Effect of Catalysts on Rate of Reaction

A catalyst is a substance that speeds up the rate of a chemical reaction but is not used up during the reaction. At the end, the catalyst is chemically unchanged — it is present in the same amount and the same chemical form as at the start, so it can be reused many times.

This is why only a small amount of catalyst is needed, even for a large reaction: the catalyst takes part in the reaction but is continuously regenerated, so the same particles are used over and over again.

What a catalyst changes — and what it does NOT change.

A catalyst only changes one thing: the rate (how fast the reaction goes). It does not change:

• the products that are made,
• the amount of product finally made,
• the enthalpy change (ΔH) of the reaction.

So if you catalyse a reaction, you reach the same final amount of product — you just reach it faster.

How you know it isn't used up. In a lab demonstration the catalyst (for example a powder) can be filtered off, dried and weighed at the end. Its mass is unchanged, which proves it was not consumed.

For particles to react they must collide with at least the activation energy (Ea) — the minimum energy a collision needs to start a reaction. Collisions with less than Ea just bounce off without reacting.

A catalyst works by providing an ALTERNATIVE REACTION PATHWAY with a LOWER ACTIVATION ENERGY. Learn this phrase exactly — it is the mark-scheme wording.

Why a lower Ea means a faster rate.

The catalyst does not give the particles more energy (the temperature is unchanged). Instead, it lowers the energy needed for a successful collision. Because the activation energy is lower:

• a greater proportion of the colliding particles now have energy ≥ Ea;
• so a higher fraction of collisions are successful;
• so there are more successful collisions per second → a faster rate.

Reading the energy-profile diagram. Both curves start at the same reactant level and end at the same product level (so ΔH is unchanged). The only difference is the height of the hump — the catalysed pathway has a smaller hump, because the activation energy is lower.

A catalyst does take part in the reaction — but by the end it is regenerated back to its original form. So although it is involved in the mechanism, its overall amount and chemical identity do not change.

The consequences for the exam:

• A tiny amount of catalyst can speed up a large reaction, because it is reused again and again.
• The catalyst is not a reactant in the balanced equation — it is often written above the arrow instead (for example, the yeast in fermentation).
• At the end you can recover the catalyst (e.g. filter it off) with its mass unchanged.

Reusability in industry. Industrial catalysts are reused continuously for months or years, which makes them very cost-effective. They can sometimes be 'poisoned' (deactivated by impurities), but in normal use they are not consumed by the reaction itself.

A favourite exam skill is to spot the catalysed reaction on a graph of product (gas volume or mass) against time.

When you compare a catalysed run with an uncatalysed run of the same reaction with the same amounts of reactants:

• the catalysed curve is steeper at the start (faster initial rate);
• the catalysed curve reaches its plateau sooner;
• both curves plateau at the SAME final amount of product.

That last point is the key: the catalyst changes how fast you get there, not how much product you get. The final amount is fixed by the moles of limiting reactant, which the catalyst does not change.

Trap to avoid. Students often say the catalysed reaction makes "more gas". It does not — it makes the same amount, just more quickly.

Enzymes are biological catalysts. They are proteins made by living cells, and they speed up the reactions of living things while not being used up — exactly like any other catalyst.

Fermentation — making alcoholic drinks.

Enzymes in yeast are used in fermentation to make alcoholic drinks. The yeast enzymes catalyse the conversion of glucose (a sugar) into ethanol (alcohol) and carbon dioxide:

$\text{glucose} \xrightarrow{\text{yeast enzymes}} \text{ethanol} + \text{carbon dioxide}$

The balanced equation is:

$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$

Conditions for fermentation:

• a warm temperature (about 30–35 °C) — warm enough for the enzymes to work quickly, but not too hot (high temperatures destroy / denature the enzymes and stop the reaction);
• no air (anaerobic) — oxygen would oxidise the ethanol;
• yeast is mixed with the sugar solution and left for the enzymes to work.

Why enzymes are special. Enzymes are extremely efficient and very specific — each enzyme catalyses only one (or a few) reactions, because its 3D shape matches only one type of molecule. They also work best at a particular temperature; too hot and they stop working permanently.