Factors Affecting Reaction Rate

Last updated: 17 August 2026

Diagnose → Learn → Master: Every study-note section includes 15 free MCQs — start with 5 Diagnostic MCQs to identify gaps in your understanding, then complete 10 Mastery MCQs near the end to check what you have learned.

A rate equation tells us mathematically how the rate of a reaction depends on concentration, but it does not by itself explain why changing the reaction conditions changes the rate. For that, we need to think about what the reacting particles are actually doing.

Concentration affects how often particles meet. Temperature affects both how frequently they collide and, much more importantly, how many particles have enough energy for their collisions to lead to reaction. A catalyst changes the route by which reactants become products.

These three effects are often mixed together in examination answers. Keeping them separate gives you a much more reliable way to explain reaction-rate changes using collision theory and the Boltzmann distribution.

Diagnostic Quiz

What Do You Already Know About Reaction Rate?

Try these five questions before reading the chapter. They check the particle ideas that you will use to explain how concentration, temperature and catalysts affect reaction rate.

5 quick questions
Focus on collision theory and activation energy
Every answer option is explained

Which statement best describes an effective collision between two reacting particles?

Any collision between the two particles

Simply colliding is not necessarily enough. Many collisions do not lead to reaction because the particles may have insufficient energy or unsuitable orientation.

A collision with sufficient energy and, where relevant, suitable orientation

Correct. An effective collision is one capable of producing reaction. The particles must have sufficient energy to overcome the activation-energy barrier and must collide suitably for the required bond rearrangement.

A collision between particles moving at exactly the same speed

The particles do not need to have the same speed. What matters is whether the collision has sufficient energy and suitable geometry.

A collision that always occurs at high concentration

High concentration increases collision frequency, but it does not guarantee that every collision is effective.

A reactant solution is made more concentrated while the temperature remains unchanged. What happens at the particle level?

The particles gain more kinetic energy

Their energy distribution is determined mainly by temperature. Increasing concentration at constant temperature does not make the particles more energetic.

The activation energy automatically decreases

Activation energy is associated with the reaction pathway. Simply increasing concentration does not provide a different pathway.

There are more reactant particles per unit volume, so collisions occur more frequently

Correct. Greater concentration places more reactant particles in a given volume, increasing collision frequency and hence the frequency of effective collisions.

Every collision becomes successful

The number of collisions increases, but individual collisions still need sufficient energy and suitable orientation to be effective.

What is the activation energy, Eₐ, of a reaction pathway?

The minimum energy that colliding particles must possess for that pathway to lead to reaction

Correct. Eₐ represents the energy barrier that must be overcome for reaction to proceed along a particular pathway.

The average kinetic energy of all particles in the sample

Average kinetic energy depends on temperature. Activation energy instead describes the energy barrier for a reaction pathway.

The total energy released by an exothermic reaction

The energy released overall is related to the enthalpy change, ΔH, not the activation energy.

The maximum possible kinetic energy of a particle

A sample contains a distribution of particle energies. Eₐ is a reaction threshold, not a maximum particle energy.

Why does increasing temperature normally increase reaction rate substantially?

The concentration of every reactant automatically increases

Increasing temperature does not necessarily increase concentration. The main explanation concerns the distribution of particle energies.

The activation energy becomes zero

Heating does not remove the activation-energy barrier. Instead, more particles become energetic enough to overcome it.

All particles acquire the same kinetic energy

Particles still have a distribution of energies. Raising the temperature changes the shape of this distribution.

A much larger fraction of particles has energy equal to or greater than Eₐ

Correct. Collision frequency also increases, but the more important effect is the substantial increase in the fraction of particles with E ≥ Eₐ.

Which statement best describes how a catalyst increases reaction rate?

It increases the kinetic energy of every reactant particle

A catalyst does not heat the particles or increase their kinetic energies.

It increases the concentration of the reactants

A catalyst can increase rate without increasing reactant concentration.

