Overview
What you’ll learn
- Distinguish physical from chemical changes using the evidence each one produces.
- Describe how surface area, concentration, temperature and catalysts change the rate of a reaction.
- Use collision theory to explain the effect of changing conditions on rate (Supplement).
- Explain reversible reactions, dynamic equilibrium and the conditions chosen for the Haber and Contact processes (Supplement).
- Define redox in terms of oxygen, hydrogen, electrons and oxidation number, and identify oxidising and reducing agents.
Syllabus checklist (0620)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 6.1Identify physical and chemical changes and describe the differences between them.
- 6.2Describe how the surface area of a solid, the concentration of a solution, the temperature and a catalyst affect the rate of a reaction.
- 6.2State that a catalyst increases the rate of a reaction and is not used up.
- 6.2Describe practical methods for investigating the rate of a reaction, including the effects of concentration and temperature.
- 6.2Interpret data, including graphs, from rate experiments.
- 6.3State that some reactions are reversible and that the products can react to re-form the reactants.
- 6.3Describe how changing the conditions can change the direction of a reversible reaction.
- 6.4Use a Roman numeral to indicate the oxidation number of an element in a compound, such as Fe in Fe₂O₃ is III.
- 6.4Define redox reactions as reactions involving oxidation and reduction.
- 6.4Define oxidation as the gain of oxygen and reduction as the loss of oxygen.
- 6.4Identify redox reactions as reactions involving the gain or loss of oxygen.
- 6.4Identify oxidation and reduction in redox reactions.
Supplement — Extended candidates only
- 6.2Describe collision theory in terms of particle collisions, activation energy and the effect of changing conditions.
- 6.2Describe and explain the effect on the rate of reaction of surface area, concentration, temperature and catalysts using collision theory.
- 6.2State that a catalyst decreases the activation energy of a reaction.
- 6.2Evaluate practical methods for investigating the rate of a reaction.
- 6.3State that a reversible reaction in a closed system reaches dynamic equilibrium, and predict and explain the effect of changing conditions.
- 6.3State the symbol equations for the Haber process (N₂ + 3H₂ ⇌ 2NH₃) and the Contact process (2SO₂ + O₂ ⇌ 2SO₃), the sources of the reactants and the typical industrial conditions.
- 6.3Explain the choice of conditions in the Haber and Contact processes in terms of rate and position of equilibrium.
- 6.4Define oxidation and reduction in terms of electron loss or gain, changes in oxidation number, and hydrogen loss or gain.
- 6.4Identify redox reactions by changes in oxidation number.
- 6.4Identify redox reactions by the colour changes seen with acidified aqueous potassium manganate(VII) (KMnO₄) or aqueous potassium iodide (KI).
- 6.4Define an oxidising agent and a reducing agent.
- 6.4Identify oxidising agents and reducing agents in reactions.
In 30 seconds
- 1A physical change alters the form or state of a substance but makes no new substance; a chemical change produces one or more new substances.
- 2Increasing the surface area, concentration or temperature makes collisions more frequent or more energetic, so the rate increases.
- 3A catalyst speeds up a reaction by lowering the activation energy and is not used up.
- 4In a reversible reaction the products can react to re-form the reactants, and in a closed system this leads to dynamic equilibrium (Supplement).
- 5The Haber process makes ammonia (N₂ + 3H₂ ⇌ 2NH₃) and the Contact process makes sulfur trioxide (2SO₂ + O₂ ⇌ 2SO₃) (Supplement).
- 6Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons.
Notes · 1 of 5
Physical and chemical changes
A physical change makes no new substance; a chemical change makes one or more new substances.
A physical change alters only the form, shape or state of a substance. No new substance is made, and the change is usually easy to reverse — for example melting ice, boiling water or dissolving sugar. A chemical change makes one or more new substances, and it is usually difficult to reverse — for example burning magnesium, rusting iron or neutralising an acid with an alkali.
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance formed? | No | Yes |
| Ease of reversal | Usually easy | Usually difficult |
| Typical examples | Melting, boiling, dissolving, breaking | Burning, rusting, neutralising, fermenting |
| Energy change | Small, if any | Often a clear energy change |
Notes · 2 of 5
The rate of a reaction
Surface area, concentration, temperature and catalysts all change how fast a reaction goes.
