Homogeneous and Heterogeneous Catalysis

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.

Key Takeaway

Table of Contents

Catalysts provide an alternative reaction pathway with a lower activation energy

A catalyst increases the rate of a reaction by providing an alternative pathway with a lower activation energy, Eₐ. The catalyst may take part in individual steps of the reaction, but it is regenerated and is not consumed overall.

A catalyst provides an alternative pathway with a lower activation energy, Eₐ.
At constant temperature, the Boltzmann distribution does not change when a catalyst is added.
A lower Eₐ means a larger fraction of particles have sufficient energy to react.
A catalyst does not change ΔH because the reactants and products remain the same.
A catalyst does not change the position of equilibrium; it allows equilibrium to be reached more quickly.
In heterogeneous catalysis, the catalyst and reactants are in different phases.
Heterogeneous catalysis commonly involves adsorption, reaction at the surface and desorption.
In homogeneous catalysis, the catalyst and reactants are in the same phase.
Homogeneous catalysts provide an alternative pathway involving one or more intermediates.
Enzymes are highly specific biological catalysts whose activity is affected by temperature and pH.

Some chemical reactions are energetically possible but occur far too slowly to be useful. The manufacture of ammonia, the removal of harmful gases from vehicle exhausts and many reactions inside living organisms all depend on catalysts to make reactions occur at useful rates.

A catalyst does not work by giving the reacting particles extra energy. Instead, it changes the route by which reactants become products. The alternative route requires a lower activation energy, so more reacting particles are able to follow it at the same temperature.

The important question is therefore not simply whether a catalyst is present, but how it provides the alternative pathway. In heterogeneous catalysis, this happens at a surface. In homogeneous catalysis, it happens through a sequence of reactions involving intermediates.

Diagnostic Quiz

What Do You Already Know About Catalysis?

Try these five questions before reading the chapter. They check the activation-energy, equilibrium and reaction-rate ideas that make catalysis easier to understand.

5 quick starting-point questions
Focus on common catalysis misconceptions
Every answer option is explained

Which statement best explains why a catalyst increases the rate of a reaction?

It increases the kinetic energy of all the reacting particles

The average kinetic energy of the particles depends on temperature. Adding a catalyst without changing temperature does not give every particle more kinetic energy.

It provides an alternative reaction pathway with a lower activation energy

Correct. The lower activation energy means that a larger fraction of particles have sufficient energy to react at the same temperature.

It increases the enthalpy change of the reaction

A catalyst does not change the enthalpies of the reactants or products, so the overall enthalpy change, ΔH, remains unchanged.

It converts an endothermic reaction into an exothermic reaction

Catalysis changes the reaction pathway, not the relative enthalpies of the reactants and products. The sign of ΔH therefore does not change.

A catalyst is added to a reaction while the temperature is kept constant. What happens to the Boltzmann distribution of particle energies?

The entire distribution shifts towards higher energy

A shift towards higher energies would result from increasing the temperature. The temperature has not changed here.

The peak becomes lower because particles are used up by the catalyst

A catalyst does not remove a significant fraction of particles from the energy distribution. It is also regenerated rather than consumed overall.

The distribution becomes narrower around the activation energy

The activation energy is not what determines the shape of the Boltzmann distribution. At a fixed temperature, the distribution remains the same.

The distribution remains unchanged, but the catalysed pathway has a lower Eₐ

Correct. Temperature determines the energy distribution. The catalyst instead lowers the activation-energy requirement by providing a different pathway.

Gaseous reactants react in the presence of a solid metal catalyst. What type of catalysis is this?

Heterogeneous catalysis

Correct. The gaseous reactants and solid catalyst are in different phases, which is the defining feature of heterogeneous catalysis.

Homogeneous catalysis

Homogeneous catalysis requires the catalyst and reactants to be in the same phase.

Enzyme catalysis

Enzyme catalysis involves biological catalysts. Nothing in the description indicates that the solid metal is an enzyme.

Uncatalysed reaction

A solid metal is explicitly acting as a catalyst, so the reaction is catalysed.

What happens when a catalyst is added to a reversible reaction that can reach equilibrium?

The equilibrium shifts towards the products

A catalyst does not favour the products. It speeds up both the forward and reverse reactions.