It provides an alternative reaction pathway with lower activation energy

Correct. A lower Eₐ means that a greater fraction of collisions has enough energy to lead to reaction at the same temperature.

It makes the enthalpy change more negative

A catalyst does not change the energies of the reactants and products, so the enthalpy change, ΔH, remains unchanged.

Collision theory and effective collisions

Chemical reactions require particles to interact so that bonds can be broken, formed or rearranged. However, not every collision produces reaction.

In a simple collision-theory model, reacting particles must:

collide with one another;
collide with sufficient energy to overcome the activation-energy barrier, Eₐ; and
where relevant, collide with a suitable orientation for the required bond rearrangement.

Reaction rate depends on effective collisions per unit time.

A condition increases reaction rate when it increases the frequency with which successful particle encounters occur.

This gives us an important distinction. The total frequency of collisions and the fraction of collisions energetic enough to react are not the same thing.

Concentration mainly affects collision frequency. Temperature changes both collision frequency and the fraction of sufficiently energetic collisions. Catalysts work differently by lowering the energy barrier of an alternative route.

Activation energy as an energy barrier

The activation energy, Eₐ, is the minimum energy that colliding particles must possess for a particular reaction pathway to lead to reaction.

We can picture Eₐ as an energy barrier separating reactants from the arrangements required for bonds to be reorganised. Collisions involving insufficient energy cannot proceed along that pathway.

Effective collision → sufficient energy and suitable collision geometry

Effect of concentration on reaction rate

Suppose two reactants are dissolved in solution. If the concentration of one reactant is increased, there are more particles of that reactant in every unit volume of solution.

The reacting particles therefore encounter one another more frequently.

The number of reactant particles per unit volume increases.
The frequency of collisions between reactant particles increases.
The frequency of effective collisions increases.
The reaction rate increases.

Concentration changes collision frequency.

At the same temperature, increasing concentration does not make individual particles more energetic. It increases the number of reactant particles available in a given volume, so collisions occur more frequently.

A particle-level example

Suppose particles A and B react:

A + B → products

If the concentration of A is increased while the other conditions remain unchanged, each B particle has more opportunities to encounter an A particle. The reaction therefore becomes faster.

Collision theory explains the qualitative increase in rate. The experimentally determined rate equation tells us the exact mathematical effect.

If rate = k[A][B], doubling [A] while keeping [B] and temperature constant doubles the rate.

If rate = k[A]²[B], doubling [A] increases the rate by a factor of 2² = 4.

This is why the statement “higher concentration gives more collisions” explains why the rate increases, while reaction order tells us how much the rate changes.

Does increasing concentration increase the rate constant?

For a particular reaction pathway at constant temperature, changing reactant concentration does not normally change the rate constant, k.

Concentration: changes the numbers of reacting particles per unit volume and therefore changes collision frequency.

Rate constant, k: remains unchanged if temperature and the reaction pathway remain unchanged.

Pressure and reactions involving gases

The same particle argument can be applied to gases. If a reacting gas mixture is compressed at constant temperature, its volume decreases while the number of gas particles remains the same.

There are therefore more reacting gas particles per unit volume. Collisions become more frequent and the reaction rate increases.

Higher pressure by compression → greater gas concentration → more frequent collisions → faster reaction

The important idea is the increased concentration of the reacting gases. Pressure itself is not a mysterious extra cause of reaction.

The Boltzmann distribution of molecular energies

Concentration can be explained mainly by counting how many particles occupy a given volume. Temperature requires another idea because particles in a sample do not all possess the same kinetic energy.

At any particular temperature, there is a distribution of molecular kinetic energies. Some particles have relatively low energies, many have intermediate energies and a smaller number have very high energies.

This spread is represented by a Boltzmann distribution, often called a Maxwell–Boltzmann distribution.