The rate of reaction is how quickly the reactants are used up or the products are formed. It can be measured by the volume of gas collected, the loss in mass as a gas escapes, or the time taken for a mixture to turn cloudy. When the rate is fast, the graph of product against time is steep; a flatter graph means the reaction is slower.
Surface area
Breaking solids up
A powder has a larger surface area than a lump, so more particles are exposed and collisions are more frequent.
Concentration
Stronger solutions
A more concentrated solution packs more reacting particles into the same volume, so collisions happen more often.
Temperature
Heating the mixture
Hotter particles move faster, so collisions become more frequent and more energetic.
Catalyst
Speeding up without being used up
A catalyst provides a route with a lower activation energy, so the reaction is faster.
- Collect the volume of gas produced in a gas syringe and record it against time.
- Measure the loss in mass as a gas escapes from the flask on a balance.
- Time how long a cross marked under the flask takes to disappear as a precipitate forms.
Notes · 3 of 5
Collision theory (Supplement)
Reacting particles must collide with at least the activation energy before a reaction can happen.
Collision theory (Supplement) explains rates in terms of particles. For a reaction to take place, reacting particles must collide, and the collision must have at least the activation energy. Most collisions do not have enough energy, so only a small fraction of them lead to a reaction.
- A higher temperature makes particles move faster, so there are more collisions and a larger fraction of them reach the activation energy.
- A higher concentration or pressure puts more particles into a given volume, so collisions become more frequent.
- A larger surface area exposes more particles to collision.
- A catalyst lowers the activation energy, so a greater proportion of collisions are successful.
Notes · 4 of 5
Reversible reactions and equilibrium
In a closed system a reversible reaction can settle at dynamic equilibrium.
Some reactions are reversible: the products can react together to re-form the reactants. We show this with the symbol ⇌. If the reaction happens in a closed system, nothing can escape, and the forward and reverse reactions can reach dynamic equilibrium (Supplement), where both reactions continue at the same rate and the amounts of reactants and products stop changing.
| Property | Haber process | Contact process |
|---|---|---|
| Equation | N₂ + 3H₂ ⇌ 2NH₃ | 2SO₂ + O₂ ⇌ 2SO₃ |
| Product | Ammonia | Sulfur trioxide |
| Source of reactants | Nitrogen from air; hydrogen from natural gas | Sulfur burned to make sulfur dioxide; oxygen from air |
| Typical conditions | About 450 °C, 200 atm, iron catalyst | About 450 °C, 1–2 atm, vanadium(V) oxide catalyst |
- Increasing the pressure shifts the equilibrium towards the side with fewer gas molecules.
- Lowering the temperature shifts the equilibrium in the exothermic direction. Both forward reactions above are exothermic, so a lower temperature gives a higher yield — but the rate would be too slow, so a compromise temperature is used.
- A catalyst speeds up both directions equally, so it does not change the position of equilibrium. It only helps equilibrium be reached sooner.
Notes · 5 of 5
Redox reactions
Oxidation and reduction always happen together, linked by the transfer of oxygen or electrons.
Redox reactions involve the transfer of oxygen or electrons. In terms of oxygen, oxidation is the gain of oxygen and reduction is the loss of oxygen. The two always happen together: a substance can only be oxidised if another is reduced at the same time.
| Property | Oxidation | Reduction |
|---|---|---|
| Oxygen | Gain of oxygen | Loss of oxygen |
| Hydrogen | Loss of hydrogen | Gain of hydrogen |
| Electrons (Supplement) | Loss of electrons | Gain of electrons |
| Oxidation number (Supplement) | Increases | Decreases |
- An oxidising agent accepts electrons and is itself reduced; it oxidises the other substance.
- A reducing agent gives away electrons and is itself oxidised; it reduces the other substance.
- Acidified potassium manganate(VII) is a useful oxidising agent: its purple colour is lost when it oxidises a reducing agent.
- Aqueous potassium iodide turns from colourless to brown when it is oxidised.
Revise
Key terms
The words examiners expect you to use precisely.
- Physical change
- A change in the form, shape or state of a substance in which no new substance is formed.
- Chemical change
- A change in which one or more new substances are formed.
