Catalysts | Chemistry

📖 In-Depth Theory

What is a Catalyst?

A CATALYST is a substance that INCREASES THE RATE of a reaction WITHOUT being used up or permanently changed itself.

Key properties:

Increases rate — provides a reaction pathway with LOWER ACTIVATION ENERGY.

Not used up — the catalyst is the same at the end of the reaction as at the start.

Present in small quantities — a small amount can catalyse a large quantity of reactants.

SPECIFIC — each catalyst only works for particular reactions (or types of reaction).

How catalysts work:

They provide an ALTERNATIVE REACTION PATHWAY with a lower activation energy.

A greater proportion of collisions now have energy ≥ the lower activation energy → faster rate.

The overall energy change (ΔH) is not affected — same reactants and products, same energy difference.

Types of Catalyst

HETEROGENEOUS CATALYSTS:

In a DIFFERENT PHASE from the reactants.

Usually a solid catalyst with gaseous or liquid reactants.

Reaction occurs on the SURFACE of the catalyst.

Example: IRON catalyst in the Haber process (N₂ + 3H₂ ⇌ 2NH₃) — solid iron, gaseous reactants.

Example: PLATINUM/RHODIUM in catalytic converters — converts CO, NO and hydrocarbons to CO₂, N₂ and H₂O.

Example: NICKEL in margarine production — hydrogenation of vegetable oils.

HOMOGENEOUS CATALYSTS:

In the SAME PHASE as the reactants (usually both dissolved in solution).

Example: hydrogen peroxide decomposition — Fe²⁺ ions in solution catalyse it.

BIOLOGICAL CATALYSTS (ENZYMES):

Protein molecules that catalyse specific biochemical reactions.

Example: amylase — catalyses starch hydrolysis. Catalase — decomposes H₂O₂.

Highly specific — one enzyme, one reaction.

Sensitive to temperature and pH — denature above ~40°C.

Industrial Importance of Catalysts

Catalysts are enormously important in industry:

HABER PROCESS (making ammonia for fertilisers):

N₂ + 3H₂ ⇌ 2NH₃ — iron catalyst.

Without catalyst: reaction too slow to be economical at practical temperatures.

With iron catalyst: reaction is fast enough at ~450°C.

CONTACT PROCESS (making sulfuric acid):

2SO₂ + O₂ ⇌ 2SO₃ — vanadium pentoxide (V₂O₅) catalyst.

CATALYTIC CONVERTERS in cars:

Platinum and rhodium catalyse conversion of toxic exhaust gases.

ECONOMIC AND ENVIRONMENTAL BENEFITS of catalysts:

Reactions can be run at LOWER TEMPERATURES → less energy needed → lower cost.

FEWER BY-PRODUCTS in many cases → cleaner processes.

Smaller reactors needed → lower capital cost.

Catalysts can be RECOVERED and REUSED → economical.

Disadvantage: catalysts can be POISONED — impurities in the reaction mixture bond to the catalyst surface and block active sites.

⚠️ Common Mistake

A catalyst is NOT used up — it is the same at the END of the reaction as at the START. It does NOT change the products or the overall energy change (ΔH). It ONLY provides an alternative pathway with LOWER ACTIVATION ENERGY — making the reaction faster, not different.

📌 Key Note

Catalyst: increases rate, not used up, lowers activation energy, does not change ΔH. Heterogeneous: different phase (solid catalyst, gas reactants). Biological: enzymes. Industrial: iron (Haber), platinum (catalytic converter), V₂O₅ (Contact process). Lower temperature possible → energy saving.

🎯 Matching Activity — Match the Catalyst to its Reaction

Match each catalyst to the industrial reaction it is used in. — drag the symbols on the right to match the component names on the left.

Iron

Drop here

Vanadium pentoxide (V₂O₅)

Drop here

Platinum / rhodium

Drop here

Nickel

Drop here

Amylase (enzyme)

Drop here

Hydrogenation — converting unsaturated oils to solid fats (margarine)

Contact process — 2SO₂ + O₂ ⇌ 2SO₃ — step in making sulfuric acid

Haber process — N₂ + 3H₂ ⇌ 2NH₃ — making ammonia for fertilisers

Biological catalyst — breaks down starch to sugars in digestion

Catalytic converter — converts CO and NOₓ to CO₂ and N₂

⭐ Higher Tier Only

Heterogeneous catalysts: reactants adsorb onto surface → bonds weaken → reaction occurs at lower Ea → products desorb. Catalyst poisoning: impurities block active sites permanently. Enzymes as biological catalysts: highly specific, optimum temperature and pH, denature above ~40°C.

🎯 Test Yourself

Question 1 of 2

1. A catalyst is added to a reaction. Which of the following is true?

2. Why can industrial processes use lower temperatures when a catalyst is present?

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🧪 Catalysts

What is a Catalyst?

Types of Catalyst

Industrial Importance of Catalysts

Mr. Badmus AI