Reading a Boltzmann distribution

On a graph of number of particles against kinetic energy:

the curve begins at the origin;
it rises to a maximum and then falls gradually towards the energy axis;
the peak represents the most probable energy;
the area under the curve represents the total number of particles in the sample; and
the area to the right of Eₐ represents particles with kinetic energy E ≥ Eₐ.

The area beyond Eₐ is crucial for reaction rate.

Particles with energies below Eₐ cannot overcome the activation-energy barrier for that pathway. A larger fraction with E ≥ Eₐ therefore means more collisions have enough energy to be effective.

Effect of temperature on reaction rate

Increasing temperature raises the average kinetic energy of the particles and changes the shape of the Boltzmann distribution.

At the higher temperature:

the peak becomes lower;
the peak moves towards higher energy;
the distribution becomes broader;
the total area under the curve remains the same if the number of particles is unchanged; and
a much larger area lies to the right of Eₐ.

Two effects therefore contribute to the higher reaction rate.

Effect 1: particles collide more frequently

At higher temperature, particles have greater average kinetic energy and generally move faster. They encounter one another somewhat more frequently, so collision frequency increases.

Effect 2: a larger fraction has sufficient energy

This is normally the more important effect.

Increasing temperature produces a substantial increase in the fraction of particles with energy equal to or greater than Eₐ.

A much larger fraction of collisions can therefore overcome the activation-energy barrier. The frequency of effective collisions rises significantly and the reaction rate increases.

Higher temperature → larger fraction with E ≥ Eₐ → more effective collisions → faster reaction

Incomplete explanation: Increasing temperature makes particles move faster, so they collide more frequently.

Better A-Level explanation: Collision frequency increases, but more importantly, a much larger fraction of particles possesses energy equal to or greater than Eₐ. The frequency of effective collisions therefore increases significantly.

Why can a small temperature increase have a large effect?

The Eₐ threshold normally lies in the high-energy tail of the Boltzmann distribution. Only a relatively small fraction of particles may initially lie beyond this threshold.

When temperature is increased, the whole distribution becomes broader and extends further towards high energies. Even though the change in average kinetic energy may appear modest, the fraction of particles beyond Eₐ can increase by a much larger proportion.

This explains why reaction rate can be highly sensitive to temperature.

Temperature and the rate constant

Increasing temperature increases the rate constant, k. At the higher temperature, a greater proportion of molecular encounters has sufficient energy for reaction.

For example, consider:

rate = k[A][B]

If [A] and [B] remain unchanged but the temperature is increased, the rate can still increase because the value of k has increased.

Effect of catalysts on reaction rate

A catalyst is a substance that increases the rate of a reaction without being consumed overall.

The phrase “not consumed overall” is important. A catalyst may participate in one or more steps of a reaction mechanism. However, it is regenerated in a later step, so there is no overall consumption of the catalyst.

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

Catalysed pathway → lower Eₐ → greater fraction of collisions can lead to reaction

What happens to the Boltzmann distribution?

If a catalyst is added without changing the temperature, the kinetic-energy distribution of the particles does not change.

Instead, the catalysed pathway has a lower Eₐ. On a Boltzmann-distribution graph, we can therefore imagine the relevant Eₐ threshold appearing further to the left.

A larger area of the existing distribution now lies beyond Eₐ. A greater fraction of collisions therefore has sufficient energy to lead to reaction.

A catalyst changes the reaction pathway, not the energies of the particles.

Adding a catalyst at constant temperature does not shift or reshape the Boltzmann distribution. The key change is the lower activation energy of the alternative pathway.

Temperature and catalysts are not doing the same thing

Increasing temperature: changes the distribution of molecular kinetic energies, giving a larger fraction of particles with E ≥ Eₐ.

Adding a catalyst: leaves the energy distribution unchanged at that temperature but provides an alternative pathway with lower Eₐ.

Does a catalyst change the rate constant?

The catalysed reaction follows a different pathway with a lower activation energy. Its rate is therefore greater under the same concentration and temperature conditions.

