What Is a Catalyst?

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

Catalysts work by providing an alternative reaction pathway with a lower activation energy, Eₐ. This allows a greater proportion of reacting particles to overcome the energy barrier and react successfully.

A catalyst may participate in intermediate steps of a reaction, but it is regenerated before the overall reaction is complete.

In simple terms:

Catalyst added → alternative pathway → lower activation energy → faster reaction

Catalysts Lower Activation Energy

Every chemical reaction has an energy barrier called the activation energy. Reacting particles must overcome this barrier before products can form.

A catalyst provides another route from reactants to products that has a lower activation-energy barrier.

Notice something very important in these reaction profiles.

The catalyzed and uncatalyzed pathways:

• start at the same reactant energy
• finish at the same product energy
• have different activation energies

Therefore:

Eₐ(catalyzed) < Eₐ(uncatalyzed)

The catalyst changes the pathway, not the beginning or end of the reaction.

What Does a Catalyst Change?

A catalyst changes the reaction pathway, activation energy and reaction rate.

It does not change:

• the energy of the reactants
• the energy of the products
• the overall enthalpy change, ΔH
• whether the reaction is exothermic or endothermic
• the equilibrium constant or equilibrium position

The catalyzed pathway may also contain additional intermediate steps and transition states.

Worked Example: Activation Energy

Suppose a reaction has:

Reactant energy = 60 kJ mol⁻¹

Uncatalyzed peak = 180 kJ mol⁻¹

Therefore:

Eₐ = 180 − 60

Eₐ = 120 kJ mol⁻¹

Now a catalyst provides a pathway whose highest point is only 110 kJ mol⁻¹.

Eₐ = 110 − 60

Eₐ = 50 kJ mol⁻¹

The catalyst has lowered the activation energy by:

120 − 50 = 70 kJ mol⁻¹

Why Does Lower Activation Energy Increase Reaction Rate?

At any particular temperature, particles do not all have exactly the same kinetic energy.

Some have relatively low energies, while others have enough energy to overcome the activation-energy barrier.

If the activation energy is lowered, more particles have sufficient energy to react successfully.

Therefore:

lower Eₐ → more successful collisions → faster reaction

Importantly, the catalyst has not given the particles extra kinetic energy. It has lowered the energy barrier they must overcome.

Catalysts and Collision Theory

According to collision theory, particles generally need to:

• collide
• collide with sufficient energy
• collide with an appropriate orientation

A catalyst increases reaction rate because its alternative mechanism requires a lower activation energy.

Some catalysts can also help arrange reacting particles in favourable positions.

A Different Reaction Pathway

It is tempting to imagine a catalyst simply pushing down the top of the original energy curve.

In reality, a catalyst can provide an entirely different reaction mechanism.

The catalyzed mechanism may involve:

• different intermediate substances
• different transition states
• additional reaction steps
• several smaller energy barriers rather than one large barrier

This is why some catalyzed reaction profiles contain several smaller peaks.

Why Isn't a Catalyst Consumed?

Consider this simplified mechanism:

A + catalyst → intermediate

Then:

intermediate + B → products + catalyst

The catalyst appears during the reaction, but it is produced again during a later step.

When the steps are combined:

A + B → products

The catalyst cancels from the overall equation.

So saying that a catalyst is "not consumed" does not mean it never participates in the reaction. It means it is regenerated overall.

Homogeneous and Heterogeneous Catalysts

A homogeneous catalyst is in the same phase as the reactants.

For example:

reactants in solution + catalyst in solution

A heterogeneous catalyst is in a different phase from the reactants.

A common industrial situation is:

gaseous reactants + solid catalyst

The reaction then occurs on the surface of the solid catalyst.

The image compares homogeneous catalysis with a heterogeneous reaction occurring on a catalyst surface.

How a Heterogeneous Catalyst Works

A simplified surface-catalyzed reaction involves several stages:

1. Reactant particles approach the catalyst.
2. Reactants attach to the surface.
3. Bonds in the reactants may weaken.
4. The particles react.
5. Products form.
6. Products leave the surface.
7. The catalyst surface becomes available again.

Attachment to the surface is called adsorption.

Release from the surface is called desorption.

Active Sites and Surface Area

Reactions do not necessarily occur equally well everywhere on a catalyst.

Particular locations where catalytic interactions occur are called active sites.

Increasing the surface area of a solid catalyst can expose more active sites.

Therefore:

greater surface area → more exposed active sites → more simultaneous catalytic interactions

This is why industrial catalysts are often porous or finely divided rather than simply being large solid blocks.

