Show Working

GCSE ChemistryAQA, Edexcel and OCR

Rates and equilibrium: GCSE questions with answers

27 chemistry questions on rates and equilibrium. Pick your answer, then tap to see if you were right and why. Every answer comes from the Show Working science bank, checked against the GCSE content every board in England teaches.

Quiz yourself: 27 questions 2 6-mark questions

01 / Quiz yourselfTry it, then tap to check

Rates and equilibrium questions to try

Choose your answer before you open it. Then read why, so it sticks.

01 / Rates and equilibriumBoth tiers

Marble chips react with dilute hydrochloric acid, giving off carbon dioxide. Which method measures the rate?

  1. AWeigh the marble chips at the start only
  2. BTest the gas with limewater at the end
  3. CMeasure the pH of the acid once at the end
  4. DMeasure the gas volume every 10 seconds
Show the answer

Answer: D Measure the gas volume every 10 seconds

To find a rate you measure how much product forms, or reactant is used up, over time. For a reaction that gives off a gas, collect it in a gas syringe or an upturned measuring cylinder and read the volume at regular times.

Checked against OCR Twenty First Century Chemistry B (J258) specification, C6.2.7

02 / Rates and equilibriumBoth tiers

A flask of marble chips and acid sits on a balance. Why does the mass go down?

  1. ACarbon dioxide escapes into the air
  2. BThe marble chips dissolve
  3. CThe acid is used up
  4. DThe carbon dioxide evaporates
Show the answer

Answer: A Carbon dioxide escapes into the air

When a gas is made in an open flask it escapes, so the flask loses mass. Reading the balance at regular times shows how fast the gas is made. A rate can be given in g/s for mass or cm³/s for gas volume.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.1

03 / Rates and equilibriumBoth tiers

Sodium thiosulfate reacts with hydrochloric acid. Why does a cross drawn under the flask disappear?

  1. AThe cross dissolves in the acid
  2. BA precipitate of sulfur forms
  3. CThe acid evaporates away
  4. DThe solution turns dark blue
Show the answer

Answer: B A precipitate of sulfur forms

The reaction makes a precipitate of sulfur, so the mixture goes cloudy and hides the cross. The shorter the time for the cross to disappear, the faster the reaction. This is a way to follow a reaction by watching it go cloudy.

Checked against Royal Society of Chemistry, The effect of temperature on reaction rate

04 / Rates and equilibriumBoth tiers

How is the mean rate of a reaction worked out?

  1. ATime taken divided by amount of product formed
  2. BAmount of product formed minus time taken
  3. CAmount of product formed times time taken
  4. DAmount of product formed divided by time taken
Show the answer

Answer: D Amount of product formed divided by time taken

Mean rate = quantity of product formed ÷ time taken, or quantity of reactant used ÷ time taken. If the gas volume is in cm³ and the time in seconds, the rate is in cm³/s.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.1

05 / Rates and equilibriumBoth tiers

On a graph of gas volume against time, what does a steeper line show?

  1. AThat the reaction has finished
  2. BA lower temperature
  3. CA faster rate of reaction
  4. DA slower rate of reaction
Show the answer

Answer: C A faster rate of reaction

The steepness (gradient) of the line shows the rate. It is steepest at the start, when there are most reactant particles, and gets less steep as reactants are used up. When the line is flat, a reactant has run out and the reaction has stopped.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.1

06 / Rates and equilibriumBoth tiers

Two experiments make the same final volume of gas, but one line is steeper at the start. What could explain this?

  1. AThe steeper one used larger marble chips
  2. BThe steeper one was at a higher temperature
  3. CThe steeper one was at a lower temperature
  4. DThe steeper one used more dilute acid
Show the answer

Answer: B The steeper one was at a higher temperature

A faster reaction gives a steeper line but, with the same amounts of reactants, the same final amount of product. The final amount only changes if you change the amount of the reactant that runs out: half the magnesium gives half the hydrogen.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.1

07 / Rates and equilibriumBoth tiers

In a disappearing cross experiment, which time shows the fastest reaction?

  1. A150 seconds
  2. B80 seconds
  3. C40 seconds
  4. D20 seconds
Show the answer

Answer: D 20 seconds

The shorter the time a reaction takes, the faster it is: rate is proportional to 1 ÷ time. So halving the time means the rate has doubled.

Checked against OCR Gateway Chemistry A (J248) specification, C5.2b

08 / Rates and equilibriumBoth tiers

Which change would make marble chips react more slowly with acid?

  1. AHeating the acid
  2. BUsing more concentrated acid
  3. CUsing larger marble chips
  4. DUsing powdered marble
Show the answer

Answer: C Using larger marble chips

A reaction goes faster when the temperature is higher, solutions are more concentrated, gases are at a higher pressure, solids are in smaller pieces or a catalyst is added. Doing the opposite slows it down.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.2

09 / Rates and equilibriumBoth tiers

According to collision theory, when do particles react?