In kinetic terms, the effective rate constant for the catalysed route is larger than for the uncatalysed route at the same temperature.

Does a catalyst change the enthalpy change?

No. A catalyst changes the pathway between reactants and products, but it does not change the energy levels of the reactants or products themselves.

Catalyst changes Eₐ, but does not change ΔH

The catalysed and uncatalysed reactions therefore have the same overall enthalpy change.

Does a catalyst change the equilibrium position?

A catalyst does not change the equilibrium composition at a particular temperature. It provides a lower-energy pathway for both the forward and reverse reactions.

The system therefore reaches equilibrium more quickly, but the position of equilibrium remains unchanged.

Homogeneous catalysis

In homogeneous catalysis, the catalyst and the reactants are in the same phase.

A common situation is an aqueous catalyst reacting with aqueous reactants. The catalyst participates in a sequence of reaction steps and is regenerated by the end of the mechanism.

Example: Fe²⁺ catalysis of the iodide–peroxodisulfate reaction

The overall reaction between peroxodisulfate ions and iodide ions is:

S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

Fe²⁺ ions can catalyse this reaction by providing an alternative sequence of steps.

First, Fe²⁺ is oxidised by S₂O₈²⁻:

S₂O₈²⁻(aq) + 2Fe²⁺(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq)

The Fe³⁺ ions formed then oxidise I⁻:

2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)

Adding these two equations gives the original overall equation. Fe²⁺ is used in the first step and regenerated in the second step.

Catalyst: Fe²⁺ is present initially, used during the mechanism and regenerated later.

Intermediate: Fe³⁺ is formed during the mechanism and subsequently consumed.

Catalysts and intermediates appear in opposite orders.

A catalyst is consumed first and regenerated later. An intermediate is produced first and consumed later. Neither appears in the overall equation after the elementary steps are added.

Heterogeneous catalysis

In heterogeneous catalysis, the catalyst and reactants are in different phases.

A common example involves gaseous reactants interacting with the surface of a solid catalyst.

The catalytic process can be described in three broad stages.

Adsorption: reactant particles become attached to active sites on the surface of the catalyst.
Surface reaction: interactions with the catalyst can weaken particular bonds, bring reactants into favourable positions and allow reaction through a lower-energy pathway.
Desorption: product particles leave the catalyst surface, releasing active sites for further reactant particles.

Adsorption is a surface process.

In heterogeneous catalysis, particles are adsorbed onto the catalyst surface. Do not confuse this with absorption, in which a substance enters the bulk of another material.

Example: iron in the Haber process

In the Haber process, gaseous nitrogen and hydrogen react in the presence of a solid iron catalyst:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

N₂ and H₂ molecules adsorb onto active sites on the iron surface. Interactions with the surface facilitate bond breaking and bond formation through an alternative lower-energy pathway.

The ammonia molecules formed then desorb, making the surface sites available again.

Why does catalyst surface area matter?

Heterogeneous catalysis occurs at the catalyst surface. Increasing the exposed surface area can therefore provide more available active sites.

More reactant particles can be adsorbed and undergo surface reaction at the same time, so catalytic activity can increase.

Catalytic converters

Catalytic converters provide another example of heterogeneous catalysis. Metals such as platinum and palladium provide surfaces on which harmful exhaust gases can react more rapidly to form less harmful substances.

Once again, the important feature is the interaction of gaseous reactant molecules with active sites on a solid surface.

Enzymes as biological catalysts

Enzymes are biological catalysts. Like other catalysts, they provide alternative reaction pathways with lower activation energies, but enzymes are usually highly specific.

An enzyme contains an active site with a particular three-dimensional arrangement. A suitable substrate can interact with this active site to form an enzyme–substrate complex.

Enzyme + substrate ⇌ enzyme–substrate complex → enzyme + product

The products are released and the enzyme is regenerated, allowing it to catalyse further reactions.

Why are enzymes specific?