The equilibrium shifts towards the reactants

A catalyst does not favour the reverse reaction either. The equilibrium position is unchanged.

Equilibrium is reached more quickly, but its position is unchanged

Correct. A catalyst increases the rates of both forward and reverse reactions, so equilibrium is established faster without changing its position.

The reverse reaction stops

A catalyst accelerates the reverse reaction as well as the forward reaction. It does not stop either direction.

Which statement correctly describes adsorption during heterogeneous catalysis?

Reactant particles enter deeply into the bulk of the catalyst

Entry into the bulk would be closer to absorption. In heterogeneous catalysis, the important process occurs at the catalyst surface.

Reactant particles become attached to active sites on the catalyst surface

Correct. Adsorption means attachment to the surface. Interactions with active sites can weaken bonds and facilitate reaction.

Product particles permanently occupy all the active sites

Products should normally desorb so that the active sites become available for further reactant particles.

The catalyst dissolves completely in the reactants

That would not describe the usual solid-surface mechanism of heterogeneous catalysis.

How a catalyst increases reaction rate

For a reaction to occur, reacting particles must be able to pass through a high-energy arrangement along the reaction pathway. The minimum energy barrier that must be overcome is called the activation energy, Eₐ.

A catalyst provides a different sequence of steps between the same reactants and products. This alternative pathway has a lower activation energy than the uncatalysed pathway.

Alternative pathway → lower Eₐ → faster reaction

It is therefore more precise to say that a catalyst provides an alternative pathway with a lower activation energy rather than saying that it simply “lowers the activation energy of the reaction”. The original uncatalysed pathway still exists.

A catalyst changes the pathway, not the starting and finishing energies.

The reactants and products are chemically the same whether or not a catalyst is used. Their enthalpies therefore remain unchanged, so the overall enthalpy change, ΔH, is also unchanged.

How would a reaction-profile diagram change?

On an energy-profile diagram, the reactant and product energy levels remain in the same positions. The catalysed pathway has a lower maximum energy than the uncatalysed pathway.

The uncatalysed pathway has the larger Eₐ.
The catalysed pathway has the smaller Eₐ.
The reactant and product energy levels are unchanged.
The value and sign of ΔH are unchanged.

Catalysts and the Boltzmann distribution

At any particular temperature, the particles in a sample do not all have the same energy. Instead, their energies follow a Boltzmann distribution.

For a particular reaction pathway, only particles with energy equal to or greater than the activation energy have sufficient energy to overcome the energy barrier. On a Boltzmann distribution, these particles are represented by the area under the curve to the right of Eₐ.

What changes when a catalyst is added?

If the temperature remains constant, the distribution of particle energies does not change. Instead, the catalysed pathway has a lower activation energy.

The Boltzmann curve remains the same shape and position.
Eₐ for the catalysed pathway lies at a lower energy.
A larger area lies to the right of the lower Eₐ.
A larger fraction of particles therefore have sufficient energy to react.
The reaction rate increases.

Increasing temperature: changes the Boltzmann distribution itself, giving a greater proportion of high-energy particles.

Adding a catalyst at constant temperature: leaves the Boltzmann distribution unchanged but provides a lower activation-energy pathway.

Do not redraw the Boltzmann curve when only a catalyst is added.

The common examination diagram should show one distribution curve and two activation-energy positions: Eₐ for the catalysed pathway lies to the left of Eₐ for the uncatalysed pathway.

Catalysts and chemical equilibrium

A catalyst can make a reversible reaction reach equilibrium more quickly, but it does not change the equilibrium position.

This is because the catalyst provides lower-activation-energy pathways for both the forward and reverse reactions.

Catalyst added → forward reaction faster + reverse reaction faster

The system therefore reaches dynamic equilibrium sooner. However, at a fixed temperature, the equilibrium composition and equilibrium constant remain unchanged.

A catalyst changes how quickly equilibrium is reached, not where equilibrium lies.

It cannot increase the equilibrium yield by shifting equilibrium towards the products. It simply allows the system to reach its existing equilibrium position in a shorter time.

Is the catalyst really unchanged?

A catalyst may be chemically changed during an individual step of the mechanism. The important point is that it is regenerated in a later step. It is therefore not consumed overall.