Catalysts in Industry

Catalysts are extremely important in industrial chemistry because manufacturers need reactions that are fast enough to be practical while controlling energy costs and unwanted products.

Three important examples are:

• iron-based catalysts in the Haber process
• vanadium(V) oxide in the Contact process
• platinum-group metals in catalytic converters

The Haber Process

The Haber process manufactures ammonia:

N₂ + 3H₂ ⇌ 2NH₃

An iron-based catalyst is used.

Nitrogen molecules contain the extremely strong N≡N triple bond. Breaking and rearranging these bonds involves a substantial activation-energy barrier.

On the catalyst surface, nitrogen and hydrogen can be adsorbed and react through a lower-energy mechanism.

The catalyst therefore allows ammonia to be produced at a much more useful industrial rate.

Catalysts and Equilibrium

The Haber reaction is reversible.

A catalyst speeds up both the forward and reverse reactions.

This means equilibrium is reached more quickly.

However:

a catalyst does not shift the equilibrium position.

It changes how quickly equilibrium is reached, not where the equilibrium eventually lies.

The Contact Process

Another important industrial example occurs during the manufacture of sulfuric acid.

One stage of the Contact process is:

2SO₂ + O₂ ⇌ 2SO₃

A commonly used catalyst is vanadium(V) oxide, V₂O₅.

The catalyst allows sulfur dioxide and oxygen to react at a useful industrial rate. The sulfur trioxide produced is then used in later stages of sulfuric acid manufacture.

Catalytic Converters

Catalysis is also used every day in vehicle exhaust systems.

A catalytic converter contains catalytic materials that commonly include platinum, palladium and rhodium.

Notice the honeycomb structure inside the converter.

The honeycomb provides a very large surface area while still allowing exhaust gases to pass through.

Catalytic converters help convert pollutants such as:

• carbon monoxide
• nitrogen oxides
• unburned hydrocarbons

into less harmful substances.

This is an excellent real-world example of heterogeneous catalysis.

Catalysts and Green Chemistry

Catalysts can make industrial chemistry more sustainable.

Using an effective catalyst may allow:

• lower operating temperatures
• reduced energy consumption
• faster production
• improved selectivity
• fewer unwanted by-products
• less chemical waste

Finding better catalysts is therefore an important area of green chemistry.

Enzymes: Biological Catalysts

Living organisms also depend on catalysts.

Biological catalysts are called enzymes.

Most enzymes are proteins. They allow biochemical reactions to occur rapidly enough to sustain life under relatively mild biological conditions.

Enzymes obey the same fundamental energetic principle as other catalysts:

they lower activation energy.

Why Do Living Organisms Need Enzymes?

Cells cannot simply heat themselves to hundreds of degrees Celsius whenever they need a chemical reaction to happen faster.

Instead, they use enzymes.

Enzymes are involved in processes such as:

• digestion
• cellular respiration
• photosynthesis
• DNA replication
• protein synthesis
• metabolism
• cell repair
• detoxification

Without enzymes, many of these reactions would occur far too slowly to sustain life.

Enzyme, Substrate and Active Site

The reactant acted upon by an enzyme is called the substrate.

An enzyme contains a particular region called its active site.

The substrate interacts with the active site to form a temporary enzyme-substrate complex.

The shape and chemical properties of the active site help determine which substrates can interact effectively with the enzyme.

The Enzyme Reaction Cycle

A simplified enzyme-controlled reaction can be represented as:

enzyme + substrate

↓

enzyme-substrate complex

↓

reaction occurs

↓

products form

↓

products leave

↓

enzyme available again

Symbolically:

E + S ⇌ ES → E + P

where:

E = enzyme
S = substrate
ES = enzyme-substrate complex
P = product

The enzyme can then catalyze another reaction.

Lock-and-Key and Induced Fit

Two models are commonly used to explain enzyme-substrate interactions.

The lock-and-key model treats the active site as having a shape complementary to the substrate.

The induced-fit model recognizes that enzymes are flexible. As a substrate begins to bind, the active site can change shape slightly to interact more effectively with it.

The lock-and-key model is useful for introducing enzyme specificity, but the induced-fit model gives a more realistic picture of many enzyme interactions.

How Do Enzymes Lower Activation Energy?

Enzymes can lower activation energy in several ways.

Depending on the enzyme, the active site may:

• bring substrates close together
• orient substrates correctly
• strain particular chemical bonds
• provide acidic or basic groups
• create a favourable local environment
• stabilize the transition state

The enzyme is therefore doing more than simply "holding" the substrate.

