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Factors Affecting Rate of Reaction

Factors affecting rate: concentration, pressure, surface area, temperature and catalysts—explained using collision-theory keywords for full marks.

  • SEC G3 Pure Chemistry 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
  • define the term catalyst and describe the effect of catalysts (including enzymes) on the rates of reactions
  • explain how pathways with lower activation energies account for the increase in rates of reactions (see also 9(b))
  • state that some compounds act as catalysts in a range of industrial processes and that enzymes are biological catalysts (see also 8.3(b), 9(b), 10(c) and 12(d))

This lesson builds on effective collisions and activation energy. Revise Collision Theory first if those ideas are unfamiliar.

1. Definition

Factors affecting rate of reaction are conditions that change how fast reactants are used up or products are formed by changing the number of effective collisions per unit time.

2. Key Ideas

  • Factors that increase rate do one (or both) of these:
    • increase collision frequency (more collisions per unit time), and/or
    • increase the proportion of collisions that are effective (energy ≥ Eₐ).
  • These are the syllabus factors:
    • Concentration (solutions) and pressure (gases): more particles per unit volume → more collisions per unit time.
    • Particle size (solids): smaller particles give a larger total surface area → more collisions at the surface per unit time.
    • Temperature: particles move faster and more particles have energy ≥ Eₐ.
    • Catalyst: lowers Eₐ (alternative pathway) → more collisions are effective.
Factor increasedApplies toWhat changes?Explanation to include
Concentrationsolutionsparticles per unit volume ↑“more frequent collisions per unit time” → “more frequent effective collisions per unit time”
Pressure (at constant T)gasesparticles per unit volume ↑“particles closer together” → “more collisions per unit time”
Smaller particle sizesolidstotal surface area and exposed particles ↑“larger total surface area” → “more collisions at the surface per unit time”
Temperaturemost reactionsspeed ↑; fraction with energy ≥ Eₐ ↑“move faster”, “more collisions per unit time”, “larger proportion have energy ≥ Eₐ”
Catalystreactions with catalystEₐ ↓“alternative pathway”, “lower Eₐ”, “higher proportion of effective collisions”

3. Detailed Explanations

Quick Recall (collision theory keywords)
  • Rate increases when there are more frequent effective collisions per unit time.
  • Effective collision needs energy ≥ Eₐ (and correct orientation for molecules).
  • Concentration/pressure/surface area mainly increase collision frequency.
  • Temperature increases collision frequency and increases the fraction with energy ≥ Eₐ.
  • A catalyst provides an alternative pathway with lower Eₐ (and is not used up).

A. Concentration (solutions)

Increasing concentration (more solute particles in the same volume of solution) increases rate because:

  • there are more reactant particles per unit volume,
  • particles are closer together,
  • so there are more frequent collisions per unit time,
  • so there are more frequent effective collisions per unit time (faster rate).
Do not write “more concentrated” without the particle idea

“More concentrated” is not the explanation. The explanation is “more reactant particles per unit volume → more collisions per unit time.”

Concentration vs Rate (Gas Volume vs Time)

Two volume-of-gas vs time curves showing higher concentration gives a steeper initial gradient and finishes sooner while reaching the same final volume.

Scroll across the graph to read all labels.

Two volume-of-gas vs time curves showing higher concentration gives a steeper initial gradient and finishes sooner while reaching the same final volume.Two volume-of-gas vs time curves showing higher concentration gives a steeper initial gradient and finishes sooner while reaching the same final volume.
Same final volume (same limiting reactant), but higher concentration gives a steeper initial gradient so the reaction finishes sooner.
Open full-size graph
View figure data
Values for Concentration vs Rate (Gas Volume vs Time)
Time (s)Lower concentrationHigher concentration
000
10612
201222
301829
402434
502937
603339
703640
803840
904040

B. Pressure (gases)

A change in pressure only affects rate if the reacting particles are gases.

Increasing gas pressure (at constant temperature) increases rate because:

  • gas particles are forced closer together (same number of particles in a smaller volume),
  • so there are more collisions per unit time,
  • so there are more frequent effective collisions per unit time.
Industry link (don’t overclaim)

In the Haber process, higher pressure increases rate because gas collisions become more frequent. If supplied data also show a yield change, describe that as a separate trend rather than treating rate and yield as the same quantity. See: Haber Process and Reversible Reactions.

C. Surface area (solids)

For reactions involving a solid, the syllabus factor is particle size; the collision-theory explanation uses the resulting change in total surface area.

Decreasing particle size (e.g., powdering a solid) increases rate because:

  • total surface area increases,
  • more solid particles are exposed to the other reactant,
  • so there are more collisions at the surface per unit time,
  • so there are more frequent effective collisions per unit time.
Keyword trap

Write “larger surface area” and “more collisions at the surface per unit time”. Writing only “smaller particles” is a half-answer.