  1. AWhen they move apart from each other
  2. BWhen they collide with enough energy
  3. COnly when they are not moving at all
  4. DWhenever they collide, at any energy
Show the answer

Answer: B When they collide with enough energy

Particles can only react when they collide, and only if the collision has at least the activation energy, the minimum energy needed for a reaction. Anything that makes these successful collisions happen more often speeds up the reaction.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.3

10 / Rates and equilibriumBoth tiers

Why does a more concentrated acid react faster?

  1. AParticles collide with more energy
  2. BParticles collide more frequently
  3. CParticles have more energy
  4. DThe activation energy is lower
Show the answer

Answer: B Particles collide more frequently

A higher concentration or pressure means more particles in the same volume. They are closer together, so they collide more often. Their energy does not change, so this is about how frequent the collisions are, not how energetic.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.3

11 / Rates and equilibriumBoth tiers

Why does raising the temperature speed up a reaction?

  1. AThere are more particles in the same volume of solution
  2. BThe activation energy becomes lower
  3. CCollisions are more frequent and more energetic
  4. DThe particles become larger
Show the answer

Answer: C Collisions are more frequent and more energetic

At a higher temperature particles move faster, so they collide more often. More importantly, a bigger fraction of the collisions have at least the activation energy, so more of them lead to a reaction.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.3

12 / Rates and equilibriumBoth tiers

Why does powdered calcium carbonate react faster than lumps?

  1. AIt has a smaller surface area to volume ratio
  2. BIt has a bigger surface area to volume ratio
  3. CIts particles have more energy
  4. DIt has a lower activation energy
Show the answer

Answer: B It has a bigger surface area to volume ratio

Only particles on the surface of a solid can collide with the other reactant. Breaking a solid into smaller pieces exposes more surface for the same volume, so collisions happen more often and the reaction is faster.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.3

13 / Rates and equilibriumBoth tiers

What does a catalyst do?

  1. AChanges which products are made
  2. BSpeeds up a reaction without being used up
  3. CSpeeds up a reaction and is used up as it does
  4. DSlows a reaction down and is used up
Show the answer

Answer: B Speeds up a reaction without being used up

A catalyst increases the rate of a reaction but is not used up: it has the same mass and is chemically unchanged at the end. It does not change the products or how much forms. Different reactions need different catalysts.

Checked against Pearson Edexcel GCSE Chemistry (1CH0) specification, statement 7.6

14 / Rates and equilibriumBoth tiers

A substance speeds up a reaction and is still there, unchanged, at the end. What is it?

  1. AA catalyst
  2. BA precipitate
  3. CA product
  4. DA reactant
Show the answer

Answer: A A catalyst

You can spot a catalyst because the reaction is faster with it and it is still there at the end, unchanged and with the same mass. It is not a reactant or a product, so it is not part of the equation, though it may be written over the arrow.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.4

15 / Rates and equilibriumBoth tiers

How does a catalyst increase the rate of a reaction?

  1. AIt raises the activation energy
  2. BIt gives the particles more energy
  3. CIt makes the particles collide with more energy each time
  4. DIt gives a route with a lower activation energy
Show the answer

Answer: D It gives a route with a lower activation energy

A catalyst provides a different pathway for the reaction with a lower activation energy. More collisions now have enough energy to react, so the rate goes up. The energies of the reactants and products, and so the overall energy change, stay the same.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.4

16 / Rates and equilibriumBoth tiers

What are enzymes?

  1. ATypes of fat
  2. BTypes of sugar
  3. CBiological catalysts
  4. DBiological fuels
Show the answer

Answer: C Biological catalysts

Enzymes are protein molecules made by living things that act as catalysts, speeding up reactions in cells without being used up. Yeast uses its enzymes to ferment sugar into ethanol, which is how alcoholic drinks are made.

Checked against Pearson Edexcel GCSE Chemistry (1CH0) specification, statement 7.8

17 / Rates and equilibriumBoth tiers

What does the symbol ⇌ show in an equation?

  1. AThe reaction is at equilibrium
  2. BThe reaction is very fast
  3. CThe reaction is exothermic
  4. DThe reaction is reversible
Show the answer

Answer: D The reaction is reversible

In a reversible reaction the products can react to give the original reactants again, shown by the ⇌ symbol. Changing the conditions, such as the temperature, can change which way the reaction goes. A catalyst speeds up both directions equally.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.1

18 / Rates and equilibriumBoth tiers

A reversible reaction is exothermic in the forward direction. What is the reverse reaction?

