Acids and alkalis · Model
Catalysts
A spatula of black powder turns a bottle that has been sitting still for a year into a reaction that is over in twenty seconds. Weigh the powder afterwards and none of it has gone.
Start here
Hydrogen peroxide slowly falls apart into water and oxygen. Left alone, a bottle takes about a year.
Tip in a spatula of manganese dioxide — a black powder that is not one of the reactants and not one of the products — and the same bottle froths over in seconds, giving off enough oxygen to relight a glowing splint. Filter the mixture afterwards and the black powder is still there, all of it, dry and weighing exactly what it did.
What did the black powder do?
It made the reaction go faster, and took no part in it. The reaction was always going to happen — hydrogen peroxide decomposes on its own — and the powder only changed how long it took. Nothing was added to the products, nothing was consumed, and the powder can be dried and used again tomorrow. That is a catalyst.
A catalyst is a substance that speeds up a chemical reaction without being used up by it. At the end there is exactly as much catalyst as there was at the start, and it can be recovered and reused.
A catalyst does not make an impossible reaction happen, and it does not change what the products are. It changes one thing only: how fast.
Your turn · five flasks
Same hydrogen peroxide in every flask. One spatula of something added to each.
0 of 5 run
Two of the five are catalysts. The other three change nothing at all — and one of those three is the flask most people bet on.
Nothing added
Sand
Manganese dioxide
Fresh liver
Dilute acid
50 cm³ of hydrogen peroxide solution in a conical flask, with a delivery tube to a gas syringe. Nothing else.
The same peroxide, with one spatula of clean dry sand stirred in.
The same peroxide, with one spatula of black manganese dioxide stirred in.
The same peroxide, with a small piece of fresh liver dropped in.
The same peroxide, with 5 cm³ of dilute acid added.
Predict before you run it.
A few bubbles. Almost nothing in a minute.
Oxygen after 60 s
2 cm³
Recovered afterwards
none added
These volumes are illustrative of what the five flasks show against each other, not measurements from one run.
The control, and the one that proves the reaction was always happening. Hydrogen peroxide decomposes into water and oxygen on its own — it is simply too slow to watch. Everything that follows is a comparison against this flask.
No change. The same few bubbles as the empty flask.
Oxygen after 60 s
2 cm³
Recovered afterwards
1.00 g of 1.00 g
These volumes are illustrative of what the five flasks show against each other, not measurements from one run.
The second control, and the important one. Sand is a solid, it is insoluble, it has a large surface area — and it does nothing. Adding a powder is not what makes a catalyst; being the right powder is.
Immediate vigorous frothing. The syringe fills in under a minute.
Oxygen after 60 s
48 cm³
Recovered afterwards
1.00 g of 1.00 g
These volumes are illustrative of what the five flasks show against each other, not measurements from one run.
A catalyst. The reaction is the same reaction giving the same products — the oxygen relights a glowing splint either way — but it is finished in a minute rather than a year. Filter, dry and weigh the powder and every milligram is still there.
Violent frothing, faster than the manganese dioxide, and the foam climbs the flask.
Oxygen after 60 s
55 cm³
Recovered afterwards
unchanged
These volumes are illustrative of what the five flasks show against each other, not measurements from one run.
Also a catalyst, and a biological one. Liver is full of catalase, an enzyme that exists to destroy hydrogen peroxide inside cells. Boil the liver first and it does nothing at all — heat destroys the enzyme's shape, and the shape is what does the work.
No change. The same few bubbles as the empty flask.
Oxygen after 60 s
2 cm³
Recovered afterwards
unchanged
These volumes are illustrative of what the five flasks show against each other, not measurements from one run.
Nothing happened, and that is worth knowing. Acid does not speed this reaction up — if anything it holds it back, which is why the bottle of hydrogen peroxide you can buy is kept slightly acidic so that it does not go off on the shelf. Pour the flask out at the end and the acid is all still there. Adding something is not the same as catalysing it: the sand made that point for a solid, and this makes it for a liquid.
Two of the five were catalysts, and one of those was alive.
The sand did nothing: adding a solid is not enough on its own. The acid did nothing either, which is the one most people get wrong — it is still sitting in the flask, unchanged, and the reaction ran at exactly the speed it ran at with nothing added at all. Being added does not make something a catalyst, and neither does coming back unchanged. Manganese dioxide and liver did both things at once: they sped the reaction up AND came back weighing the same.