Examples of Enzymes

Catalase

One particularly useful example is catalase.

Hydrogen peroxide can be produced during cellular metabolism. Because it can damage cells, organisms need to break it down efficiently.

Catalase speeds up:

2H₂O₂ → 2H₂O + O₂

The oxygen produced can be observed as bubbles.

Investigating Catalase

Catalase activity can be investigated experimentally using materials such as potato or liver.

Hydrogen peroxide is added to the catalase-containing material, and the oxygen produced can be collected.

The volume of oxygen produced over a measured period can be used to determine reaction rate.

For example:

reaction rate = volume of O₂ produced ÷ time

Raw and Cooked Potato

A simple catalase experiment can also demonstrate enzyme denaturation.

Raw potato contains active catalase.

If potato is heated strongly, the structure of the enzyme can be disrupted.

When hydrogen peroxide is added, untreated potato typically produces more vigorous oxygen bubbling than heavily heated potato because heating can denature the catalase.

Temperature and Enzyme Activity

Temperature has a major effect on enzyme-controlled reactions.

At relatively low temperatures, particles have less kinetic energy and enzyme-substrate collisions occur less frequently.

As temperature rises, collision frequency increases and the reaction generally becomes faster.

Eventually an optimum temperature is reached.

Above the optimum, activity can decrease rapidly because excessive heat disrupts the enzyme's three-dimensional structure.

Importantly, there is no universal optimum temperature for all enzymes. Different enzymes and organisms are adapted to different conditions.

Enzyme Denaturation

An enzyme's function depends on its three-dimensional structure.

High temperatures can disrupt interactions that maintain this structure.

The active site changes shape, so the substrate may no longer bind effectively.

This process is called denaturation.

Therefore:

high temperature → structural change → active site altered → fewer successful interactions → lower enzyme activity

Low temperature is different. It generally slows an enzyme-controlled reaction but does not necessarily denature the enzyme.

Temperature and Catalysts Are Different

Increasing temperature and adding a catalyst can both increase reaction rate, but they work differently.

Increasing temperature → increases particle kinetic energy

Adding a catalyst → lowers activation energy

Heating does not normally lower the activation energy of a particular reaction pathway.

pH and Enzyme Activity

Enzymes can also be strongly affected by pH.

Different enzymes have different optimum pH values.

Digestive enzymes provide good examples.

Pepsin operates effectively under acidic conditions in the stomach.

Salivary amylase operates closer to neutral conditions.

Enzymes operating farther along the digestive system may function best under more alkaline conditions.

Changing pH can affect electrical charges and interactions within an enzyme, which can alter its shape and activity.

Enzymes in Industry

Enzymes are not restricted to living cells. Humans use them extensively in technology and industry.

Applications include:

• biological detergents
• cheese making
• baking
• brewing
• fruit juice production
• lactose-free dairy products
• pharmaceuticals
• biotechnology

One major advantage is specificity. An enzyme can sometimes produce a desired chemical change while minimizing unwanted side reactions.

Lactase and Lactose-Free Milk

Lactase catalyzes the breakdown of lactose.

Lactose is converted into the simpler sugars glucose and galactose.

This reaction can be used commercially to manufacture lactose-free milk.

Immobilized Enzymes

Industrial enzymes can sometimes be attached to or trapped within a solid material. These are called immobilized enzymes.

One method traps enzymes inside small alginate beads.

For example, lactase-containing beads can be packed into a column.

Milk enters the column and flows around the beads.

As it passes through:

lactose → glucose + galactose

The lactase remains in the column while the treated milk flows out.

Advantages of Immobilized Enzymes

Immobilized enzymes can offer several advantages:

• the enzyme can be reused
• enzyme and product are easier to separate
• continuous processing is possible
• product contamination by enzyme is reduced
• the enzyme may be more stable under some conditions

However, immobilization can also require specialized equipment and may sometimes reduce the rate at which substrate reaches the enzyme.

Catalysts Can Lose Activity

Although catalysts are not consumed in the intended overall reaction, they do not necessarily work forever.

Catalysts can become less effective because of:

• contamination
• overheating
• structural changes
• chemical damage
• blockage of active sites

This is called catalyst deactivation.

Catalyst Poisoning

If another substance binds strongly to the active sites of a heterogeneous catalyst, reactants may no longer be able to adsorb there.

This is known as catalyst poisoning.

The sequence is:

impurity attaches → active sites blocked → less adsorption → fewer catalytic reactions → lower reaction rate

This does not contradict the statement that catalysts are not consumed overall. The catalyst has instead become deactivated.

