the number in kJ.
9
PSIn the same diagram, what is the energy change? Write the number in kJ, with the sign.
10
RExplain why reactions need a minimum energy to start, in one sentence. Accept any sentence that says particles must collide with enough energy to break bonds.
11
RExplain why a catalyst makes a reaction faster, using activation energy, in one sentence. Accept any sentence that says a lower activation energy means more particles have enough energy to react.
12.4

Bond energies and the energy balance [H]

GCSE
[H] Calculate the overall energy change of a reaction from bond energies
Video lesson: Bond energies and the energy balance [H]Coming soon
Context. A student wants to know whether making water from hydrogen and oxygen gives out energy. “Can we work it out from the bonds?” she asks. Yes: breaking bonds takes in energy and making bonds gives out energy, so the overall change is the difference.

[H] Higher tier only. Breaking bonds takes in energy (endothermic); making bonds gives out energy (exothermic). The overall energy change = energy taken in to break bonds - energy given out to make bonds. A negative answer means the reaction is exothermic.

BondBond energy (kJ/mol)
H-H436
Cl-Cl242
H-Cl431
O=O498
O-H464
C-H413
C=O805
Bond energies in kJ/mol (average values)

Key ideas

  • Energy in (breaking) minus energy out (making) = overall energy change.
  • Negative: exothermic. Positive: endothermic.
  • Count every bond broken on the reactant side, and every bond made on the product side.
  • Bond energies are in kJ/mol: multiply by the number of bonds first.
Worked example 12.4.1
[H] For H2 + Cl2 -> 2HCl, the bonds broken are H-H (436) and Cl-Cl (242); the bonds made are two H-Cl (431 each). Calculate the overall energy change in kJ/mol. Write the number.

Method. Energy in minus energy out.

  1. Broken. Energy in = 436 + 242 = 678 kJ/mol.
    678
  2. Made. Energy out = 2 x 431 = 862 kJ/mol.
    862
  3. Change. Overall = 678 - 862 = -184 kJ/mol.
    -184
  4. Write the answer.
    -184

Check. The reaction is exothermic. ✓

Worked example 12.4.2
[H] For 2H2 + O2 -> 2H2O, the bonds broken are two H-H and one O=O; four O-H bonds are made. Calculate the overall energy change in kJ/mol. Write the number.

Method. Use the bond energies for each side.

  1. Broken. 2 x 436 + 498 = 1370.
    1370
  2. Made. 4 x 464 = 1856.
    1856
  3. Change. 1370 - 1856 = -486 kJ/mol.
    -486
  4. Write the answer.
    -486

Check. Exothermic: more energy is released than taken in. ✓

Worked example 12.4.3
[H] Is the reaction in the first example exothermic or endothermic? Write the word.

Method. Look at the sign.

  1. Sign. The overall change is negative.
    -184
  2. Decide. So the reaction is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. More energy is given out making bonds than taken in breaking them. ✓

Watch out

Watch out: the bond energy values are per mole of bonds. Multiply each bond energy by the number of bonds of that type before you add.

Exam tip

Exam tip. Make two columns: bonds broken and bonds made. Add each column, then subtract.

Exercise 12D  Bond energies and the energy balance [H]

1
F[H] Is breaking a bond endothermic or exothermic? Write the word.
2
F[H] Is making a bond endothermic or exothermic? Write the word.
3
F[H] What is the bond energy of H-H, in kJ/mol? Write the number.
4
F[H] What is the energy needed to break two H-H bonds, in kJ/mol? Write the number.
5
F[H] What is the energy released when two H-Cl bonds are made, in kJ/mol? Write the number.
6
PS[H] Calculate the overall energy change for H2 + Cl2 -> 2HCl (broken 678, made 862). Write the number in kJ/mol.
7
PS[H] For a reaction where energy in is 1000 kJ/mol and energy out is 1200 kJ/mol, is it exothermic or endothermic? Write the word.
8
PS[H] Calculate the overall energy change for CH4 + 2O2 -> CO2 + 2H2O, using broken: 4 C-H (413 each) and 2 O=O (498 each); made: 2 C=O (805 each) and 4 O-H (464 each). Write the number in kJ/mol.
9
R[H] Explain why the overall energy change of a reaction is the difference between the bonds broken and the bonds made, in one sentence. Accept any sentence that says energy is taken in to break bonds and released when bonds form.
12.5

Chemical cells and fuel cells

GCSE
Explain how a cell produces electricity, and describe a hydrogen fuel cell
Video lesson: Chemical cells and fuel cellsComing soon
Context. A torch uses a battery, and a car uses a fuel cell in some prototypes. “How do these produce electricity from chemicals?” asks a pupil. In a cell, a chemical reaction moves electrons through an external circuit.