Catalyst consumed in one step → catalyst regenerated in a later step

Heterogeneous catalysis

In heterogeneous catalysis, the catalyst and reactants are in different phases. A common example involves gaseous reactants reacting on the surface of a solid metal catalyst.

The catalyst surface is important because it contains active sites where reactant particles can attach and react. The general mechanism can be organised into three stages.

Stage	What happens?	Why it matters
1. Adsorption	Reactant particles become attached to active sites on the catalyst surface.	Interactions with the surface can weaken bonds and hold reacting species in suitable positions.
2. Surface reaction	Adsorbed particles react while attached to the surface.	The surface provides an alternative route with a lower activation energy.
3. Desorption	Product particles detach from the catalyst surface.	Active sites become available for further reactant particles.

Step 1: adsorption

Adsorption is the attachment of particles to a surface. Reactant molecules collide with the catalyst and may become attached to active sites through temporary interactions with atoms on the catalyst surface.

These interactions can weaken bonds within the reactant molecules. Adsorption may also hold reacting species close together and in positions that make reaction more favourable.

Adsorption: particles attach to the surface of a material.

Absorption: particles enter the bulk or interior of a material.

The surface process involved in heterogeneous catalysis is adsorption.

Step 2: reaction on the surface

Once the reactants have adsorbed, bonds in the reactants can weaken while new bonds begin to form. The catalyst surface therefore provides a different sequence of bond-breaking and bond-forming steps from the uncatalysed reaction.

Since this surface pathway has a lower activation energy, reaction occurs more readily.

Step 3: desorption

The product particles eventually detach from the catalyst surface. This is called desorption.

Desorption is essential because the active sites must become available again. Otherwise, the catalyst surface would gradually become covered and unable to accommodate further reactant particles.

Remember the sequence: adsorption → surface reaction → desorption.

Reactants attach to active sites, react through the lower-energy surface pathway, and the products then leave. The active sites are available for another catalytic cycle.

Iron as a heterogeneous catalyst in the Haber process

The Haber process is an important example of heterogeneous catalysis. Nitrogen and hydrogen are gases, while the catalyst is solid iron.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Since the reactants and catalyst are in different phases, the process is heterogeneous.

What happens at the iron surface?

Adsorption: N₂ and H₂ molecules become adsorbed onto active sites on the iron surface.
Bond weakening: interactions between the adsorbed molecules and iron weaken bonds within the reactants, including the strong N≡N bond.
Surface reaction: the adsorbed species rearrange and N–H bonds form through a lower-energy pathway.
Desorption: NH₃ molecules leave the surface, freeing the active sites.

Exam-style explanation: N₂ and H₂ adsorb onto active sites on the iron surface. Interactions with the surface weaken bonds in the reactants and allow reaction through a pathway with lower activation energy. NH₃ then desorbs, leaving the active sites available for further reaction.

Heterogeneous catalysis in catalytic converters

Catalytic converters in vehicle exhaust systems use solid metal catalysts to increase the rates of reactions that convert harmful exhaust gases into less harmful products.

Metals such as platinum and palladium provide surfaces on which exhaust gases can adsorb and react.

One reaction involving carbon monoxide and nitrogen monoxide can be represented as:

2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)

The general catalytic sequence is the same:

CO and NO adsorb onto active sites on the metal surface.
The adsorbed species react through a lower-energy pathway.
CO₂ and N₂ desorb from the surface.

The important transferable idea is therefore not simply to memorise the metal used in a catalytic converter. You should recognise the example as heterogeneous catalysis and explain it using adsorption, surface reaction and desorption.

Catalyst poisoning

A heterogeneous catalyst can work effectively only if reactants can reach its active sites. If another substance binds strongly to the catalyst surface, it may block those sites.

This reduction in catalytic activity is called catalyst poisoning.

The catalyst does not necessarily need to be completely destroyed. If enough active sites become occupied by strongly adsorbed impurities, fewer sites remain available for the intended reactants.

Catalyst poisoning reduces the number of available active sites.

Fewer reactant particles can adsorb onto the catalyst surface, so fewer catalytic reaction events can occur per unit time and the reaction rate decreases.