D. Temperature

For most reactions, increasing temperature increases rate because:

  • particles have more kinetic energy (energy of motion) and move faster,
  • so there are more collisions per unit time, and
  • a larger proportion of particles have energy ≥ Eₐ, so a larger proportion of collisions are effective,
  • so there are more frequent effective collisions per unit time.
Temperature explanation is two-part

Full marks needs both:

  • collision frequency increases, and
  • a larger fraction has energy ≥ Eₐ.

E. Catalysts

A catalyst is a substance that increases the rate of reaction and is chemically unchanged at the end of the reaction.

A catalyst increases rate by providing an alternative pathway with a lower activation energy, Eₐ.

Exothermic energy profileExothermic energy profile. Reactants are at higher energy than products, so the enthalpy change is negative. The activation energy is measured from the reactant energy level to the peak. A dashed catalysed pathway has a lower peak but the same reactant and product energy levels.EnergyProgress of reactionReactantsProductsEₐΔH < 0catalysed pathwayuncatalysed pathway
A catalyst provides an alternative pathway with lower activation energy. Reactant and product energy levels—and therefore ΔH—do not change.

So:

  • Eₐ is lower,
  • so a larger proportion of particles have energy ≥ Eₐ,
  • so a larger proportion of collisions are effective,
  • so there are more frequent effective collisions per unit time (faster rate).
Catalyst vs temperature (do not mix)

Temperature increases particle energy.
A catalyst does not “give energy” — it lowers Eₐ.

Syllabus link: catalysts (industrial + enzymes)

The syllabus also expects you to know that some compounds act as industrial catalysts, such as iron in the Haber process, and that enzymes are biological catalysts.

Observation, particle model, energy profile

A faster visible change is the macroscopic observation. More frequent effective collisions are the particle-level explanation. A lower peak on the catalysed pathway is the symbolic energy-profile representation. Do not claim that a catalyst changes Δ H or the final amount of product.

4. Common Mistakes

  • Writing “rate increases because concentration/pressure is higher” with no particle explanation.
  • Saying pressure affects all reactions (false): it affects rate only when reacting particles are gases.
  • Using “particle size” without linking it to surface area and collisions at the surface.
  • Explaining temperature using only “more collisions” and forgetting “larger proportion have energy ≥ Eₐ”.
  • Saying “a catalyst is used up” or “a catalyst gives particles energy” (both false).
  • Confusing “faster rate” with “more products”. Rate describes how quickly change occurs; any yield conclusion must come from the reaction information or supplied data.

5. Exam Tips

3-mark explanation template (copy this)
  1. State the particle change: “more particles per unit volume / larger surface area / particles move faster / Eₐ is lower”.
  2. State collision result: “more collisions per unit time”.
  3. Finish: “more frequent effective collisions per unit time, so rate increases”.
  • If the question asks for a “fair test”, only change one variable (e.g., same volume and concentration of acid, same mass of solid, same temperature).
  • If the factor is temperature or catalyst, you must mention Eₐ somewhere for full marks.

6. Worked Examples

Modelled example 1

Pressure (gases only)

Core

Problem

Explain why increasing pressure increases the rate of reaction between N₂(g) and H₂(g) at constant temperature.
Study the worked solution

  1. State the particle change

    Method

    Bring the gas particles closer together.

    Reason

    At constant temperature, increasing pressure means the same gas particles occupy a smaller volume.

    Working

    More reacting particles per unit volume.

  2. Infer the collision change

    Method

    Increase collisions per unit time.

    Reason

    Closer gas particles encounter one another more frequently.

    Working

    Higher collision frequency.

  3. Conclude with effective collisions

    Method

    State that effective collisions occur more frequently.

    Reason

    A greater number of collisions per second gives more successful collisions per second under otherwise unchanged conditions.

    Working

    More frequent effective collisions per unit time, so rate increases.

Guided practice 2

Surface area (marble + acid)

About 5 min

Problem

Equal masses of large marble chips and powdered CaCO₃(s) react with identical dilute HCl(aq) portions. Explain why the powder reacts faster.

Complete the solid-interface chain

Total surface area
Exposed CaCO3 particles
Surface collisions per unit time

Hints

Hint 1: particle size to area
Smaller particles increase total surface area without changing the marble mass.
Hint 2: interface
Acid particles react only where they can reach exposed marble particles.
View solution step by step

  1. Translate particle size to surface area

    Method

    Give powder the larger total surface area.

    Reason

    Dividing the same mass into smaller particles exposes more solid.

