What is a catalyst?
A catalyst is a substance that changes the speed of a chemical reaction (usually makes it faster) but is not used up.
- Its mass is the same at the start and at the end.
- Its chemical nature is the same at the end (its look may change a little, for example a powder may become finer).
- A small amount is enough, because it is used again and again.
- It does not change which products form or how much product you finally get; it only gets you there sooner.
Speeding up a reaction with a catalyst is called catalysis.
Making oxygen in the lab
A decomposition reaction is one where one substance breaks down into two or more simpler ones.
From hydrogen peroxide
2H₂O₂ → 2H₂O + O₂ (catalyst: MnO₂)
Without MnO₂ the bubbles are very slow. With a pinch of MnO₂, oxygen comes off fast at room temperature. No heating is needed.
From potassium permanganate
2KMnO₄ → K₂MnO₄ + MnO₂ + O₂ (needs heating)
Here heat breaks the solid down. A plug of cotton wool at the mouth of the tube stops powder getting into the delivery tube.
Collecting the gas
- Over water (downward displacement of water): oxygen does not dissolve much in water, so it pushes the water out of an upside-down jar. This gives fairly pure gas.
- Upward displacement of air: oxygen is a little denser than air, so a jar kept mouth-up fills from the bottom.
Test for oxygen: a glowing splint relights in it.
How does a catalyst work?
Particles must hit each other with enough energy to react. This smallest energy is the activation energy, like a hill to climb.
A catalyst gives the reaction a different route (pathway) with a lower hill. At the same temperature, many more collisions now have enough energy, so the reaction is faster.
Catalyst versus heating
Heating also speeds up a reaction, but in a different way: it gives particles more energy so more of them can climb the same hill. A catalyst lowers the hill instead. Both together give the fastest rate.
Surface catalysts
Many solid catalysts work on their surface in three steps: (1) reactant molecules stick to the surface (adsorption), (2) their bonds weaken and new bonds form, (3) the products leave (desorption), and the surface is free for the next molecules. That is why powders work better than lumps: more surface.
When the catalyst is in a different state from the reactants (solid catalyst, gas or liquid reactants) it is called heterogeneous. When it is in the same state (all dissolved together) it is homogeneous.
Some substances slow reactions down; they are called inhibitors (negative catalysts), for example preservatives in food.
Catalysts in living things and in industry
- Enzymes are protein catalysts in cells. Amylase in saliva breaks starch into sugar; catalase in liver breaks down hydrogen peroxide. Each enzyme works best at a certain temperature and pH.
- Ammonia (for fertiliser) is made from nitrogen and hydrogen with an iron catalyst.
- Catalytic converters in vehicles use platinum, palladium and rhodium to turn carbon monoxide and nitrogen oxides into carbon dioxide and nitrogen.
- Hydrogenation of vegetable oils uses a nickel catalyst.
Catalysts save energy and money because reactions run fast at lower temperatures.
Try it: a safe catalase test
With an adult, put a slice of raw potato in a little 3% hydrogen peroxide (the kind sold as a mild antiseptic). Watch the bubbles. Now try a slice of boiled potato. Fewer bubbles? Boiling destroys the enzyme catalase. Then open the last 3D step and compare "catalyst on" and "off".
Key formulas and definitions
- 2H₂O₂ → 2H₂O + O₂ (MnO₂ catalyst)
- 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂ (heat)
- Catalyst: same mass and same chemical nature before and after
- Catalyst lowers activation energy; heating gives particles more energy
- Rate roughly doubles for every 10 °C rise (rule of thumb)
Worked examples
1. 0.5 g of MnO₂ is added to hydrogen peroxide. After the reaction it is filtered, washed and dried. What mass is recovered and why?
0.5 g. A catalyst is not used up in the reaction.
2. Why is a powdered catalyst better than one lump of the same mass?
The powder has much more surface. The reaction happens on the surface, so more reactant molecules can stick and react at the same time.
3. Does adding MnO₂ give more oxygen from 50 mL of hydrogen peroxide?
No. The same total oxygen forms; it only forms faster. A catalyst does not change the amount of product.
4. A reaction makes 10 mL of gas per minute at 25 °C. Using the rough rule, estimate the rate at 45 °C without a catalyst.
45 − 25 = 20 °C, two steps of 10 °C. Rate doubles twice: 10 × 2 × 2 = 40 mL per minute.
5. Explain why oxygen can be collected over water.
Oxygen dissolves only slightly in water, so it pushes the water out of the jar and collects as a gas.
Common mistakes
- Saying the catalyst "does not take part". It does take part, but it is given back unchanged at the end.
- Thinking a catalyst gives more product. It only makes the same product faster.
- Mixing up a catalyst and heating. Heating gives particles more energy; a catalyst lowers the energy needed.
- Writing MnO₂ as a reactant in the equation. Write it above the arrow.