Homogeneous catalysis

In homogeneous catalysis, the catalyst and the reactants are in the same phase. They may, for example, all be gases or all be dissolved in the same aqueous solution.

There is no separate solid surface on which adsorption must occur. Instead, the catalyst usually reacts with one of the reactants to form another species. This species then reacts in a later step, eventually regenerating the catalyst.

Reactant + catalyst → intermediate → products + regenerated catalyst

When the elementary steps are added together, the catalyst does not appear in the overall equation because it is regenerated.

Heterogeneous catalysis: catalyst and reactants are in different phases, and reaction commonly occurs at active sites on a surface.

Homogeneous catalysis: catalyst and reactants are in the same phase, and the catalyst participates in a sequence of reactions involving intermediates.

Nitrogen dioxide as a homogeneous catalyst

Nitrogen dioxide can catalyse the oxidation of sulfur dioxide by oxygen. Since the reacting species and catalyst are all gases, this is homogeneous catalysis.

The overall oxidation is:

SO₂(g) + ½O₂(g) → SO₃(g)

In the catalysed pathway, NO₂ first reacts with SO₂:

SO₂(g) + NO₂(g) → SO₃(g) + NO(g)

The NO produced then reacts with oxygen:

NO(g) + ½O₂(g) → NO₂(g)

Adding the two steps and cancelling NO and NO₂ gives the overall reaction:

SO₂(g) + ½O₂(g) → SO₃(g)

Which species is the catalyst?

NO₂ is consumed in the first step and regenerated in the second step. It therefore acts as the catalyst.

NO is formed in the first step and consumed in the second. In this sequence, NO is an intermediate.

Follow each species through the mechanism.

A catalyst is present at the beginning of the catalytic cycle, is consumed in one step and is regenerated later. An intermediate is produced during the mechanism and then consumed in a later step.

Fe²⁺ and Fe³⁺ ions as a homogeneous catalytic system

The reaction between peroxodisulfate ions, S₂O₈²⁻, and iodide ions, I⁻, provides another important example of homogeneous catalysis.

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

The direct reaction is relatively slow because both reactants are negatively charged. As S₂O₈²⁻ and I⁻ approach one another, electrostatic repulsion makes successful reaction less favourable.

The Fe²⁺/Fe³⁺ redox couple provides an alternative route in two steps.

Step 1: Fe²⁺ reacts with S₂O₈²⁻

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

Fe²⁺ is oxidised to Fe³⁺, while S₂O₈²⁻ is reduced to SO₄²⁻.

Step 2: Fe³⁺ reacts with I⁻

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

Fe³⁺ is reduced back to Fe²⁺, while I⁻ is oxidised to I₂.

Adding the two equations gives:

S₂O₈²⁻(aq) + 2Fe²⁺(aq) + 2Fe³⁺(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq) + 2Fe²⁺(aq) + I₂(aq)

The Fe²⁺ and Fe³⁺ terms cancel, leaving:

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

Why is the catalysed route easier?

The uncatalysed reaction involves direct reaction between S₂O₈²⁻ and I⁻, two negatively charged ions. The alternative route instead involves reactions between oppositely charged ions:

S₂O₈²⁻ reacts with Fe²⁺.
I⁻ reacts with Fe³⁺.

The iron ions can therefore transfer electrons through two more favourable reaction steps rather than requiring the two negatively charged reactants to react directly.

Exam-style explanation: Fe²⁺/Fe³⁺ provides an alternative redox pathway. Fe²⁺ is oxidised to Fe³⁺ by S₂O₈²⁻, and Fe³⁺ is subsequently reduced back to Fe²⁺ by I⁻. The iron species is therefore regenerated overall, while the alternative pathway has a lower activation energy.

Catalyst or intermediate?

In a multistep reaction mechanism, both catalysts and intermediates may disappear when the individual equations are added. You therefore cannot identify them simply by checking whether they appear in the overall equation.

Species	What happens in the mechanism?	Overall result
Catalyst	Consumed in an earlier step and regenerated in a later step	Not consumed overall
Intermediate	Formed in an earlier step and consumed in a later step	Does not appear in the overall equation

Worked example

Consider this simplified mechanism:

A + X → AX

AX + B → AB + X

X is consumed in the first step and regenerated in the second. X is therefore the catalyst.