A chemical cell has two different metals (electrodes) in an electrolyte. The more reactive metal loses electrons more readily, so it is the negative terminal. Electrons flow through the wire from the more reactive metal to the less reactive one. A greater difference in reactivity gives a bigger voltage. A fuel cell uses a fuel, such as hydrogen, and oxygen, and it produces electricity and water. It does not need to be recharged while fuel is supplied.

A zinc-copper cellzinc (-)copper (+)electrolyteelectrons flow
A zinc-copper cell: zinc is the negative electrode
hydrogen infuel cellwater + electricityoxygen (from air) goes in as well
A hydrogen-oxygen fuel cell

Key ideas

  • A cell has two electrodes of different metals in an electrolyte.
  • The more reactive metal is the negative terminal.
  • Electrons flow from the negative to the positive electrode through the wire.
  • A hydrogen fuel cell produces electricity and water; the only product is water.
Worked example 12.5.1
In a zinc-copper cell, which metal is the negative terminal? Write the name.

Method. The more reactive metal is negative.

  1. Compare. Zinc is more reactive than copper.
    zinc more reactive
  2. Decide. So zinc is the negative terminal.
    zinc
  3. Write the answer.
    zinc

Check. Electrons flow from zinc to copper through the wire. ✓

Worked example 12.5.2
Which product does a hydrogen fuel cell produce, apart from electricity? Write the name.

Method. Recall the reaction.

  1. Recall. Hydrogen reacts with oxygen to form water.
    water
  2. Decide. So the product is water.
    water
  3. Write the answer.
    water

Check. The only product is water, so it is clean. ✓

Worked example 12.5.3
Why does a more reactive pair of metals give a bigger voltage? Write one reason.

Method. Compare the metals.

  1. Look. A bigger difference in reactivity gives electrons more reason to flow.
    bigger difference
  2. Conclude. So the voltage is bigger.
    bigger voltage
  3. Write the answer.
    a bigger difference in reactivity gives a bigger voltage

Check. The metals in the cell are the electrodes. ✓

Watch out

Watch out: the more reactive metal is the negative terminal, and electrons flow from it. Do not reverse the direction of the electrons.

Exam tip

Exam tip. In a cell question, say which metal is more reactive, which is negative, and which way the electrons flow.

Exercise 12E  Chemical cells and fuel cells

1
FIn a cell, which terminal does the more reactive metal form? Write the word.
2
FIn which direction do electrons flow through the wire of a cell? Write the words.
3
FWhat is the electrolyte in a cell? Write the words.
4
FWhat is the only product of a hydrogen-oxygen fuel cell, apart from electricity? Write the word.
5
FWhat two substances are used in a hydrogen fuel cell? Write the two names.
6
FWhich metal is more reactive, zinc or copper? Write the name.
7
PSWhich is the negative electrode in a cell with iron and copper electrodes? Write the name of the metal. (Iron is more reactive.)
8
RExplain why a fuel cell is useful, in one sentence. Accept any sentence that says it produces electricity with only water as the product, with no pollution at the point of use.
9
RExplain why a cell stops working when the reactants are used up, in one sentence. Accept any sentence that says the chemical reaction stops when a reactant runs out.

Chapter summary

SkillMethod
ExothermicEnergy released; temperature rises; products lower than reactants.
EndothermicEnergy taken in; temperature falls; products higher than reactants.
Temperature changeQ = m x 4.2 x change in temperature (J); divide by 1000 for kJ.
Activation energyThe minimum energy needed to start a reaction; the peak on an energy diagram.
[H] Bond energiesEnergy in (breaking) minus energy out (making) = overall change; negative means exothermic.
CellsThe more reactive metal is negative; electrons flow from it through the wire.
Fuel cellHydrogen and oxygen react; electricity and water are produced.