Enzyme Inhibition

Enzymes can also have their activity reduced by substances called inhibitors.

An inhibitor may:

• block the active site
• compete with the substrate
• bind elsewhere on the enzyme
• change enzyme shape
• interfere with the catalytic mechanism

Enzyme inhibition is important in medicine, metabolism and biochemical regulation.

Comparing Industrial Catalysts and Enzymes

Despite their differences, both rely on the same fundamental idea:

alternative reaction pathway → lower activation energy → faster reaction

Worked Example: Catalyst and ΔH

Consider a reaction with:

Reactants = 80 kJ mol⁻¹

Products = 30 kJ mol⁻¹

Uncatalyzed peak = 220 kJ mol⁻¹

Catalyzed peak = 140 kJ mol⁻¹

Without catalyst:

Eₐ = 220 − 80

Eₐ = 140 kJ mol⁻¹

With catalyst:

Eₐ = 140 − 80

Eₐ = 60 kJ mol⁻¹

But the enthalpy change remains:

ΔH = 30 − 80

ΔH = −50 kJ mol⁻¹

Therefore, the catalyst has changed Eₐ, but it has not changed ΔH.

Did You Know?

An iron-based catalyst inside an ammonia plant and an enzyme inside one of your cells may look completely different, but energetically they perform the same fundamental job.

Both provide a reaction mechanism with a lower activation-energy barrier.

That makes catalysis an important link between chemical energetics, reaction kinetics, industrial chemistry and biology.

Common Misconceptions

"A catalyst gives particles more energy."

No. A catalyst lowers the activation-energy barrier.

"A catalyst changes ΔH."

No. The energy difference between reactants and products remains unchanged.

"A catalyst never participates in the reaction."

It may participate in intermediate steps, but it is regenerated overall.

"A catalyst shifts equilibrium."

No. It allows equilibrium to be reached faster.

"A catalyst lasts forever."

Not necessarily. Catalysts can become poisoned or otherwise deactivated.

"Enzymes supply energy to cells."

No. Enzymes lower activation energy.

"All enzymes work best at 37°C."

No. Optimum temperature varies considerably between enzymes and organisms.

"All enzymes work best at pH 7."

No. Different enzymes can have very different optimum pH values.

Key Terms

Catalyst – A substance that increases reaction rate without being consumed overall.

Activation energy, Eₐ – The energy barrier that must be overcome for a reaction to occur.

Alternative pathway – A different reaction route with a lower activation-energy barrier.

Reaction mechanism – The sequence of elementary stages through which a reaction occurs.

Homogeneous catalyst – A catalyst in the same phase as the reactants.

Heterogeneous catalyst – A catalyst in a different phase from the reactants.

Active site – A location where catalytic interactions occur.

Adsorption – Attachment of particles to a surface.

Desorption – Release of particles from a surface.

Enzyme – A biological catalyst.

Substrate – A reactant acted upon by an enzyme.

Enzyme-substrate complex – The temporary combination formed when a substrate interacts with an enzyme.

Induced fit – A model in which substrate binding causes a change in the enzyme's active site.

Denaturation – Loss of an enzyme's functional three-dimensional structure.

Immobilized enzyme – An enzyme attached to or trapped within a solid support.

Catalyst poisoning – Reduction in catalytic activity caused by substances blocking or altering active sites.

Inhibitor – A substance that reduces enzyme activity.

Key Takeaways

• Catalysts increase reaction rate without being consumed overall.
• They provide an alternative reaction pathway with lower activation energy.
• Lower Eₐ allows more reacting particles to overcome the energy barrier.
• Catalysts do not give particles extra kinetic energy.
• Catalysts do not change ΔH.
• Catalysts may participate in intermediate stages but are regenerated.
• Heterogeneous catalysts often operate through adsorption at surface active sites.
• Greater catalyst surface area can expose more active sites.
• Important industrial examples include the Haber process, Contact process and catalytic converters.
• Catalysts speed both forward and reverse reactions, allowing equilibrium to be reached faster without shifting its position.
• Enzymes are biological catalysts.
• Enzymes have active sites that interact with substrates.
• The induced-fit model recognizes that an enzyme can change shape during substrate binding.
• Temperature and pH can strongly affect enzyme activity.
• Excessive heating can cause enzyme denaturation.
• Immobilized enzymes can be retained and reused in industrial processes.
• Catalysts can become deactivated even though they are not consumed by the intended reaction.
• Catalysis connects energetics, kinetics, industry and biology.