The rate and extent of chemical change: catalysts and their action

What you'll learn

This revision guide covers everything you need to know about catalysts and their action for AQA GCSE Chemistry. You'll understand how catalysts increase reaction rates without being used up, how they work at a molecular level, and their crucial role in industrial processes and biological systems. This topic links directly to reaction rates and is essential for both Paper 1 and Paper 2.

Key terms and definitions

Catalyst — a substance that increases the rate of a chemical reaction without being chemically changed or used up by the reaction

Activation energy — the minimum amount of energy that colliding particles must have for a reaction to occur

Enzyme — a biological catalyst made from protein that speeds up reactions in living organisms

Active site — the specific region on an enzyme where substrate molecules bind and the reaction occurs

Transition metal — an element from the central block of the periodic table (such as iron, nickel, or platinum) that commonly acts as a catalyst

Surface area — the amount of exposed area of a solid substance; catalysts often work by providing a large surface for reactions to occur

Inhibitor — a substance that slows down or prevents a catalytic reaction by blocking the active site or interfering with the catalyst

Turnover — the ability of a catalyst to be used repeatedly in multiple reaction cycles without being consumed

Core concepts

How catalysts work

Catalysts work by providing an alternative reaction pathway with a lower activation energy than the uncatalysed reaction. This means more colliding particles have sufficient energy to react, so the reaction rate increases.

Key points about catalysts:

• They are not used up during the reaction
• They can be recovered chemically unchanged at the end
• Only small amounts are needed because they work repeatedly
• They do not change the products of a reaction
• They do not change the position of equilibrium (in reversible reactions)
• They help the reaction reach equilibrium faster
• Different reactions require different catalysts — they are specific

The catalyst remains unchanged at the end of the reaction. This means it has the same mass and chemical properties, though it may appear physically different (for example, the surface might look worn).

Energy profiles and catalysts

Energy profile diagrams show how energy changes during a chemical reaction. When a catalyst is present, the diagram shows a lower activation energy peak.

For an exothermic reaction with a catalyst:

• The reactants start at a higher energy level
• The curve rises to a lower peak (lower activation energy)
• The products end at the same energy level as without a catalyst
• The overall energy change (ΔH) remains the same
• The difference is only in the height of the activation energy barrier

For an endothermic reaction with a catalyst:

• The reactants start at a lower energy level
• The curve rises to a lower peak than without a catalyst
• The products end at the same higher energy level
• Again, ΔH is unchanged
• Only the activation energy is reduced

This is crucial: catalysts do not change the energy of reactants or products, only the activation energy.

Types of catalysts

Heterogeneous catalysts

These are catalysts in a different physical state (phase) from the reactants. Most industrial catalysts are heterogeneous — typically solid catalysts with gaseous or liquid reactants.

Examples you must know:

• Iron in the Haber process (making ammonia): N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
• Platinum or palladium in catalytic converters in car exhausts
• Vanadium(V) oxide in the Contact process (making sulfuric acid)
• Nickel in hydrogenation reactions (converting vegetable oils to margarine)

Heterogeneous catalysts work by:

1. Reactant molecules adsorbing (sticking) onto the catalyst surface
2. Bonds in reactant molecules weakening
3. Atoms or molecules reacting on the surface
4. Product molecules desorbing (leaving) the surface
5. The catalyst surface becoming available for more reactions

Homogeneous catalysts

These are catalysts in the same physical state as the reactants. They are less common at GCSE level but important to understand.

Example: Using acids as catalysts in esterification reactions where both the acid catalyst and organic reactants are liquids.

Biological catalysts (enzymes)

Enzymes are protein molecules that act as catalysts in living organisms. They are highly specific — each enzyme catalyses only one type of reaction.

Key features of enzymes:

• Made from long chains of amino acids folded into specific 3D shapes
• Have an active site with a particular shape
• Only substrate molecules with a complementary shape can bind to the active site
• Often described using the "lock and key" model
• Work best at optimum temperature (around 37°C for human enzymes)
• Work best at optimum pH (varies for different enzymes)
• Can be denatured (permanently damaged) by high temperatures or extreme pH values

When denatured, the enzyme's active site changes shape permanently, so substrate molecules can no longer bind. The enzyme can no longer function as a catalyst.

Examples of enzyme-catalysed reactions:

• Amylase breaks down starch into sugars (in digestion)
• Catalase breaks down hydrogen peroxide into water and oxygen (in cells)
• Proteases break down proteins into amino acids (in digestion)

Industrial importance of catalysts

Catalysts are essential in industrial chemical production because they:

• Increase the rate of reaction, producing more product per unit time
• Allow reactions to occur at lower temperatures, reducing energy costs
• Reduce the carbon footprint by requiring less fuel combustion
• Can be used repeatedly, making processes more economical
• Improve sustainability of chemical manufacturing

The Haber process

This produces ammonia (NH₃) for fertilizers:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Conditions:

• Temperature: approximately 450°C
• Pressure: approximately 200 atmospheres
• Catalyst: iron

Without the iron catalyst, this reaction would be far too slow at 450°C. The catalyst allows a reasonable rate at a relatively low temperature (considering the reaction).