Only substrates capable of making suitable interactions with a particular active site bind effectively. This gives enzymes their characteristic specificity.

Why are enzymes sensitive to temperature?

Increasing temperature initially tends to increase reaction rate because particles move more rapidly and a larger fraction has sufficient energy for reaction.

However, enzymes are proteins whose catalytic activity depends on their three-dimensional structure. At sufficiently high temperatures, interactions maintaining that structure can be disrupted.

The enzyme becomes denatured. The shape and chemical environment of the active site change, so the substrate no longer binds effectively and the reaction rate falls.

Why can pH affect enzyme activity?

Changes in pH can alter the charges on groups within a protein and disrupt interactions that help maintain its structure or active-site chemistry.

An enzyme therefore normally operates most effectively within a particular range of pH values.

Moderate temperature increase: can increase the rate of an enzyme-catalysed reaction.

Excessively high temperature: can denature the enzyme and reduce the number of functional active sites, causing the rate to decrease.

Comparing concentration, temperature and catalysts

Concentration, temperature and catalysts can all make a reaction faster, but an examination explanation should state exactly what each factor changes.

Factor	What changes?	Why does rate increase?	Effect on k
Higher concentration	More reactant particles per unit volume	Collision frequency and hence effective-collision frequency increase	Normally unchanged at constant temperature
Higher gas pressure by compression	Concentration of reacting gas particles increases	Gas particles collide more frequently	Unchanged if temperature is unchanged
Higher temperature	Boltzmann energy distribution changes	Collision frequency increases and, more importantly, a much larger fraction of particles has E ≥ Eₐ	Increases
Catalyst	An alternative reaction pathway with lower Eₐ becomes available	A greater fraction of collisions can overcome the activation-energy barrier	Greater rate is obtained for the catalysed route

A reliable examination method

When asked to explain why a reaction becomes faster, avoid jumping directly to the phrase “more successful collisions”. Show the complete chain of reasoning.

For concentration, begin with more particles per unit volume.
For temperature, emphasise the larger fraction with E ≥ Eₐ.
For a catalyst, begin with the alternative pathway and lower Eₐ.

Build the explanation as a chain.

Change in conditions → particle-level consequence → change in frequency of effective collisions → change in reaction rate

Common mistakes with factors affecting reaction rate

Common mistake	Why it is wrong	Correct approach
Saying higher concentration gives particles more energy	Concentration changes the number of particles per unit volume, not their kinetic-energy distribution	Explain that collision frequency increases
Saying increasing concentration increases k	At constant temperature, concentration affects the concentration terms in the rate equation	Keep k constant if temperature and the reaction pathway are unchanged
Saying higher temperature only makes particles collide more frequently	This misses the much larger effect on the fraction of particles with sufficient energy	Emphasise the increase in the fraction with E ≥ Eₐ
Saying the total area under a Boltzmann curve increases with temperature	The total number of particles has not changed	Keep the total area constant while changing the shape of the curve
Saying all particles have more than Eₐ at higher temperature	Even at a higher temperature, the sample still contains a distribution of energies	State that a larger fraction has E ≥ Eₐ
Saying a catalyst gives reactant particles more energy	The kinetic-energy distribution is unchanged at the same temperature	State that an alternative pathway with lower Eₐ is provided
Saying a catalyst lowers Eₐ for the original pathway	The catalysed reaction follows an alternative pathway	Describe the alternative pathway as having lower Eₐ
Saying a catalyst takes no part in the reaction	A catalyst may participate in elementary steps	State that it is regenerated and not consumed overall
Confusing adsorption with absorption	Heterogeneous catalysis normally involves attachment to a surface	Use adsorption for attachment to the catalyst surface
Saying a catalyst changes ΔH	The energies of the reactants and products are unchanged	State that Eₐ changes but ΔH remains unchanged
Saying a catalyst shifts the equilibrium position	It speeds up both forward and reverse reactions and does not alter the equilibrium composition at the same temperature	State that equilibrium is reached faster but its position is unchanged

Factors-affecting-rate checklist

Concentration

State that particles per unit volume increase.
Explain that collision frequency increases.
Connect this to more effective collisions per unit time.
Do not say that particle energy or k increases.