AX is formed in the first step and consumed in the second. AX is therefore an intermediate.

Adding the two steps gives:

A + B → AB

The order matters.

A catalyst is used first and regenerated later. An intermediate is formed first and consumed later. Both may be absent from the overall equation.

Enzymes as biological catalysts

Enzymes are biological catalysts. Most enzymes are proteins whose three-dimensional structures allow them to catalyse particular reactions with high specificity.

The reactant acted upon by an enzyme is called the substrate. The substrate binds to a particular region of the enzyme called the active site.

The lock-and-key model

In the lock-and-key model, the active site has a shape that is complementary to the substrate. A substrate with a suitable shape can fit into the active site and form an enzyme–substrate complex.

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

The enzyme is regenerated at the end of the process and can therefore catalyse further reaction cycles.

The particular shape of the active site also helps to explain enzyme specificity. A substrate that does not fit the active site appropriately cannot interact with it effectively.

Effect of temperature on enzyme activity

Increasing temperature initially increases the kinetic energy of enzyme and substrate particles. They collide more frequently, so enzyme–substrate complexes can form more frequently and the reaction rate increases.

At sufficiently high temperatures, however, interactions responsible for maintaining the enzyme's three-dimensional structure are disrupted. The shape of the active site changes and the substrate no longer fits effectively.

The enzyme is then described as denatured.

Effect of pH on enzyme activity

Changes in pH can also affect interactions within the enzyme and change its three-dimensional shape. If the active site is altered sufficiently, the substrate can no longer bind effectively and catalytic activity decreases.

Incorrect wording: the enzyme “dies” at high temperature.

Correct wording: an enzyme is a molecule rather than a living organism. At sufficiently high temperature, it becomes denatured, changing the shape of its active site.

Comparing homogeneous and heterogeneous catalysis

Feature	Heterogeneous catalysis	Homogeneous catalysis
Phases	Catalyst and reactants are in different phases	Catalyst and reactants are in the same phase
Where reaction occurs	Usually at active sites on a solid surface	Within the same gaseous or liquid phase
Key mechanism	Adsorption → surface reaction → desorption	Reaction through intermediates in a catalytic cycle
How the catalyst becomes available again	Products desorb and free the active sites	The catalytic species is regenerated chemically in a later step
Example	Fe in the Haber process	Fe²⁺/Fe³⁺ in the S₂O₈²⁻/I⁻ reaction

A reliable way to identify the type of catalysis

Identify the phase of the reactants.
Identify the phase of the catalyst.
If the phases differ, think about surface adsorption.
If the phases are the same, look for a sequence of reactions and catalyst regeneration.

Do not stop at “the catalyst lowers Eₐ”.

That explains why catalysis increases rate, but an examination question about heterogeneous or homogeneous catalysis usually expects you to explain how the alternative pathway is produced.

Common mistakes with catalysis

Common mistake	Why it is wrong	Correct approach
Saying that a catalyst gives particles more energy	Particle energies are determined by temperature	State that the catalyst provides an alternative pathway with lower Eₐ
Redrawing the Boltzmann curve when only a catalyst is added	The temperature has not changed	Keep the same curve and show a lower Eₐ for the catalysed pathway
Saying that a catalyst changes ΔH	The reactants and products retain the same enthalpies	Change the pathway and Eₐ, not the overall ΔH
Saying that a catalyst shifts equilibrium towards the products	Both forward and reverse reactions are accelerated	State that equilibrium is reached faster but its position is unchanged
Confusing adsorption with absorption	Heterogeneous catalysis involves attachment to a surface	Use adsorption for reactants attaching to active sites
Stopping a heterogeneous mechanism after adsorption	Products must leave before active sites can be reused	Describe adsorption, surface reaction and desorption
Saying that catalyst poisoning means the catalyst has been used up	Poisoning usually involves blockage of active sites	Explain that fewer active sites remain available for reactant adsorption
Calling every species that cancels from a mechanism a catalyst	Intermediates also cancel from the overall equation	Check whether the species is consumed then regenerated, or formed then consumed
Forgetting to regenerate a homogeneous catalyst	A catalyst cannot be consumed overall	Identify the later step that reforms the catalytic species
Saying that an enzyme dies at high temperature	An enzyme is a molecule rather than a living organism	State that the enzyme becomes denatured and its active site changes shape

Catalysis checklist

Explain the rate increase

State that an alternative pathway is provided.
State that the alternative pathway has a lower Eₐ.
Keep the Boltzmann distribution unchanged at constant temperature.
Explain that a larger fraction of particles can overcome Eₐ.