    Working

    More CaCO₃ particles are exposed.

  2. Translate area to rate

    Method

    Increase collisions at the surface per unit time.

    Reason

    More exposed sites allow more frequent effective acid–marble collisions.

    Working

    More frequent effective collisions per unit time, so powder reacts faster.

Common misconception 3

Concentration vs volume (spot the mistake)

Find and correct the mistake

Learner claim

A student says, “Using 50 cm³ of acid instead of 25 cm³ makes the reaction faster because concentration is higher.” Explain the error and state how to test a concentration effect.

Separate amount of solution from particles per volume

Doubling volume alone
Valid concentration change

View solution step by step

  1. Identify the variable error

    Method

    Reject volume as proof of higher concentration.

    Reason

    Concentration is the number of solute particles per unit volume, not the total volume present.

    Working

    50 cm³ of a given solution has the same concentration as 25 cm³ of that solution.

  2. State the correct concentration mechanism

    Method

    Use acid with more reactant particles per unit volume.

    Reason

    Closer, more numerous acid particles collide with the other reactant more frequently.

    Working

    Higher concentration → more frequent effective collisions per unit time → faster rate.

Examiner practice 4

Temperature (full marks)

4 marks

Examination question

Explain, using collision theory, why increasing temperature increases the rate of reaction. [4 marks]

Write the complete causal chain

View solution step by step

  1. Kinetic energy and speed

    1 mark

    Method

    State that particles gain kinetic energy and move faster.

    Reason

    Temperature measures the average kinetic energy of the particle population.

    Working

    Higher temperature → faster particle motion.

  2. Collision frequency

    1 mark

    Method

    Increase collisions per unit time.

    Reason

    Faster-moving particles meet more often.

    Working

    Collision frequency increases.

  3. Activation threshold

    1 mark

    Method

    Increase the proportion with energy at least Eₐ.

    Reason

    A larger fraction of collisions can overcome the activation-energy barrier.

    Working

    Larger proportion with energy ≥ Eₐ.

  4. Rate conclusion

    1 mark

    Working

    More frequent effective collisions per unit time, so rate increases.

Challenge 5

Catalyst (what changes, what doesn’t)

Minimal support

Mechanism transfer

A student says, “A catalyst increases rate because it increases collision frequency and gives particles more energy.” Correct both claims using mark-scheme wording, and state what remains unchanged on an energy profile.

Distinguish pathway from particle energy

Catalysed pathway
Reactant particle energy at same temperature
Reactant/product levels and ΔH

Hints

Hint 1: change the barrier
A catalyst changes the route available between the same reactants and products.
Hint 2: compare with temperature
Temperature changes the particle-energy distribution; a catalyst does not.
View solution step by step

  1. Change the reaction pathway

    Method

    Provide an alternative pathway with lower activation energy.

    Reason

    A lower threshold lets a larger proportion of the existing collisions have enough energy.

    Working

    Lower Eₐ → larger effective-collision proportion.

  2. Reject extra particle energy

    Method

    Keep particle kinetic energies unchanged at the same temperature.

    Reason

    The catalyst changes the pathway, not the temperature.

    Working

    It does not necessarily raise total collision frequency or give particles energy.

  3. Keep the energy difference stable

    Method

    Leave reactant level, product level and Δ H unchanged.

    Reason

    Only the activation-energy peak is lowered; the reaction endpoints are identical.

    Working

    More frequent effective collisions per unit time, so rate increases without changing Δ H.

7. Mind Stretchers

Mind stretcher 1: Same concentration, different ratesExtension

Question: Two reactions use solutions at the same concentration and temperature, but Reaction A is much slower. Give two possible reasons (collision theory) without changing concentration or temperature.

Show Answer

Possible answers (any two, explained):

  • Reaction A has a higher activation energy, so a smaller fraction of particles have energy ≥ Eₐ → fewer effective collisions.
  • Reaction A has no catalyst (Reaction B has a catalyst), so Eₐ is higher → fewer effective collisions.
  • Reaction A involves a solid with smaller surface area (large lumps instead of powder), so fewer collisions occur at the surface per unit time.

Mind stretcher 2: Choose the correct explanation sentenceExtension

Question: Pick the best final sentence for a pressure question: (A) “More collisions occur.” (B) “There are more frequent effective collisions per unit time.” Why?

Show Answer

(B) is better because it uses the mark-scheme target: rate depends on effective collisions, not just total collisions. You should still include the particle reason first (particles closer together → collision frequency increases).

8. Quiz

Practise this lesson

The shared K324 / 6092 practice includes concentration, pressure, surface area, temperature, catalysts and experimental design.

K324 / 6092 Practice