Note the total oxygen. Every flask that was left long enough produced the same amount in the end. A catalyst changes when the reaction finishes, not how much it makes.
Key fact
A catalyst speeds up a reaction and is not used up by it. The mass of catalyst at the end equals the mass at the start, and the products are unchanged.
Three judgements
Why anybody cares
0 of 3 decided
A catalytic converter in a car exhaust contains platinum and rhodium — expensive metals. Why is the cost tolerable?
Because a catalyst is not consumed, a few grams coating a honeycomb can process the exhaust of a car for its entire life. If the metal were a reactant it would need topping up like fuel, and no car would be affordable. The converter turns carbon monoxide and unburnt fuel into carbon dioxide and water on the way out of the engine.
A factory reaction runs at 450 °C without a catalyst and 250 °C with one, producing the same amount of product. Which matters more to the company?
The temperature. The yield is the same either way, so the catalyst is not making more of anything — it is making the same amount for less fuel, every hour, for decades. Two hundred degrees across an industrial plant is an enormous quantity of energy, which is why almost every large-scale process is catalysed.
Would adding a catalyst make copper react with dilute hydrochloric acid?
No, and this is the limit of what a catalyst is. Copper is below hydrogen in the reactivity series, so there is no reaction to speed up. A catalyst shortens the time a possible reaction takes; it cannot make an impossible one occur, however much you add.
Think again
“A catalyst is used up slowly, which is why it wears out.”
Catalytic converters really do have to be replaced eventually. Commit before you read on.
The balance settles it. Weigh the manganese dioxide before, filter it out afterwards, dry it and weigh it again: the mass is the same, and the same spatula of powder will run the reaction again tomorrow. Nothing is being slowly consumed.
Catalysts do fail in the real world, but not by being used up. They get poisoned — coated by something else that sticks to the surface and blocks it — or physically clogged, or damaged by heat. That is why leaded petrol destroys a catalytic converter: the lead covers the surface the reaction needs. Blocked is not the same as consumed.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What does a catalyst do?
Rung 2 · The one that catches people
Two flasks of hydrogen peroxide are left until the reaction is completely finished. One had a catalyst, one did not. How do the volumes of oxygen compare?
Rung 3 · Explain
Describe an experiment that would prove a black powder is acting as a catalyst in the decomposition of hydrogen peroxide, and say what results would settle it.
Rung 4 · Take it somewhere new
Catalytic converters stop working if a car runs on leaded petrol, but the platinum inside has not been consumed. Explain how a catalyst can fail without being used up, and what that tells you about how catalysts work.
Key note
A catalyst speeds up a chemical reaction without being used up. It can be filtered out at the end, weighs the same as it did at the start, and can be used again. It does not change the products and it does not make impossible reactions happen — it only changes the rate. Enzymes are the catalysts living things use.
Going further
The great majority of everything manufactured by the chemical industry passes over a catalyst at some point. The reason is money and energy: a reaction that needs 400 °C without a catalyst and 200 °C with one costs half as much to run, every hour, for the life of the plant. The ammonia process that makes the world's fertiliser runs over iron, and the catalyst is the difference between a reaction that is possible and a reaction that is worth doing.
Your own body is running thousands of catalysed reactions at this moment, at 37 °C, which is a temperature at which almost none of them would otherwise proceed usefully. The catalysts are enzymes — large protein molecules, each shaped to fit one particular reaction. The liver in the flask worked because it is full of catalase, an enzyme whose job is to destroy hydrogen peroxide before it damages cells. Boil the liver first and nothing happens: heat wrecks the shape, and an enzyme with the wrong shape is no longer a catalyst.
Before this lesson
Next in this unit
At GCSE this becomes
- Catalysts lowering activation energy, reaction profiles with and without one, and enzymes as biological catalysts with optimum temperatures and pH.
Where to next
- Next: Energy and changes of state
Energy changes in reactions
- Previous: Making a pure dry salt
Ask Mr Badmus AI
Still not sure how something can change a reaction and not take part?
Hydrogen peroxide at the strength used here bleaches skin and damages eyes, and the liver flask froths over: it is run in a large flask standing in a tray, with eye protection on. Manganese dioxide is harmful if swallowed and the recovered powder is not handled with bare hands.
Lesson content © MrBadmusAI.