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K326 / K328 / 2027
Rate of Reactions

Full chapter

Rate of Reactions

Explain why reactions speed up, and read what a progress graph actually proves.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

Explain a rate change using collisions

Changing a condition changes how often particles react.

Make a guess. You are not marked.

Marble chips react with excess dilute hydrochloric acid. The experiment is repeated with the same mass of marble as a fine powder. What changes?

  1. No change in rate or total gas
  2. Faster reaction, more gas in total
  3. Faster reaction, same total volume of gas
Show the answer

Faster reaction, same total volume of gas

Powder exposes more surface, so acid particles collide with it more often. The mass of marble is unchanged, so the total carbon dioxide is unchanged.

Reaction rate measures how quickly a reactant is used up or a product forms. Reacting particles must collide, and collisions must have enough energy to lead to reaction. An explanation should connect the changed condition to particle behaviour, then to the number of successful collisions per unit time.

Four factors and their explanations
ChangeParticle explanationImportant control
Higher solution concentrationMore reacting particles per unit volume; more frequent collisionsKeep total solution volume and temperature controlled
Higher gas pressure at fixed temperatureGas particles are closer together; more collisions per unit volume per secondApplies to gaseous reactants, not simply pressing on a solid
Smaller solid pieces, same total massGreater exposed surface area offers more places for collisionsGrinding does not create more total moles of solid
Higher temperatureParticles gain kinetic energy and move faster. A greater proportion of particles have energy equal to or greater than the activation energy (the minimum energy needed to react), so the frequency of effective collisions increasesDo not explain temperature solely as an increase in concentration

A larger lump and equal-mass powder can produce the same final amount of gas while reacting at different speeds. In contrast, adding more of a limiting reactant can change the final amount. Keep the question "how fast?" separate from "how much?".

Check your understanding

Why does powdered calcium carbonate react faster than the same mass of chips in excess acid?

  1. The powder has more particles of carbonate, so it makes more carbon dioxide in total.
  2. The powder has lower activation energy, because grinding weakens the bonds in it.
  3. The powder has more surface area, so there are more collisions with acid each second.
Show the answer

The powder has more surface area, so there are more collisions with acid each second.

Acid particles can only hit the exposed surface. Powder exposes far more surface, so effective collisions are more frequent. The final volume of CO2 is the same, because the mass of carbonate is the same.

Exam-style question

Magnesium ribbon reacts with dilute hydrochloric acid. The experiment is repeated with more concentrated acid, keeping everything else the same. Explain, in terms of particles, why the reaction is faster. [2]

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Exam-style question

Calcium carbonate chips react with dilute hydrochloric acid. The experiment is repeated at a higher temperature, keeping everything else the same. Explain, in terms of particles, why the reaction is faster. [3]

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02

Choose a changing quantity

Match the measurement to the reaction.

Ways to follow progress
MeasurementSuitable situationRate evidence
Gas volume against timeGas-producing reaction; gas can be collectedVolume increase per unit time
Mass against timeA gas escapes from an open reaction vesselMass loss per unit time
Time to a fixed visible endpointA precipitate obscures a mark or a colour reaches a chosen endpointShorter time means a faster average approach to that same endpoint

For a gas syringe, check airtight connections and free plunger movement. For mass loss, prevent splashes so that lost liquid is not mistaken for escaped gas. Start timing consistently. A subjective colour or visibility endpoint is less precise than a clear instrumental measurement, so use the same criterion and repeat.

Worked exampleCalculate an average rate

A reaction produces 36 cm3 of gas in its first 40 s.

Try it first. Then show the worked stepsHide the worked steps
  1. Average rate = change in gas volume / time interval.
  2. 36/40 = 0.90 cm3 s-1.
  3. This is an average over 40 s; the rate may have fallen throughout that interval.
Answer

0.90 cm3 s-1. State both interval and units.

Try it step by step

Work it out in steps: Change in the measured quantity, then Time interval, then Average rate.

  1. Problem 1. A gas syringe reads 12.0 cm3 at 20 s and 42.0 cm3 at 80 s. Find the average rate between 20 s and 80 s, in cm3 s-1 to 3 significant figures.
  2. Problem 2. A flask on a balance loses carbon dioxide. Its mass is 120.84 g at 0 s and 120.24 g at 120 s. Find the average rate of mass loss, in g s-1 to 2 significant figures (the mass lost, 0.60 g, has only 2).
  3. Problem 3. A gas syringe reads 15.0 cm3 at 10 s and 39.0 cm3 at 50 s. Find the change in volume, the time interval and the average rate. Give the rate in cm3 s-1 to 3 significant figures.
Show the answer

Problem 1.

Change in the measured quantity: Change in volume = 42.0 - 12.0 = 30.0 cm3.

Time interval: Time interval = 80 - 20 = 60 s.

Average rate: Average rate = 30.0 / 60 = 0.500 cm3 s-1 over that interval.

Problem 2.

Change in the measured quantity: Mass lost = 120.84 - 120.24 = 0.60 g.

Time interval: Time interval = 120 - 0 = 120 s.

Average rate: Average rate = 0.60 / 120 = 0.0050 g s-1. Two significant figures, because 0.60 g has two.