Can you…?

  • I can classify reactions as exothermic or endothermic from temperature change and examples
  • I can calculate the energy transferred in a temperature-change practical
  • I can interpret an energy level diagram and identify the activation energy
  • [H] I can calculate the overall energy change from bond energies
  • I can explain how a cell works, and describe a hydrogen fuel cell

Key terms

exothermic
a reaction that transfers energy to the surroundings, so the temperature rises
endothermic
a reaction that takes in energy from the surroundings, so the temperature falls
activation energy
the minimum energy needed for a reaction to start
energy level diagram
a diagram showing the energy of the reactants and products
bond energy
[H] the energy needed to break one mole of a particular bond, in kJ/mol
fuel cell
a cell that produces electricity from a fuel, such as hydrogen, reacting with oxygen

Exercise 12Z  Check-up

1
Is a reaction that releases energy exothermic or endothermic? Write the word.
2
Name the minimum energy needed for a reaction to start. Write the words.
3
Calculate the energy transferred when 50 g of solution rises by 4.0 degrees C, in J. Write the number.
4
Which metal is the negative terminal in a cell made of zinc and copper? Write the name.
5
What is the only product of a hydrogen fuel cell, apart from electricity? Write the word.
6
[H] Calculate the overall energy change for 2H2 + O2 -> 2H2O (broken 1370, made 1856). Write the number in kJ/mol.
7
An energy diagram shows reactants at 80 kJ and products at 120 kJ. Is the reaction exothermic or endothermic? Write the word.
8
Explain why a catalyst does not change the energy change of a reaction, in one sentence. Accept any sentence that says it only lowers the activation energy, and the reactant and product energies stay the same.
9
Explain why a cell with zinc and copper has a voltage, in one sentence. Accept any sentence that says the metals differ in reactivity, so electrons flow from zinc to copper.
10
[H] Is making a bond endothermic or exothermic? Write the word.

Total 24 marks. Allow about 24 minutes.

Exercise 12X  Exam-style questions

1
PSA reaction causes the temperature of the solution to rise from 20.0 degrees C to 26.0 degrees C. Is the reaction exothermic or endothermic? Write the word, and the temperature change.[2 marks]
2
PSCalculate the energy transferred when 80 g of solution rises by 5.0 degrees C, in J. Write the number.[2 marks]
3
FDraw or describe an energy level diagram for an exothermic reaction, and state whether the products are higher or lower than the reactants. Write the word: higher or lower.[2 marks]
4
RExplain why activation energy is needed even for an exothermic reaction, in one sentence. Accept any sentence that says particles must have enough energy to break bonds before the reaction can start.[2 marks]
5
PS[H] Calculate the overall energy change for H2 + Cl2 -> 2HCl, using bond energies H-H 436, Cl-Cl 242, H-Cl 431. Write the number in kJ/mol.[3 marks]
6
R[H] Explain, using bond energies, why a reaction is exothermic when more energy is released making bonds than is taken in breaking them. Write one or two sentences. Accept any answer that compares energy in and energy out.[3 marks]
7
RExplain how a cell produces electricity, in two sentences. Accept any answer that says a chemical reaction releases electrons at the more reactive metal, which flow through the wire to the other electrode.[3 marks]
8
RDescribe the main advantage of a hydrogen fuel cell over burning hydrogen for electricity, in one sentence. Accept any sentence that says a fuel cell produces electricity directly, with water as the only product.[2 marks]
9
PSIn an energy diagram, the reactants are at 150 kJ, the peak is at 200 kJ and the products are at 100 kJ. Find the activation energy and the energy change. Write two numbers, activation energy then energy change.[3 marks]
10
RExplain why the temperature of the solution is measured at its highest point in the temperature-change practical, in one sentence. Accept any sentence that says the highest point shows the full energy given out.[2 marks]
← Chapter 11: Electrolysis and oxidation and reductionChapter 13: The rate of chemical reactions and catalysts →

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