Explain heterogeneous catalysis

Check that catalyst and reactants are in different phases.
Describe adsorption onto active sites.
Explain reaction on the catalyst surface.
Finish with product desorption and release of active sites.

Explain homogeneous catalysis

Check that catalyst and reactants are in the same phase.
Write the separate reaction steps clearly.
Identify any intermediate formed.
Show explicitly that the catalyst is regenerated.

Check your final explanation

Do not change ΔH or the equilibrium position.
Do not confuse adsorption with absorption.
Distinguish catalysts from intermediates.
Use “denatured”, not “dead”, when describing enzymes.

Mastery Quiz

Homogeneous and Heterogeneous Catalysis Mastery Quiz

Now apply the chapter to energy distributions, catalytic surfaces and multistep mechanisms. These questions focus on reasoning rather than simple recall.

10 application-focused questions
Includes mechanisms and catalytic cycles
Every answer option is explained

A reaction is carried out twice at the same temperature, once without a catalyst and once with a catalyst. Which statement correctly compares the two experiments?

The catalysed reaction has particles with a higher average kinetic energy

Average kinetic energy depends on temperature. Since both experiments are at the same temperature, this does not explain the faster catalysed reaction.

The catalysed reaction has a different Boltzmann distribution

The Boltzmann distribution is determined by temperature, so it is the same for both experiments.

The catalysed pathway has a lower Eₐ, so a larger fraction of particles can react

Correct. The same energy distribution is compared with a lower activation-energy threshold, so a larger fraction of particles have sufficient energy for the catalysed pathway.

The catalyst makes ΔH more negative

ΔH depends on the enthalpy difference between reactants and products and is unaffected by the alternative pathway.

A gaseous reactant is converted into products on a solid metal catalyst. Which sequence best represents a complete heterogeneous catalytic cycle?

Adsorption → surface reaction → desorption

Correct. Reactants first attach to active sites, reaction occurs on the surface, and products then leave so that the active sites can be reused.

Desorption → adsorption → permanent binding

Reactants must adsorb before surface reaction can occur. Permanent binding would also prevent the catalytic site from being reused.

Absorption → precipitation → evaporation

These are not the characteristic stages of heterogeneous surface catalysis. The relevant process is adsorption, not absorption.

Desorption → surface reaction → adsorption

The sequence is reversed. Reactants must first become attached to the catalyst before reacting on its surface.

A small amount of an impurity binds very strongly to the surface of a solid catalyst. The reaction rate decreases greatly. What is the best explanation?

The impurity increases the activation energy of every particle

Activation energy describes a reaction pathway, not an energy permanently assigned to individual particles.

The impurity blocks active sites, so fewer reactant particles can adsorb

Correct. This is catalyst poisoning. Strongly adsorbed impurities occupy active sites and reduce the area of effective catalytic surface available.

The impurity shifts the equilibrium towards the reactants

Loss of catalytic activity affects the rate at which equilibrium is reached, not the equilibrium position itself.

The impurity increases the temperature of the catalyst

The stated problem is strong surface binding. The rate falls because active sites are blocked, not because the catalyst has necessarily changed temperature.

Which explanation best describes the role of iron in the Haber process?

Iron shifts the N₂ + 3H₂ ⇌ 2NH₃ equilibrium towards NH₃

Iron does not change the equilibrium position. It increases the rates of both forward and reverse reactions.

Iron supplies nitrogen atoms that become part of NH₃

The nitrogen atoms in NH₃ come from N₂. Iron acts as a catalyst and is not incorporated into the ammonia product.

Iron raises the kinetic energy of N₂ and H₂ molecules

The kinetic-energy distribution depends on temperature. Iron instead provides a catalytic surface and a lower-energy reaction pathway.

