Catalysts

Section 1 of 7What a catalyst does

Speeds up the reaction

• Speeds up the reaction A catalyst increases the rate of a chemical reaction without being used up.
• It is chemically the same, and has the same mass, at the end as at the start.

Small amounts are enough

• Small amounts are enough Because it is not consumed, a small quantity can catalyse a very large amount of reactant.

Not part of the equation

• Not part of the equation Catalysts do not appear as reactants or products.
  • They are written above or below the arrow, for example iron in the Haber process: N₂ + 3H₂ ⇌ 2NH₃.

Specific

• Specific Different reactions need different catalysts, such as iron for ammonia and vanadium(V) oxide for the contact process.

Section 2 of 7How a catalyst works

1Alternative pathway

• Alternative pathway The catalyst gives the reactants a different route from reactants to products.

2Lower activation energy

• Lower activation energy The minimum energy that colliding particles need is reduced.

3More successful collisions

• More successful collisions A bigger proportion of collisions now have enough energy to react.

4Faster reaction

• Faster reaction More successful collisions per second means a greater rate of reaction.

Section 3 of 7Reading the evidence

Feature	Without catalyst	With catalyst
Activation energy	Higher	Lower (peak of the reaction profile is lower)
Initial gradient of a volume-time graph	Shallower	Steeper
Time to finish	Longer	Shorter, the curve levels off sooner
Final amount of product	Same	Same, because the amount of reactant is unchanged
Overall energy change	Same	Same, only the pathway differs

Section 4 of 7Why industry uses catalysts

Catalysts in industry

Faster production

• Faster production More product is made in a given time.

Lower temperature needed

• Lower temperature needed Reactions can run at lower temperatures, which cuts fuel costs and saves energy.

Surface area

• Surface area Catalysts are often used as pellets, beads or gauzes so that more of the surface is exposed to the reacting gases.

Reusable

• Reusable The catalyst can be used again and again, though it may eventually become poisoned by impurities.

Section 5 of 7Biological catalysts

Enzymes

• Enzymes Enzymes are large protein molecules that act as catalysts in living things.
• They speed up reactions such as digestion and respiration at body temperature.

Section 6 of 7Key terms

Term	Meaning
Catalyst	A substance that increases the rate of a reaction without being used up.
Activation energy	The minimum energy colliding particles must have in order to react.
Reaction profile	A diagram showing the energy of the reactants, the peak of the reaction, and the products.
Enzyme	A protein that acts as a biological catalyst.
Successful collision	A collision in which particles have at least the activation energy and react.

Section 7 of 7Exam technique

1

Saying a catalyst gives the particles more energy. It does not; it lowers the activation energy instead.

2

Claiming a catalyst increases the yield or the amount of product. The final amount of product stays the same.

3

Thinking a catalyst is used up. It is unchanged at the end and can be reused.

4

Writing a catalyst as a reactant in the balanced equation.

5

Stating that a catalyst increases the number of collisions. It increases the proportion of collisions that are successful.