Chemistry · Unit 11

Rates of Reaction

How fast reactions go

Thermodynamics tells you whether a reaction can happen; kinetics tells you whether it will happen in a useful amount of time. A great many industrially important reactions are both favourable and far too slow.

The unit covers measuring rate, collision theory as the explanation underneath, the factors that change rate, catalysts, and rate equations and orders.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-3

What this unit covers

Measuring Rate
Collision Theory
Factors Affecting Rate
Catalysts
Rate Equations and Orders

Where this fits

Needs Chemical Reactions. Pairs with Chemical Equilibria.

Where people slip

A catalyst does not change how much product you get, only how quickly you get there. It lowers the activation energy and is unchanged at the end.

How the unit is structured

Rates of Reaction runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Catalysts

Multiple choiceLevel 1
1. What is a catalyst?
- A substance that speeds up a reaction without being used upcorrect
- A substance that is used up to release energy
- A reactant that turns into product
- A substance that slows a reaction down
A catalyst speeds up a reaction but is not used up, so it can be recovered unchanged at the end.
Fact or fibLevel 2
2. A single catalyst will speed up any reaction you add it to.
Answer: False
Catalysts are specific, so a given catalyst only speeds up its own particular reaction, not all of them.
Fill the blankLevel 2
3. A catalyst works by providing a different reaction pathway with a lower ____ energy.
- activationcorrect
- kinetic
- potential
- nuclear
By lowering the activation energy, a catalyst lets more collisions succeed, so the reaction speeds up.
Match the pairsLevel 3
4. Match each catalyst to the reaction it is used to speed up.
Answer: Iron = Haber process making ammonia; Manganese(IV) oxide = Decomposing hydrogen peroxide; Platinum = Catalytic converter in cars; Nickel = Hardening vegetable oils
Iron catalyses the Haber process, manganese(IV) oxide the decomposition of hydrogen peroxide, platinum catalytic converters, and nickel the hydrogenation of oils.
Picture questionLevel 3
5. 📈 A rate-time graph for a reaction starts low, rises to a peak, then falls back towards zero. Which kind of reaction shows this unusual hump-shaped curve?
- An autocatalytic reactioncorrect
- A simple first-order decay
- A zero-order reaction
- A reaction already at equilibrium
In autocatalysis the rate rises as the catalytic product builds up, peaks, then falls as the reactants run out.
Put in orderLevel 3
6. Put the stages of an enzyme-catalysed reaction (Michaelis-Menten model) in the order they happen.
Answer: Enzyme and substrate collide and bind -> An enzyme-substrate complex forms -> The substrate is converted into product -> The product leaves, freeing the enzyme
The enzyme binds substrate to form an enzyme-substrate complex, converts it to product, then releases the product and is reused.

Collision Theory

Fact or fibLevel 1
7. Every single collision between particles results in a reaction.
Answer: False
Only collisions with enough energy and the right orientation react, so most collisions are unsuccessful.
Fill the blankLevel 1
8. According to collision theory, reacting particles must first ____ with each other before they can react.
- collidecorrect
- freeze
- dissolve
- evaporate
Particles can only react if they collide, so all reactions depend on collisions between particles.
Build the sentenceLevel 2
9. Build the definition: activation energy is...
Answer: the minimum energy needed to start a reaction
Activation energy is the minimum energy that colliding particles need in order to react.
Choose all that applyLevel 2
10. Which conditions are needed for a collision between particles to actually cause a reaction? (Select all that apply)
- The particles collide with at least the activation energycorrect
- The particles collide with the correct orientationcorrect
- The particles are the same colour
- The particles are moving in the same direction
A successful collision needs enough energy (at least the activation energy) and the correct orientation of the particles.
Guess the numberLevel 3
11. An Arrhenius plot of ln k against 1/T has a gradient of -6000 K. Using gradient = -Ea/R with R = 8.31 J mol-1 K-1, calculate the activation energy Ea in kJ/mol.
Answer: 50 kJ/mol
Ea = -gradient x R = 6000 x 8.31 = 49900 J/mol, which is about 50 kJ/mol.
Match the pairsLevel 3
12. Match each symbol in the Arrhenius equation k = A e^(-Ea/RT) to its meaning.
Answer: k = Rate constant; A = Frequency factor; Ea = Activation energy; R = Gas constant
k is the rate constant, A the frequency factor, Ea the activation energy and R the gas constant.

