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Rate of Reactions

10. Rate of Reactions

  • SEC G3 Combined Science Chemistry component 2027
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Learning objectives

  • describe the effect of concentration, pressure, particle size and temperature on the rates of reactions and explain these effects in terms of collisions between reacting particles
  • interpret data obtained from experiments concerned with rate of reaction.

A reaction rate tells you how quickly a measurable quantity changes. This lesson keeps three ideas connected: what you observe, what the graph shows, and how particle collisions explain the pattern.

1. Definition

The rate of reaction is the change in the amount of a reactant used up or product formed per unit time.

average rate = (change in measured quantity)/(time taken)

The unit depends on the measured quantity. Examples include cm³ s⁻¹ for gas volume and g s⁻¹ for mass loss.

2. Key Ideas

EvidenceRate meaningParticle meaning
Steeper volume–time curveMore gas forms per unit timeReacting particles collide more frequently
Steeper downward mass–time curveGas escapes more quicklyReactant is converted to gaseous product more quickly
Same final volume, reached soonerFaster rate, same final amountThe starting conditions changed collision frequency, not the limiting amount
Curve becomes horizontalRate is zeroA reactant has been used up or the reaction has effectively stopped
Keep rate separate from final amount

The gradient describes rate. The final height describes total gas produced. A steeper curve can finish at the same height.

3. Detailed Explanations

Measuring a reaction rate

  • Gas syringe: record gas volume at regular time intervals.
  • Mass loss: record the decrease in mass as a gaseous product escapes. Use cotton wool to reduce spray while allowing gas to leave.
  • Fixed visual change: time how long a marked cross takes to disappear as a precipitate forms. Compare only experiments with the same end-point.
Gas-syringe method for measuring reaction rateA conical flask containing marble chips and dilute hydrochloric acid is sealed with a bung and connected by a delivery tube to a graduated gas syringe. Carbon dioxide collected is recorded at regular time intervals.marble chips + dilute HClgraduated gas syringerecord CO₂ volume / cm³ at regular timesairtight delivery tube
A gas syringe gives a series of gas-volume readings, not just one finishing time. Check the bung and delivery tube for leaks before comparing rate curves.

Same final amount, different reaction rates

Two gas-volume against time curves. The faster reaction has the steeper initial gradient and reaches the same final volume earlier.

Scroll across the graph to read all labels.

Two gas-volume against time curves. The faster reaction has the steeper initial gradient and reaches the same final volume earlier.Two gas-volume against time curves. The faster reaction has the steeper initial gradient and reaches the same final volume earlier.
Both reactions produce 40 cm³ of gas. The steeper curve represents the faster rate.
Open full-size graph
View figure data
Values for Same final amount, different reaction rates
Time (s)FasterSlower
000
10167
202814
303621
404028
504034
604040

The four prescribed factors

  1. Higher concentration: there are more reactant particles per unit volume, so collisions occur more frequently and rate increases.
  2. Higher pressure for reacting gases: the gas particles are closer together, so collisions occur more frequently and rate increases.
  3. Smaller solid particles: the same mass has a larger total surface area, so collisions at the solid surface occur more frequently and rate increases.
  4. Higher temperature: particles move faster, so collisions occur more frequently and rate increases.
Course stopping point

For this G3 Science topic, use particle motion and collisions between reacting particles. Catalyst definitions, activation energy, alternative pathways and collision orientation belong to G3 Chemistry.

Planning a fair investigation

Change one factor. Keep the amounts of reactants and all other rate factors constant. Measure the same quantity at regular times, repeat each condition, identify anomalies and calculate a mean from justified valid results.

4. Common Mistakes

  • Saying a larger volume of acid means a higher concentration. Concentration is particles per unit volume.
  • Saying pressure affects a reaction between solutions. Pressure is a rate factor here only when reacting particles are gases.
  • Saying smaller solid particles have less mass when the experiment uses the same total mass.
  • Saying the faster experiment makes more product without checking the final graph value.
  • Comparing times when the measured end-points are different.

5. Exam Tips

Build the factor explanation

State what changes at particle level, state that collisions occur more frequently per unit time, then link this to the higher reaction rate.

Exam question 1: Control a concentration investigationCore

A student changes acid concentration and measures hydrogen volume every 10 s. Name three variables that should be controlled.

Show Answer

Keep the volume of acid, mass and surface area of the magnesium, and temperature constant. Use the same apparatus and timing method in each run.

6. Worked Examples

Modelled example 1

Calculate an average rate

Core

Problem

A reaction produces 36 cm³ of gas in 45 s. Calculate the average rate.
Study the worked solution

  1. Identify change and time

    Method

    Use the 36 cm³ product increase over 45 s.

    Reason

    Reaction rate compares a measurable change with its elapsed time.

    Working

    Change = 36 cm³; time = 45 s.

  2. Calculate and report

    Method

    Divide gas volume by time.