- Rate of reaction
- How quickly reactants are used up or products are formed during a reaction.
- Catalyst
- A substance that increases the rate of a reaction by lowering the activation energy, without being used up.
- Activation energy
- The minimum energy that colliding particles must have for a reaction to take place.
- Reversible reaction
- A reaction in which the products can react together to re-form the reactants, shown with the symbol ⇌.
Show 3 more key termsShow fewer key terms
- Dynamic equilibrium
- The state of a reversible reaction in a closed system when the forward and reverse reactions continue at equal rates, so the amounts of substances stay constant.
- Oxidation number
- A number that shows how oxidised an element is within a compound, written with Roman numerals in a name such as iron(III).
- Oxidising agent
- A substance that accepts electrons, causing another substance to be oxidised, and is itself reduced.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Saying a catalyst is "used up" during a reaction.
A catalyst is not consumed; it takes part in the reaction but is regenerated, so it can be recovered at the end.
Instead: A catalyst increases the rate but is not used up and does not change the yield.
Mistake: Believing that a higher temperature only makes particles collide more often.
Raising the temperature also gives the particles more kinetic energy, so a larger fraction of collisions reach the activation energy.
Instead: Temperature increases both the collision frequency and the proportion of successful collisions.
Show 8 more mistakesShow fewer mistakes
Mistake: Thinking a catalyst changes how much product is formed.
A catalyst speeds up both the forward and the reverse reaction equally, so it does not move the position of equilibrium.
Instead: A catalyst only changes the rate; the equilibrium yield is unchanged.
Mistake: Calling dissolving a chemical change.
Dissolving changes only the form of the substance — no new substance is made, and the solute can usually be recovered by evaporation.
Instead: Dissolving is a physical change; rusting and burning are chemical changes.
Mistake: Mixing up oxidation and reduction.
It is easy to confuse the two definitions, especially when switching between oxygen and electrons.
Instead: Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons.
Mistake: Saying that both reactions stop at equilibrium.
Equilibrium is dynamic, so both directions keep happening even though the amounts no longer change.
Instead: At dynamic equilibrium the forward and reverse rates are equal, but they are not zero.
Mistake: Expecting a change in pressure to affect the equilibrium of a solution.
Only gases are squeezed into a smaller volume by pressure, so dissolved species are barely affected.
Instead: Pressure shifts an equilibrium towards the side with fewer gas molecules, which matters only when gases are involved.
Mistake: Confusing a shift in the position of equilibrium with a change in rate.
A shift changes the amounts present at equilibrium, while rate only changes how quickly equilibrium is reached.
Instead: A catalyst changes the time to reach equilibrium; temperature and pressure changes move the position itself.
Mistake: Insisting oxidation must always involve oxygen.
The oxygen definition is only one of several; reactions with no oxygen present can still be redox.
Instead: Oxidation is loss of electrons (or gain of oxygen, or loss of hydrogen); reduction is the reverse.
Mistake: Using “the particles have more energy” as the explanation for every factor that speeds up a reaction.
Only a temperature rise changes the energy of the particles; concentration, pressure and surface area change how often they collide.
Instead: Match the explanation to the factor: collision frequency for concentration, pressure and surface area; collision energy for temperature; lower activation energy for a catalyst.
Check yourself
Quick quiz
18 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Identify a physical change
Which of the following is a physical change?
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Questions
Frequently asked questions
What is the difference between a physical change and a chemical change?
A physical change alters only the form, shape or state of a substance and makes no new substance, so it is usually easy to reverse. A chemical change makes one or more new substances and is usually difficult to reverse.
Why does raising the temperature speed up a reaction?
Hotter particles have more kinetic energy and move faster. This makes collisions more frequent and, more importantly, means a larger fraction of collisions have at least the activation energy, so more of them are successful.
What does dynamic equilibrium mean?
In a closed system, a reversible reaction can reach a state where the forward and reverse reactions continue at equal rates. The amounts of reactants and products then stay constant, even though both reactions are still happening.
What is the difference between an oxidising agent and a reducing agent?
An oxidising agent accepts electrons, so it oxidises another substance and is itself reduced. A reducing agent gives away electrons, so it reduces another substance and is itself oxidised.
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Topic 7: Acids, bases and salts