Problem 3.

Change in the measured quantity: Change in volume = 39.0 - 15.0 = 24.0 cm3.

Time interval: Time interval = 50 - 10 = 40 s.

Average rate: Average rate = 24.0 / 40 = 0.600 cm3 s-1.

Check your understanding

Two experiments reach the same end-point, one in 20 s and one in 40 s. Which is faster?

  1. Neither, because both reached the very same end-point.
  2. The 40 s one, because it ran for longer and so reacted more.
  3. The 20 s one, because the same change happened in less time.
Show the answer

The 20 s one, because the same change happened in less time.

For the same amount of change, rate is proportional to 1/time. 1/20 is twice 1/40, so the 20 s experiment is twice as fast.

03

Read slope and final amount separately

A plateau means product formation has stopped, not that nothing happened.

Make a guess. You are not marked.

Magnesium ribbon (in excess) reacts with 50 cm3 of dilute hydrochloric acid, and the gas volume is plotted against time. The experiment is repeated with 50 cm3 of acid at half the concentration. How does the new curve compare?

  1. Just as steep, but it levels off at half the volume
  2. Less steep, and it levels off at half the volume
  3. Less steep at the start, but the same final volume
Show the answer

Less steep, and it levels off at half the volume

Fewer acid particles per cm3 means fewer collisions per second. The acid is the limiting reactant and there are half as many moles of it, so half as much gas forms.

Same yield, different rates

The faster experiment reaches 44 cubic centimetres earlier; both finish at the same volume.

Illustrative results: equal amounts of magnesium, acid in excess, same temperature; concentration differs.

The gradient of a product-volume graph gives rate. A steeper initial slope means faster initial reaction. The curve becomes less steep as reactants are consumed. A horizontal line means no further gas is collected; a reactant may have been exhausted. A tangent estimates the instantaneous rate at a point; a line joining two measured points gives the average over that interval.

For a mass-loss graph, the mass decreases, so the slope is negative. Use the magnitude of the decrease per time for a positive reaction-rate value. Compare final gas volumes only at the same temperature and pressure. A leak can reduce apparent volume without changing the actual chemical yield.

Worked exampleDiagnose a changed curve

A repeat reaches its plateau sooner but at the same final volume. What can be concluded?

Try it first. Then show the worked stepsHide the worked steps
  1. It formed the measured product more quickly.
  2. The same final amount formed under the stated gas conditions.
  3. A rate factor such as temperature, concentration or surface area may explain it; the curve alone does not identify which.
Answer

Faster rate, same final amount. Do not claim that a catalyst or temperature change is uniquely proved.

Check your understanding

A lower final volume of gas is recorded after a bung leaks. Does that prove less product formed?

  1. No. Some gas probably escaped before it could be collected.
  2. No. A leak lets in extra air, which adds to the gas volume collected.
  3. Yes. The final volume measures the amount of product made.
Show the answer

No. Some gas probably escaped before it could be collected.

A leak changes the measurement, not the chemistry. Separate apparatus faults from chemical explanations before drawing a conclusion.

Exam-style question

Magnesium ribbon reacts with excess dilute hydrochloric acid. The volume of hydrogen is recorded every 30 s.
Time / s: 0, 30, 60, 90, 120, 150, 180
Volume / cm3: 0, 20, 34, 43, 48, 50, 50
(a) Plot the results and draw a suitable line. [2]
(b) Use your graph to find the volume at 45 s. [1]
(c) State the time at which the reaction stopped, and explain why it stopped. [1]
(d) The experiment is repeated with the same mass of magnesium and acid that is twice as concentrated, still in excess. Sketch the new curve on the same axes and explain its shape. [2] [6]

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Check your understanding

Excess zinc reacts with 50 cm3 of dilute sulfuric acid, so the acid is limiting. It is repeated with 50 cm3 of acid of half the concentration. How does the new volume-time curve compare?

  1. It starts just as steeply and levels off at the same volume
  2. It starts less steeply and levels off at the same volume
  3. It starts less steeply and levels off at half the volume
  4. It starts more steeply and levels off at half the volume
Show the answer

It starts less steeply and levels off at half the volume

Fewer acid particles per unit volume means fewer effective collisions, so the starting gradient is lower. Half the concentration in the same volume is half the moles of the limiting acid, so half the hydrogen forms.

Quick revision

Revisit the essentials, then return to an explanation when you need it.

Exam definitions: learn these in mark-scheme words
TermDefinition to write
Rate of reactionThe change in the amount (or concentration) of a reactant or product per unit time.
Effective collisionA collision between reacting particles that have energy equal to or greater than the activation energy, so that they react.

Rate describes change per unit time. Concentration, pressure, temperature and exposed surface area affect successful collisions. On a progress graph, slope indicates rate and the final level indicates measured amount.

Scope

Learning outcomes

10. Rate of Reactions (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 10(a) Explain rate factors

    • Concentration
    • Gas pressure
    • Particle size/surface area
    • Temperature
    • Collisions between reacting particles

    Explain a rate change using collisions

  2. 10(b) Interpret rate data

    • Volume/mass/time measurements
    • Gradient and plateau
    • Measurement limitations

    Choose a changing quantityRead slope and final amount separately