Temperature

State that particles have greater average kinetic energy.
Recognise that collision frequency increases.
Emphasise the much larger fraction with E ≥ Eₐ.
Conclude that k and reaction rate increase.

Catalysts

State that an alternative pathway is provided.
State that the alternative pathway has lower Eₐ.
Do not say that the catalyst gives particles more energy.
Remember that the catalyst is regenerated overall.

Catalytic mechanisms

Homogeneous catalyst and reactants are in the same phase.
Heterogeneous catalysis occurs at a phase boundary or surface.
Use adsorption, surface reaction and desorption where appropriate.
For enzymes, connect specificity to the active site.

Mastery Quiz

Factors Affecting Reaction Rate Mastery Quiz

Now apply collision theory, Boltzmann distributions and catalysis to unfamiliar situations. These questions focus on the reasoning expected in A-Level Chemistry.

10 application-focused questions
Concentration, temperature and catalysis are integrated
Every answer option is explained

A reaction has the rate equation rate = k[A][B]. The concentration of A is doubled while [B] and the temperature remain constant. Which statement is correct?

The rate doubles because k doubles

The rate does double, but k does not change merely because [A] changes at constant temperature. The increase comes from the [A] term in the rate equation.

The rate doubles while k remains unchanged

Correct. The reaction is first order with respect to A. Doubling [A] doubles the rate, while k remains constant because the temperature and reaction pathway are unchanged.

The rate increases by a factor of four while k remains unchanged

A factor of four would result if the reaction were second order with respect to A. Here the rate is directly proportional to [A].

Neither the rate nor k changes

Increasing [A] increases the frequency of encounters involving A and changes the rate according to the rate equation.

Two Boltzmann distributions describe the same number of particles at temperatures T₁ and T₂, where T₂ > T₁. Which feature must the T₂ curve have?

A higher and narrower peak with a smaller total area

Higher temperature produces a broader distribution with a lower peak. The total area remains the same for the same number of particles.

The same shape but with the Eₐ line moved to the right

Raising temperature changes the shape of the energy distribution. Eₐ for the same reaction pathway is not shifted simply because the temperature rises.

A lower, broader peak with a larger fraction of particles beyond Eₐ

Correct. At the higher temperature the distribution spreads towards higher energies, so a substantially larger fraction of particles has E ≥ Eₐ.

A greater total area because hotter particles occupy more space

The area represents the number of particles, not their physical volume. The same sample contains the same number of particles.

A catalyst is added to a reaction mixture without changing its temperature. Which description of a Boltzmann-distribution diagram is correct?

The distribution remains unchanged, but the catalysed Eₐ is lower

Correct. The particle-energy distribution depends on temperature. A catalyst instead provides an alternative pathway with a lower activation energy.

The entire distribution moves to higher energy

This would imply that the particles have become more energetic. A catalyst does not increase their kinetic energies at constant temperature.

The distribution becomes narrower because collisions become more organised

Catalysis does not narrow the kinetic-energy distribution. It changes the available reaction pathway.

The total area under the curve becomes larger

Adding a small amount of catalyst does not represent an increase in the number of reactant particles shown by their Boltzmann distribution.

A gaseous reaction is carried out in a sealed container. The gases are compressed to half their original volume at the same temperature. Why does the reaction rate normally increase?

Compression increases Eₐ

Increasing Eₐ would make reaction more difficult. Compression does not normally change the reaction pathway in this way.

Every gas particle gains kinetic energy

The temperature is unchanged, so the kinetic-energy distribution is not increased simply by this compression.

The rate constant doubles

At the same temperature and for the same pathway, k remains unchanged.

The concentration of the gas particles increases, so collisions occur more frequently

Correct. The same particles occupy a smaller volume, increasing their concentration and collision frequency.