N₂ and H₂ adsorb onto iron, their bonds are weakened, reaction occurs and NH₃ desorbs

Correct. This describes the essential heterogeneous catalytic sequence on the iron surface.

In a catalytic converter, 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g) occurs on a solid metal surface. Which statement is correct?

The process is homogeneous because all four substances in the equation are gases

The phase of the catalyst must also be considered. The gaseous reactants interact with a solid catalyst, so the catalysis is heterogeneous.

The products must remain permanently adsorbed for the catalyst to work

Products must desorb so that active sites become available for further reactant molecules.

CO and NO can adsorb onto the solid surface, react there and then desorb as products

Correct. The solid catalyst provides active sites for adsorption and surface reaction, followed by desorption of CO₂ and N₂.

The catalyst increases the equilibrium constant for the reaction

A catalyst changes reaction rates but does not change an equilibrium constant at a fixed temperature.

Consider the mechanism:

SO₂ + NO₂ → SO₃ + NO
NO + ½O₂ → NO₂

Which statement correctly identifies NO₂ and NO?

NO₂ is the catalyst and NO is an intermediate

Correct. NO₂ is consumed in the first step and regenerated in the second. NO is formed in the first step and consumed in the second.

NO is the catalyst and NO₂ is an intermediate

This reverses their roles in the mechanism as written. NO₂ is present first and regenerated later.

Both NO and NO₂ are products of the overall reaction

Both species cancel when the elementary equations are added, so neither appears in the overall reaction.

Both NO and NO₂ are consumed overall

They cancel from the overall equation. NO₂ is regenerated and NO is produced and then consumed.

The following two reactions form a catalytic cycle:

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

What shows that the iron species is acting catalytically?

Fe²⁺ is converted permanently into Fe³⁺

Fe³⁺ is converted back into Fe²⁺ in the second step. The conversion is therefore not permanent.

Iron appears in the overall equation as a product

The Fe²⁺ and Fe³⁺ terms cancel when the two elementary equations are added.

The iron ions cycle between Fe²⁺ and Fe³⁺ and are regenerated overall

Correct. Fe²⁺ is converted into Fe³⁺ and Fe³⁺ is converted back into Fe²⁺. The iron redox couple therefore provides a catalytic cycle without being consumed overall.

Iron changes S₂O₈²⁻ into I⁻

S₂O₈²⁻ is reduced to SO₄²⁻, while I⁻ is oxidised to I₂. The iron ions transfer electrons between the two redox processes.

Why can the Fe²⁺/Fe³⁺ pathway increase the rate of the reaction between S₂O₈²⁻ and I⁻?

It makes S₂O₈²⁻ and I⁻ lose their negative charges before reacting

The ions retain their identities until they undergo the redox reactions. Their charges are not simply removed beforehand.

It replaces the direct reaction between two negative ions with steps involving oppositely charged ions

Correct. S₂O₈²⁻ reacts with Fe²⁺ and I⁻ reacts with Fe³⁺. These alternative electron-transfer steps avoid the less favourable direct approach of two negatively charged reactants.

It changes the overall reaction into an equilibrium reaction

Catalysis changes the pathway. It does not need to convert the overall reaction into an equilibrium process.

It increases the concentration of S₂O₈²⁻ and I⁻

The catalyst provides an alternative mechanism; its effect does not depend on increasing the initial concentrations of the two reactants.

A reaction occurs by the mechanism:

P + C → PC
PC + Q → R + C

Which statement is correct?

PC is the catalyst because it cancels from the overall equation

Cancellation alone does not identify a catalyst. PC is formed in the first step and consumed in the second, so it is an intermediate.

C is an intermediate because it is present in both steps

C is consumed first and regenerated later. That pattern identifies it as a catalyst, not an intermediate.

Both C and PC are catalysts because both cancel

They have different roles. C is consumed and regenerated, whereas PC is formed and then consumed.

C is the catalyst and PC is an intermediate

Correct. C is consumed in the first step and regenerated in the second. PC is produced in the first step and consumed in the second.

An enzyme-catalysed reaction becomes very slow after the enzyme is heated strongly. Which explanation is most appropriate?

The enzyme has been denatured, changing the shape of its active site

Correct. High temperature can disrupt interactions maintaining the enzyme's three-dimensional structure. The substrate can then no longer bind effectively to the altered active site.