    Reason

    The average is taken across the full stated interval.

    Working

    36/45 = 0.80 cm³ s⁻¹.

Guided practice 2

Plan a concentration comparison

About 6 min

Problem

A student investigates how hydrochloric-acid concentration affects its reaction with equal masses of marble chips. Choose a suitable measurement and identify two important controls.

Change concentration only

Measurement
Marble mass
Marble surface area

Hints

Hint 1: product
The reaction produces carbon dioxide.
Hint 2: fair test
Keep every rate factor except concentration unchanged.
View solution step by step

  1. Choose the evidence

    Method

    Record carbon-dioxide volume at fixed time intervals using a gas syringe.

    Reason

    The readings produce a volume–time curve whose gradient represents rate.

    Working

    gas volume / cm³ against time / s

  2. Control the comparison

    Method

    Keep marble mass, marble surface area, acid volume and temperature constant.

    Reason

    Each could otherwise change rate or final gas amount.

    Working

    change concentration only; hold all other rate factors constant

Common misconception 3

Compare two curves

Find and correct the mistake

Learner claim

Curve A reaches 50 cm³ in 40 s and Curve B reaches 50 cm³ in 90 s. A student says, “A is faster, so it must make more gas.” Diagnose the claim and compare both rate and final amount.

Read gradient and final height separately

Faster curve
Final gas amount

View solution step by step

  1. Compare rates

    Method

    Choose Curve A as faster.

    Reason

    It reaches the same fixed gas volume in less time and therefore has the steeper gradient.

    Working

    A reaches 50 cm³ in 40 s; B takes 90 s.

  2. Compare final amounts

    Method

    State that both produce 50 cm³ in total.

    Reason

    The final graph height represents accumulated product, not how quickly it formed.

    Working

    Same final amount; different rates.

Examiner practice 4

Explain the temperature effect

4 marks

Examination question

A reaction is faster at 40 °C than at 20 °C. Explain this difference using the G3 Science particle model. [4 marks]

Build a linked particle explanation

View solution step by step

  1. Change particle motion

    1 mark

    Method

    State that reacting particles move faster at the higher temperature.

    Reason

    Heating increases their motion.

    Working

    higher temperature → faster-moving particles

  2. Change encounters

    1 mark

    Method

    State that particles meet more often.

    Reason

    Faster motion produces more collisions in a given time.

    Working

    faster motion → more collisions

  3. Use the time phrase

    1 mark

    Method

    State that collisions occur more frequently per unit time.

    Reason

    Rate compares change with time, so the time link matters.

    Working

    collision frequency per second increases

  4. Reach the rate

    1 mark

    Method

    Conclude that reactant is used or product is formed more quickly.

    Reason

    More frequent reacting-particle collisions increase the measured rate.

    Working

    more product per unit time → higher rate

Challenge 5

Explain a surface-area effect

Minimal support

Factor transfer

Explain why 5.0 g of powdered marble reacts faster than 5.0 g of large marble chips with the same acid. Keep the explanation at G3 Science particle-collision depth.

Build the surface-to-rate chain

Powder total surface area
Acid–marble collisions per unit time

Hints

Hint 1: same mass
The amount of marble is controlled; focus on how much solid is exposed.
View solution step by step

  1. Compare exposed marble

    Method

    Give the powdered sample a larger total surface area.

    Reason

    Breaking the same 5.0 g mass into smaller particles exposes more marble particles.

    Working

    Powder has more accessible surface.

  2. Connect exposure to rate

    Method

    Increase collisions at the marble surface per unit time.

    Reason

    More exposed sites allow acid particles to meet marble particles more frequently.

    Working

    More frequent collisions per unit time, so the reaction rate is higher.

7. Mind Stretchers

Mind stretcher 1: Separate rate from final amountExtension

Two experiments reach the same final gas volume, but one curve is initially twice as steep. What can and cannot be concluded?

Show Answer

The steeper curve shows a higher initial rate. The matching final volumes show the same final amount of gas. The graph alone does not identify which rate factor was changed.

Mind stretcher 2: Evaluate an anomalous repeatExtension

Times for a cross to disappear are 42 s, 43 s and 71 s. Identify the anomaly and state the next step.

Show Answer

71 s is anomalous because it is far from the two concordant values. Check whether the quantities, temperature, mixing and viewing end-point were controlled, repeat the condition, then calculate a mean from justified valid results.

Mind stretcher 3: Keep the explanation at course depthExtension

A student writes, “Heating increases concentration.” Correct the explanation using this course’s particle model.

Show Answer

Heating does not increase concentration. At higher temperature, particles move faster and collide more frequently per unit time, so the reaction rate increases.

8. Quiz

Check 1: Check your understandingCore

Check rate units, graphs, fair tests and collision explanations, then practise whichever part felt least secure.

Check your understanding

Practise reaction rates or try a mixed set.