  1. AExothermic, by the same amount
  2. BEndothermic, by the same amount
  3. CEndothermic, by a smaller amount
  4. DEndothermic, by a larger amount
Show the answer

Answer: B Endothermic, by the same amount

If a reversible reaction gives out energy in one direction, it takes in exactly the same amount of energy in the other direction. So an exothermic forward reaction has an endothermic reverse reaction.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.2

19 / Rates and equilibriumBoth tiers

Why can a reversible reaction in a closed system never turn all its reactants into products?

  1. AThe reactant particles never collide at all
  2. BProducts keep escaping from the closed system
  3. CSome products keep turning back into reactants
  4. DThe catalyst is used up partway through
Show the answer

Answer: C Some products keep turning back into reactants

As soon as some products form, they start to react to give back reactants. In a closed system the two reactions balance at equilibrium, so a mixture of reactants and products is always left and the yield is never 100%.

Checked against OCR Twenty First Century Chemistry B (J258) specification, C6.3.1

20 / Rates and equilibriumBoth tiers

What is true when a reversible reaction is at dynamic equilibrium?

  1. AThe amounts of reactants and products are equal
  2. BBoth reactions have stopped
  3. CThe forward and reverse rates are equal
  4. DOnly the forward reaction is happening
Show the answer

Answer: C The forward and reverse rates are equal

At dynamic equilibrium the forward and reverse reactions are still going on, but at exactly the same rate. Products form as fast as they break down, so the amount of each substance stays constant, though the amounts are not usually equal.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.3

21 / Rates and equilibriumBoth tiers

Where can a reversible reaction reach equilibrium?

  1. AOnly when a catalyst is used
  2. BIn a closed system
  3. CWhere a gas can escape
  4. DIn an open beaker
Show the answer

Answer: B In a closed system

Equilibrium needs a closed system, where no reactants or products can get in or out. If a product such as a gas escapes, it cannot react back, so the reaction never settles at equilibrium.

Checked against OCR Gateway Chemistry A (J248) specification, C5.3b

22 / Rates and equilibriumHigher only

A change is made to a system at equilibrium. How does the system respond?

  1. AIt shifts to counteract the change
  2. BIt shifts so as to make the change bigger
  3. CBoth reactions stop
  4. DNothing happens to it
Show the answer

Answer: A It shifts to counteract the change

If the conditions of a system at equilibrium change, the position of equilibrium shifts to oppose the change (Le Chatelier’s principle). The amounts of reactants and products adjust until a new equilibrium is reached.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.4

23 / Rates and equilibriumHigher only

The concentration of a reactant is increased at equilibrium. What happens?

  1. ANothing changes
  2. BThe reaction stops
  3. CMore reactants form
  4. DMore products form
Show the answer

Answer: D More products form

Adding more of a reactant shifts the equilibrium to the right, so more product forms until a new equilibrium is reached. Removing a product has the same effect, which is why industry often removes the product as it is made.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.5

24 / Rates and equilibriumHigher only

The forward reaction is exothermic. What happens to the amount of product if the temperature is raised?

  1. AIt decreases
  2. BIt stays the same
  3. CIt becomes zero
  4. DIt increases
Show the answer

Answer: A It decreases

Raising the temperature shifts the equilibrium in the endothermic direction, which takes in the extra energy. If the forward reaction is exothermic, the reverse is endothermic, so less product forms. Lowering the temperature favours the exothermic direction.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.6

25 / Rates and equilibriumHigher only

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). What does raising the pressure do to the position of equilibrium?

  1. AShifts it towards nitrogen and hydrogen
  2. BShifts it towards ammonia
  3. CStops the forward reaction
  4. DHas no effect on the position
Show the answer

Answer: B Shifts it towards ammonia

Raising the pressure shifts the equilibrium towards the side with fewer gas molecules. In the ammonia reaction there are 4 molecules on the left and 2 on the right, so more ammonia forms. If both sides have the same number of gas molecules, pressure has no effect.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.2.7

26 / Rates and equilibriumHigher only

What effect does a catalyst have on the position of equilibrium?

  1. AIt increases the yield of product
  2. BNone, it speeds up both directions equally
  3. CIt shifts it towards the reactants
  4. DIt shifts it towards the products so more forms
Show the answer

Answer: B None, it speeds up both directions equally

A catalyst lets equilibrium be reached sooner but does not change where it lies. To get the most product, choose conditions that favour the forward reaction, but very high pressures need strong, expensive equipment and bring safety risks.

Checked against OCR Twenty First Century Chemistry B (J258) specification, C6.3.3

27 / Rates and equilibriumBoth tiers

A flask of marble chips and excess acid sits on a balance. How does the reading change as the reaction goes on?