The enzyme has shifted the reaction equilibrium completely towards the reactants

Enzymes are catalysts and do not alter the equilibrium position. The decrease in rate results from loss of the functional active-site shape.

All substrate molecules have lost their kinetic energy

Heating does not remove kinetic energy from substrate molecules. The key problem is structural change to the enzyme.

The enzyme has been permanently converted into the reaction product

An enzyme normally acts as a catalyst and is regenerated. Denaturation changes its structure rather than converting it into the reaction product.

Frequently asked questions

Questions students often ask about catalysis

These questions connect the different catalytic mechanisms to activation energy, equilibrium and reaction kinetics — the areas where the terminology can easily become mixed up.

Does a catalyst actually lower the activation energy?

The most precise description is that a catalyst provides an alternative pathway with a lower activation energy. The original uncatalysed pathway and its activation energy still exist. Reactants simply have access to another, easier route to products.

Why does the Boltzmann curve stay the same when a catalyst is added?

The Boltzmann distribution depends on temperature. If a catalyst is added without changing the temperature, the energies of the particles are still distributed in the same way. What changes is the activation energy required for the catalysed pathway.

Does a catalyst increase the equilibrium yield of a product?

No. A catalyst increases the rates of both the forward and reverse reactions. The same equilibrium position is reached more quickly, but the equilibrium composition at a fixed temperature is unchanged.

Why must products desorb from a heterogeneous catalyst?

If products remained attached to the catalyst surface, they would continue occupying active sites. Desorption frees those sites so that new reactant particles can adsorb and another catalytic cycle can occur.

Does stronger adsorption always make a catalyst more effective?

No. Reactants need to interact with the surface strongly enough for bonds to be weakened and reaction to occur, but products must also be able to leave. If a species binds too strongly and remains on the surface, it can block active sites and reduce catalytic activity.

Why is adsorption different from absorption?

Adsorption occurs when particles become attached to a surface. Absorption involves particles entering the bulk of another material. Heterogeneous catalysis depends on adsorption because the catalytic reaction occurs at surface active sites.

How can I tell whether a species is a catalyst or an intermediate?

Look at the order in which it appears. A catalyst is consumed in an earlier step and regenerated in a later step. An intermediate is formed in an earlier step and consumed later. Both may disappear when the elementary equations are added, so cancellation alone is not enough.

Are Fe²⁺ and Fe³⁺ both involved in the same catalytic cycle?

Yes. The iron species cycles between the +2 and +3 oxidation states. Fe²⁺ can be oxidised to Fe³⁺ by S₂O₈²⁻, and Fe³⁺ can then be reduced back to Fe²⁺ by I⁻. The Fe²⁺/Fe³⁺ redox couple therefore provides the alternative pathway and is regenerated overall.

Why are variable oxidation states useful in homogeneous catalysis?

A species that can change readily between oxidation states can accept electrons in one step and donate them in another. This allows electron transfer to occur through a sequence of lower-energy reactions while the original catalytic oxidation state is regenerated.

Can a catalyst appear in a mechanism even though it is absent from the overall equation?

Yes. A catalyst participates directly in individual elementary steps. Because it is regenerated later, the catalyst appears on both sides when the elementary equations are added and therefore cancels from the overall equation.

Why are enzymes usually specific to particular substrates?

An enzyme has an active site with a particular three-dimensional shape. Only substrates with a suitable complementary shape can bind effectively, so an enzyme normally catalyses a limited range of reactions.

Why can both high temperature and unsuitable pH reduce enzyme activity?

Both can disrupt interactions that maintain the enzyme's three-dimensional structure. If the shape of the active site changes, the substrate binds less effectively and the catalytic reaction becomes slower.

Catalysis becomes much easier to understand when every example is traced back to the same central idea: an alternative pathway with a lower activation energy. Heterogeneous catalysts create that pathway through adsorption, reaction and desorption at a surface, while homogeneous catalysts take part in reaction mechanisms and are regenerated through catalytic cycles. Enzymes apply the same principle to biological reactions with remarkable specificity. Once you can identify how the alternative pathway is created and how the catalyst is regenerated, unfamiliar catalysis questions become much more systematic.