Fe²⁺ catalyses the reaction between S₂O₈²⁻ and I⁻ by the steps:

S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

Which statement correctly identifies Fe²⁺ and Fe³⁺?

Fe³⁺ is the catalyst and Fe²⁺ is the intermediate

Fe²⁺ is present before the mechanism, is consumed and then regenerated. Fe³⁺ is instead produced and subsequently consumed.

Fe²⁺ is the catalyst and Fe³⁺ is an intermediate

Correct. Fe²⁺ is used in the first step and regenerated in the second. Fe³⁺ is formed in the first step and consumed in the second.

Both Fe²⁺ and Fe³⁺ are catalysts

Fe³⁺ does not begin the catalytic cycle and become regenerated in the way required of the catalyst here. It is an intermediate.

Neither species is involved in the reaction mechanism

Both appear explicitly in the elementary steps even though neither appears in the overall reaction equation.

Which sequence best describes the action of a typical solid heterogeneous catalyst on gaseous reactants?

Absorption → evaporation → condensation

These are not the characteristic stages of heterogeneous surface catalysis.

Desorption → adsorption → surface reaction

Products cannot desorb before the reactants have first interacted with the catalyst surface and reacted.

Adsorption → surface reaction → desorption

Correct. Reactants first attach to the catalyst surface, react at active sites and then leave the surface as products.

Ionisation → neutralisation → precipitation

These terms describe different chemical processes and do not represent the general mechanism of heterogeneous catalysis.

A catalyst is added to a reversible exothermic reaction already at a fixed temperature. Which combination is correct?

ΔH becomes more negative and equilibrium shifts towards products

A catalyst does not change reactant or product energy levels and therefore does not change ΔH or the equilibrium position.

ΔH remains unchanged but equilibrium shifts towards products

The first part is correct, but a catalyst does not favour the forward equilibrium reaction over the reverse reaction.

ΔH changes but the equilibrium position remains unchanged

The catalyst changes the pathway but not the energy difference between reactants and products, so ΔH does not change.

ΔH and the equilibrium position both remain unchanged, but equilibrium is reached faster

Correct. A catalyst lowers the activation barriers for both directions and allows equilibrium to be established more rapidly without changing its position.

An enzyme-catalysed reaction becomes faster as the temperature rises from 20 °C to 35 °C but much slower after prolonged heating at 80 °C. What is the best explanation?

Moderate heating increases reaction rate, but very high temperature can denature the enzyme

Correct. At first, greater kinetic energy increases reaction rate. At very high temperature, disruption of the enzyme structure changes its active site and reduces catalytic activity.

The activation energy becomes larger whenever temperature increases

Heating does not normally increase the activation energy of the catalytic pathway. The later decrease arises from enzyme denaturation.

The substrate concentration must become zero at 80 °C

The observation does not imply that all substrate has disappeared. Loss of enzyme structure provides the relevant explanation.

Enzymes only act as catalysts below room temperature

Many enzymes operate above room temperature. Their activity depends on the particular enzyme and on whether its structure remains intact.

Which is the strongest explanation for the increase in rate when a reaction mixture is heated from 298 K to 318 K?

The particles move faster, so the reaction becomes faster

This is partly true but incomplete. It does not explain the major change in the fraction of sufficiently energetic collisions.

A larger fraction of particles has E ≥ Eₐ, so the frequency of effective collisions increases substantially

Correct. This directly links the Boltzmann distribution to activation energy and effective-collision frequency.

Eₐ decreases because the particles are hotter

Increasing temperature does not by itself provide a different reaction pathway or lower the activation energy.

The total number of particles increases

Heating a fixed sample does not create additional reactant particles. It redistributes their kinetic energies.

Four experiments use the same reaction. Which change can increase reaction rate while leaving the reactant concentrations and temperature unchanged?