Catalytic converters

Catalytic converters in vehicle exhaust systems use platinum, palladium, and rhodium catalysts to convert harmful gases into less harmful ones:

• Carbon monoxide → carbon dioxide: 2CO + O₂ → 2CO₂
• Nitrogen oxides → nitrogen: 2NO → N₂ + O₂
• Unburned hydrocarbons → carbon dioxide and water

The precious metal catalysts are coated on a ceramic honeycomb structure to provide a large surface area.

Catalyst poisoning

Catalysts can be poisoned by impurities that bind strongly to the catalyst surface, blocking active sites. This reduces the catalyst's effectiveness.

For example:

• Sulfur compounds poison iron catalysts in the Haber process
• Lead compounds poison catalytic converters (which is why unleaded petrol must be used)

Poisoned catalysts may need to be replaced, increasing costs and waste.

Worked examples

Example 1: Energy profile interpretation

Question: The diagram shows an energy profile for a reaction with and without a catalyst.

[Energy profile showing two curves - one higher peak (uncatalysed) and one lower peak (catalysed), with reactants at 50 kJ and products at 30 kJ]

a) What is the activation energy without a catalyst? (1 mark) b) What is the activation energy with a catalyst? (1 mark) c) Is this reaction exothermic or endothermic? Explain your answer. (2 marks) d) Explain why the catalyst increases the rate of reaction. (2 marks)

Model answers:

a) 90 kJ (or 40 kJ above reactants) ✓

b) 60 kJ (or 10 kJ above reactants) ✓

c) Exothermic ✓ because the products have less energy than the reactants / energy is released ✓

d) The catalyst lowers the activation energy ✓ so more particles have sufficient energy to react when they collide / a greater proportion of collisions result in a reaction ✓

Example 2: Catalysts in industry

Question: Ammonia is manufactured using the Haber process.

a) Name the catalyst used in the Haber process. (1 mark) b) State two advantages of using a catalyst in this industrial process. (2 marks) c) Explain why only a small mass of catalyst is needed. (2 marks)

Model answers:

a) Iron ✓

b) Any two from:

• Increases the rate of reaction / produces ammonia faster ✓
• Allows a lower temperature to be used / reduces energy costs ✓
• Reduces fuel consumption / reduces carbon emissions ✓
• Makes the process more economical / profitable ✓

c) The catalyst is not used up / consumed in the reaction ✓ so it can be used repeatedly / for many reaction cycles ✓

Example 3: Enzymes

Question: Enzymes are biological catalysts.

a) Describe the structure of an enzyme molecule. (2 marks) b) Explain why enzymes are specific to particular reactions. (2 marks) c) A student investigates the effect of temperature on an enzyme. At 70°C, the enzyme stops working. Explain why. (3 marks)

Model answers:

a) Enzymes are proteins ✓ made from chains of amino acids folded into a specific three-dimensional shape / with a specific active site ✓

b) The active site has a specific shape ✓ that only substrate molecules with a complementary shape can fit into / bind to ✓

c) High temperature denatures the enzyme ✓ the active site changes shape (permanently) ✓ so substrate molecules can no longer bind / fit / the enzyme cannot catalyse the reaction ✓

Common mistakes and how to avoid them

• Mistake: Saying catalysts are "used up slowly" or "consumed in small amounts" Correction: Catalysts are NOT used up at all — they can be recovered chemically unchanged at the end of the reaction

• Mistake: Stating that catalysts change the products of a reaction Correction: Catalysts only affect the rate — they never change what products are formed

• Mistake: Claiming catalysts provide energy for reactions Correction: Catalysts lower the activation energy needed; they don't supply energy themselves

• Mistake: Drawing energy profiles where the catalyst changes the energy level of products or reactants Correction: Only the height of the activation energy peak changes; start and end points remain the same

• Mistake: Confusing "active site" with "activation energy" Correction: Active site is a physical location on an enzyme; activation energy is the minimum energy needed for a reaction

• Mistake: Writing that catalysts "speed up particles" or "make particles move faster" Correction: Catalysts provide an alternative pathway with lower activation energy; they don't affect particle speed (that's temperature)

Exam technique for "The rate and extent of chemical change: catalysts and their action"

• Command word "explain": You must give reasons or mechanisms. For catalysts, always mention alternative pathway AND lower activation energy AND why this increases rate (more successful collisions)

• Energy profile questions: Always label both axes (energy on y-axis, progress of reaction on x-axis), mark activation energy clearly with arrows, and show the catalyst pathway as a lower peak but same start/end points

• Industrial catalyst questions: Name the specific catalyst (iron for Haber, platinum/palladium/rhodium for catalytic converters), state advantages (faster rate, lower temperature, reduced costs), and link to sustainability where appropriate

• Enzyme questions: Mention protein structure, active site, specificity, and denaturation. Use correct terminology — "complementary shape" not "same shape," "denature" not "die" or "break"

Quick revision summary

Catalysts increase reaction rates by providing an alternative pathway with lower activation energy without being used up. They work repeatedly in small amounts and don't change products or energy changes. Industrial catalysts (like iron in the Haber process) reduce costs and environmental impact. Enzymes are biological catalysts with specific active sites that can be denatured by extreme conditions. Energy profiles show catalysts lowering the activation energy peak while keeping reactant and product energy levels unchanged.