A catalyst speeds up a reaction by giving it an alternative pathway with a lower activation energy, and it is not used up. It does not change how much product forms. Those three points are what a full-mark answer usually contains.
This lesson sits in rates of reaction, after powder and lumps, and it introduces the idea of activation energy in the simplest way.
What is activation energy?
Activation energy is the minimum energy that colliding particles need for a reaction to happen. A collision with less energy than this bounces apart without reacting.
A catalyst offers a different route that needs less energy. With a lower energy requirement, a greater proportion of collisions are successful, so the rate goes up even though the temperature and concentration are unchanged.
What does a catalyst change and not change?
| Quantity | Does a catalyst change it? |
|---|---|
| Rate of reaction | Yes, it increases |
| Activation energy | Yes, the alternative pathway has a lower value |
| Amount of product (final yield of an irreversible reaction) | No |
| Mass of the catalyst at the end | No, it is recovered (its form may change physically) |
| The balanced equation | No, the catalyst is not written as a reactant |
The catalyst can take part in steps along the way, but it is regenerated by the end. That is why the same small amount can be used again.
Worked example
Invented data. Hydrogen peroxide breaks down as:
2H₂O₂ → 2H₂O + O₂
A school-style investigation, carried out only in a supervised laboratory, compares two samples each containing 0.0200 mol of hydrogen peroxide. One has a solid catalyst added. The other has none.
Step 1, use the ratio. H₂O₂ : O₂ is 2 : 1, so oxygen formed = 0.0200 ÷ 2 = 0.0100 mol.
Step 2, convert to volume. At room temperature and pressure, 0.0100 × 24 = 0.24 dm³ = 240 cm³.
Step 3, compare the two results. The catalysed sample reaches 240 cm³ in a shorter time. The uncatalysed sample also reaches 240 cm³ eventually.
Step 4, check the catalyst. Invented measurement: the catalyst had mass 0.50 g before and 0.50 g after it was filtered, washed and dried. It was not used up.
Step 5, write the explanation. “The catalyst provides an alternative pathway with a lower activation energy, so more collisions are successful each second and the rate increases. The same amount of hydrogen peroxide gives the same volume of oxygen.”
What mistake should you watch for?
Mistaken answer: “The catalyst is used up, which is why the reaction slows down and stops.”
The reaction slows because the reactant is being used up, not the catalyst. If the catalyst were consumed, its mass at the end would be smaller. A catalyst can become coated or clumped, and its physical form can change, but it is chemically the same.
Another slip is to say a catalyst “gives the particles more energy”. The particles have the same energy. The pathway needs less.
Check yourself
1. State two things a catalyst does not change in the example above.
Show answer
Any two of these: the final volume of oxygen (240 cm³), the amount of hydrogen peroxide needed, the chemical identity and mass of the catalyst.
2. 0.0400 mol of hydrogen peroxide decomposes completely with or without a catalyst. What volume of oxygen forms at room temperature and pressure, using 24 dm³ per mole?
Show answer
O₂ = 0.0400 ÷ 2 = 0.0200 mol. Volume = 0.0200 × 24 = 0.480 dm³ = 480 cm³.
3. Correct this sentence: “A catalyst is a reactant that is used up quickly.”
Show answer
“A catalyst is not a reactant and is not used up. It speeds up the reaction through a pathway with a lower activation energy and is unchanged chemically at the end.”
Where does this lead next?
Now combine every idea so far in judging a fair comparison in a dataset. Check gas volumes with the mole and equation-ratio tutor and confirm equations with the equation balance reasoning trainer.
If you would like your catalyst wording checked against real questions, our teachers can do that in online one-to-one Chemistry tuition.