Diluting one reactant with water

Dilution changes reactant concentration and would usually reduce the frequency of reactant collisions.

Heating the mixture

Heating can increase reaction rate, but the temperature would no longer be unchanged.

Adding a suitable catalyst

Correct. A suitable catalyst can provide an alternative lower-Eₐ pathway without changing the reactant concentrations or temperature.

Removing half of one reactant

This changes the amount and generally the concentration of that reactant, so the stated conditions would not remain unchanged.

Frequently asked questions

Questions students often ask about factors affecting reaction rate

These questions clarify the distinctions that most often cause confusion when concentration, temperature, rate equations and catalysis are studied together.

Why is saying “there are more collisions” sometimes not enough in an examination?

You should explain why the frequency of effective collisions changes. Higher concentration gives more particles per unit volume. Higher temperature gives a much larger fraction of particles with E ≥ Eₐ. A catalyst provides an alternative pathway with lower Eₐ.

Does doubling concentration always double reaction rate?

No. The exact effect depends on the order of reaction with respect to that reactant.

If the reaction is first order, doubling the concentration doubles the rate. If it is second order, the rate increases by a factor of four. If it is zero order, changing that concentration does not change the rate.

Why does temperature change k but concentration normally does not?

Changing concentration at constant temperature changes how frequently the reacting particles encounter one another. This effect appears through the concentration terms in the rate equation.

Changing temperature alters the distribution of molecular energies and therefore the proportion of encounters capable of overcoming the activation-energy barrier. The rate constant consequently changes.

Does increasing temperature lower the activation energy?

No. For the same reaction pathway, Eₐ remains the same. Raising the temperature changes the energy distribution so that a greater fraction of particles has E ≥ Eₐ.

Does a catalyst reduce the energy of the reactants?

No. The energy level of the reactants is unchanged. A catalyst provides a different route from reactants to products with a lower activation-energy barrier.

Why does a catalyst not change ΔH?

ΔH depends on the energy difference between the reactants and products. A catalyst changes only the pathway connecting them. The initial and final energy levels are unchanged, so ΔH is unchanged.

Does a catalyst change the position of equilibrium?

No. A catalyst provides an alternative lower-energy pathway for both the forward and reverse reactions. Equilibrium is therefore reached more quickly, but the equilibrium composition at that temperature is unchanged.

What is the difference between a homogeneous and a heterogeneous catalyst?

In homogeneous catalysis, the catalyst and reactants are in the same phase. In heterogeneous catalysis, the catalyst and reactants are in different phases and the reaction commonly occurs at a surface or phase boundary.

What is the difference between a catalyst and an intermediate?

A catalyst is present initially, is consumed in one step and regenerated later. An intermediate is produced during one step and consumed in a later step.

Neither appears in the final overall equation after the elementary steps are combined.

Why is adsorption important in heterogeneous catalysis?

Adsorption brings reacting particles into contact with active sites on the catalyst surface. Interactions with the surface can weaken bonds, hold reactants in favourable positions and provide a lower-energy route for bond rearrangement.

Why does increasing the surface area of a solid catalyst often increase its effectiveness?

A larger exposed surface normally provides more accessible active sites. More reactant particles can therefore be adsorbed and react at the surface at the same time.

Why can an enzyme-catalysed reaction slow down when the temperature becomes too high?

At first, increasing temperature tends to increase reaction rate. At sufficiently high temperature, however, the enzyme can become denatured. Its active-site structure changes, so the substrate no longer binds effectively and catalytic activity falls.

Concentration, temperature and catalysts can all increase reaction rate, but their particle-level explanations are different. Concentration changes how frequently reactant particles encounter one another. Temperature changes the molecular energy distribution and greatly increases the fraction able to overcome Eₐ. Catalysts provide an alternative route with a lower activation energy. Keeping these three mechanisms separate gives you a reliable framework for explaining reaction rates, interpreting Boltzmann distributions and understanding catalytic mechanisms throughout A-Level Chemistry.