  1. AIt rises quickly, then more slowly, then stays the same
  2. BIt falls quickly, then more slowly, then stays the same
  3. CIt falls at a steady rate until it reaches zero
  4. DIt falls slowly at first, then faster and faster
Show the answer

Answer: B It falls quickly, then more slowly, then stays the same

A graph of product against time is steepest at the start and flattens when a reactant runs out. A faster reaction gives a steeper line, but the same amount of the reactant that runs out gives the same final amount. A mass loss graph has the same shape, upside down.

Checked against AQA GCSE Chemistry (8462) specification, section 4.6.1.1

02 / Longer answersMarked by levels

6-mark questions on this topic

Write your answer first, on paper. Then open it and tick off the points you made. How 6-mark questions are marked.

6 marks / Collision theory and rateBoth tiers

Calcium carbonate reacts with dilute hydrochloric acid. A student finds that powdered calcium carbonate reacts faster than lumps, and that the reaction is faster in warm acid than in cold acid.

Explain these results using collision theory.

What a full answer includes
  • Particles react only when they collide with at least the activation energy.
  • Powder has a larger surface area to volume ratio than lumps of the same mass.
  • So more particles of calcium carbonate are exposed to the acid.
  • This gives more frequent collisions, so the rate increases.
  • At a higher temperature the particles move faster, so they collide more often.
  • They also have more energy, so a greater proportion of collisions have at least the activation energy.
  • So there are more successful collisions per second and the rate increases.

Model answer

According to collision theory, particles can only react when they collide, and the collision must have at least a certain minimum amount of energy, called the activation energy. The rate of reaction depends on how many successful collisions happen each second. When the calcium carbonate is powdered, it has a much larger surface area to volume ratio than the same mass in lumps. More of its particles are on the surface, exposed to the acid particles, so collisions happen more frequently and the reaction is faster. When the acid is warmed, the particles gain energy and move faster. This means they collide more often. More importantly, a greater proportion of the particles now have energy equal to or greater than the activation energy, so a larger fraction of the collisions are successful. Both effects mean there are more successful collisions per second, so the rate of reaction increases at the higher temperature.

Where marks slip

Saying particles collide “more” is not enough; say more frequent collisions, or more collisions per second. For temperature, give both reasons: more frequent collisions and more collisions with enough energy.

6 marks / Changing conditions in ammonia productionHigher only

Ammonia is made in a reversible reaction that reaches equilibrium: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic.

Explain how increasing the pressure and increasing the temperature would each affect the yield of ammonia at equilibrium and the rate of the reaction.

What a full answer includes
  • There are 4 molecules of gas on the left and 2 on the right.
  • Increasing the pressure moves the position of equilibrium to the side with fewer gas molecules, the right.
  • So the yield of ammonia increases at higher pressure.
  • Higher pressure also increases the rate because the gas particles are closer together and collide more often.
  • The forward reaction is exothermic, so increasing the temperature moves the equilibrium in the endothermic direction, to the left.
  • So the yield of ammonia decreases at higher temperature.
  • But a higher temperature increases the rate, because particles collide more often and with more energy.
  • So a compromise temperature is used: fast enough, with an acceptable yield.

Model answer

The system responds to a change in conditions by counteracting it. In the equation there are four molecules of gas on the left (one nitrogen and three hydrogen) and only two molecules of ammonia on the right. If the pressure is increased, the position of equilibrium moves towards the side with fewer gas molecules, which reduces the pressure, so it shifts to the right and the yield of ammonia increases. A higher pressure also increases the rate, because the gas particles are closer together and collide more frequently. The forward reaction is exothermic, so the reverse reaction is endothermic. If the temperature is increased, the equilibrium moves in the endothermic direction to take in the extra energy, so it shifts to the left and the yield of ammonia decreases. However, a higher temperature makes the rate faster, because the particles move faster, collide more often and more of the collisions have at least the activation energy. This means there is a conflict for temperature, so a compromise temperature is chosen that gives a reasonable rate with an acceptable yield.

Where marks slip

Pupils often say a higher temperature increases the yield because it speeds up the reaction. Rate and equilibrium position are different things: for an exothermic forward reaction, heating lowers the yield.

03 / In the appComing to Show Working

All 34 rates and equilibrium facts in Show Working

In the app, choose your exam board and course and you only see what your board asks for. Tap through questions, or open Read the answers to see every answer in the topic at once, covered until you tap. Science is coming to the app soon.

Coming soon to the App Store

More GCSE chemistry topics

Atoms and the periodic tableBonding, structure and propertiesChemical changesEnergy changesOrganic chemistryChemical analysisChemical and allied industriesEarth and the atmosphereRequired practicalsAll of GCSE Chemistry

Our own questions, written for every board. Show Working is independent and not affiliated with any exam board.

Show WorkingComing soon to the App Store