Factors Affecting Rate

Build the sentenceLevel 1
13. Build the sentence about how concentration affects rate.
Answer: higher concentration gives more frequent collisions
A higher concentration means more frequent collisions, which speeds the reaction up.
Choose all that applyLevel 1
14. Which of these factors can change the rate of a chemical reaction? (Select all that apply)
- Temperaturecorrect
- Concentrationcorrect
- Surface area of a solidcorrect
- The colour of the beaker
Temperature, concentration and the surface area of a solid all affect rate, but the colour of the container does not.
Fact or fibLevel 1
15. Diluting an acid with water will make it react faster with a metal.
Answer: False
Diluting lowers the concentration, so there are fewer collisions per second and the reaction slows down.
Fill the blankLevel 1
16. The concentration of a solution is usually measured in ____ per cubic decimetre (mol/dm3).
- molescorrect
- grams
- litres
- degrees
Concentration is measured in moles per cubic decimetre, written as mol/dm3.
Guess the numberLevel 2
17. As a rough rule of thumb, a reaction's rate roughly doubles for every rise in temperature of about how many degrees Celsius?
Answer: 10 degrees C
A common approximation is that the rate of many reactions doubles for roughly every 10 degrees C rise in temperature.
Multiple choiceLevel 2
18. Why does powdered calcium carbonate react faster with acid than large lumps of it?
- Powder has a larger surface area, so more particles are exposed to collide
- Powder is a different chemical from the lumps
- Powder is always at a higher temperature
- Powder has a lower activation energy
Powder has a much larger surface area, so more particles are exposed for the acid to collide with.

Measuring Rate

Fact or fibLevel 1
19. On a graph of product formed against time, a steeper line means a faster reaction.
Answer: True
A steeper gradient means more product forms each second, so the reaction is faster.
Fill the blankLevel 1
20. A reaction that gives off a gas can be followed by collecting the gas in a gas ____ and recording its volume over time.
- syringecorrect
- burette
- funnel
- condenser
A gas syringe collects the gas released so its volume can be read at regular time intervals.
Multiple choiceLevel 1
21. What does the rate of a chemical reaction tell you?
- How quickly reactants are used up or products are formedcorrect
- How much product could form in total
- How much energy is released
- Whether the reaction can be reversed
The rate of reaction measures how quickly reactants are used up or products are formed over time.
Odd one outLevel 2
22. Which of these is NOT a valid way to measure the rate of a reaction?
- Counting the number of atoms in one moleculecorrect
- Measuring the volume of gas produced over time
- Recording the loss in mass over time
- Timing how long a cross takes to disappear
Counting atoms in a molecule tells you nothing about speed, unlike gas volume, mass loss or timing a colour change.
Picture questionLevel 2
23. 💉 This piece of apparatus collects a gas and measures its volume as a reaction proceeds. What is it?
- Gas syringecorrect
- Burette
- Thermometer
- Colorimeter
A gas syringe collects gas produced and lets you read its volume directly at set times.
Put in orderLevel 2
24. Put these steps in the right order for measuring rate by the volume of gas produced.
Answer: Connect the reaction flask to a gas syringe -> Add the reactants and start the timer -> Record the gas volume at regular time intervals -> Plot volume of gas against time on a graph
You set up the apparatus, start the reaction and timer, record gas volume at intervals, then plot the results.

Rate Equations and Orders

Fill the blankLevel 2
25. In the rate equation rate = k[A]^m[B]^n, the powers m and n are called the ____ of reaction.
- orderscorrect
- coefficients
- masses
- concentrations
The powers m and n are the orders of reaction with respect to A and B, and can only be found by experiment.
Build the sentenceLevel 3
26. Build the rate equation for the SN2 hydrolysis of a primary haloalkane.
Answer: rate = k [haloalkane] [OH-]
SN2 is second order overall: rate = k[haloalkane][OH-], being first order in each reactant.
Choose all that applyLevel 3
27. Which of these can appear in an experimentally determined rate equation? (Select all that apply)
- A reactant involved up to and including the rate-determining stepcorrect
- A catalyst that takes part in the slow stepcorrect
- A reactant that only reacts in steps after the rate-determining step
- A product that plays no part in the mechanism
Species involved up to and including the rate-determining step, including catalysts, can appear; those acting only after it do not.
Fact or fibLevel 3
28. A first-order reaction has a half-life that stays constant as the reaction proceeds.
Answer: True
A constant half-life, independent of concentration, is the signature of a first-order reaction.
Guess the numberLevel 3
29. A reaction has the rate equation rate = k[A]^2[B]. What is the overall order of reaction?
Answer: 3
The overall order is the sum of the individual orders, so 2 + 1 = 3.
Match the pairsLevel 3
30. Match each overall order of reaction to the correct units of its rate constant k (rate in mol dm-3 s-1).
Answer: Zero order = mol dm-3 s-1; First order = s-1; Second order = mol-1 dm3 s-1; Third order = mol-2 dm6 s-1
Each extra order of concentration divides into k, changing its units by a factor of mol-1 dm3.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Rates of Reaction

120 questions across 20 steps. Start with step one and crawl at your own pace.

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Read about Rates of Reaction

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

What Is Activation Energy? Why Reactions Need a Starting PushLearn why chemical reactions need a minimum energy barrier, how temperature affects collisions, and how catalysts lower it.August 14, 2026 · 6 min read

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