GCSE · Chemistry · Edexcel · Spec 1CH0

Catalysts

Hydrogen peroxide breaks down slowly. Add a pinch of manganese(IV) oxide and it fizzes with oxygen at once, yet afterwards the manganese(IV) oxide is still there. How?

Chemistry · Catalysts

One reaction, two routes over the hill

View: Without a catalyst. Showing 2 layers: Axes and energy levels, Without a catalyst

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Without a catalyst, the reacting particles face this hill. Its height, measured from the reactants' energy level to the top of the curve, is the activation energy.

A reaction profile plots energy against the progress of a reaction. Switch views, then tap each part to see what it shows.

Follow the chain

Reason it through

Why does a lower activation energy make the reaction faster?

Link 1 of 4

First link · your turn

The catalyst gives the reaction a new route. What is different about that route?

Locked — reveal the link above first

A catalyst at work

Tap each substance to see its part in the reaction. Then find the catalyst.

Tap a molecule to see what it does in the reaction.

Check your thinking

What does a catalyst really do?

A student adds a catalyst to a reaction and writes: "The catalyst helps the reaction." Helps how, exactly?

Which is closest to what you think right now?

It makes the reaction give a bigger amount of product.It gets used up, so you have to keep adding more.It gives the particles extra energy so they react.It opens a route with a lower activation energy and isn't used up.

How sure are you?

I'm sureI think soI'm guessing

It makes the reaction give a bigger amount of product.

This is a common misconception

Tempting, because fast fizzing can look like 'more'. But a catalyst does not change the amount of product formed. The same amount forms; it just forms faster. Faster fizzing means the oxygen arrives sooner, not that there is more of it.

It gets used up, so you have to keep adding more.

This is a common misconception

A catalyst is not used up. It's still there when the reaction is complete, so it doesn't need to be replaced. That's why the equation puts it above the arrow rather than with the reactants.

It gives the particles extra energy so they react.

This is a common misconception

The particles get nothing extra. The catalyst provides an alternative pathway with a lower activation energy. The hill comes down, so more of the particles already there have enough energy to react.

It opens a route with a lower activation energy and isn't used up.

This matches the evidence

Yes. The alternative pathway has a lower activation energy, so more collisions are successful and the rate increases. The catalyst is not used up, and the amount of product stays the same.

Faster, not more. Not used up. A lower hill, not more energetic particles.

Predict, then check

Two different catalysts, one reaction. Commit before you look.

Two tubes hold the same hydrogen peroxide. Manganese dioxide (another name for manganese(IV) oxide) goes into one: rapid bubbling of oxygen. Copper oxide goes into the other: only a few bubbles. What should you conclude?

Put it into words

Why catalysts matter

Enzymes are biological catalysts. Reactions in cells happen at a much higher rate with enzymes than without them; without enzymes, many reactions would not happen quickly enough to keep cells alive. In industry, catalysts let products be made efficiently and cost-effectively. In the Haber process, an iron catalyst increases the rate at which ammonia is made from nitrogen and hydrogen. In a car's catalytic converter, platinum and rhodium catalyse the reaction of carbon monoxide with nitrogen monoxide, which forms carbon dioxide and nitrogen and reduces pollution from cars.

Using the information above, explain why catalysts are so useful. Include one example from living cells, one from industry and one from cars. [4 marks]

Your answer

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The mark scheme

Enzymes are biological catalysts: they make reactions in cells happen at a much higher rate. (1 mark)
Without enzymes, many reactions would not happen quickly enough to keep cells alive. (1 mark)
In industry, a catalyst (such as iron in the Haber process, making ammonia) increases the rate, so products are made efficiently and cost-effectively. (1 mark)
In a catalytic converter, platinum and rhodium speed up the reaction of carbon monoxide with nitrogen monoxide, reducing pollution from cars. (1 mark)

Model answer

Catalysts speed up reactions without being used up. In living cells, enzymes are biological catalysts: they make reactions happen at a much higher rate, and without them many reactions would not happen quickly enough to keep cells alive. In industry, catalysts mean products are made efficiently and cost-effectively; for example, an iron catalyst increases the rate of ammonia production from nitrogen and hydrogen in the Haber process. In cars, the platinum and rhodium in a catalytic converter catalyse the reaction of carbon monoxide with nitrogen monoxide to carbon dioxide and nitrogen, which reduces pollution.

Cells, factories and car exhausts rely on the same trick: a catalyst speeds up a reaction and is not used up.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

What a catalyst is, and what it does not do.
How to read activation energy on a reaction profile, with and without a catalyst.
Why a lower activation energy makes a reaction faster.
How catalysts compare, and why they matter in cells, industry and cars.

The big picture

A catalyst increases the rate of a reaction without being used up. It provides an alternative pathway with a lower activation energy, so more particles have enough energy, more collisions succeed and the rate rises. The amount of product doesn't change.

Key points

1A catalyst increases the rate of a reaction and is not used up, so it doesn't need replacing when the reaction is complete.

2A catalyst does not change the amount of product: the same amount forms, faster.

3Activation energy is the minimum energy particles must have to react when they collide.

4A catalyst provides an alternative pathway with a lower activation energy, so more particles have enough energy, more collisions are successful and the rate increases.

5On a reaction profile, a catalyst leaves the reactants' and products' energy levels unchanged but lowers the peak.

6Catalysts vary in effectiveness and are specific to particular reactions. Enzymes are biological catalysts.

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Worked example

Problem

Adding potassium iodide to hydrogen peroxide makes oxygen bubble off much faster. Explain why, and explain why the total amount of oxygen formed does not increase.

⚠ Watch out

Measuring the activation energy from the bottom of the graph, or from the products. On a reaction profile it runs from the reactants' energy level up to the top of the curve, with or without a catalyst.

🧠

Memory hook

Same start, same finish, lower hill. A catalyst changes the road, not the destination.

✓

Check yourself

Without looking back, sketch one reaction profile showing a reaction with and without a catalyst. Mark the activation energy on both curves, and circle the two parts of your sketch that must not move.

Flashcards

(14)

What is a catalyst?

A substance that increases the rate of a chemical reaction without being used up.

Does a catalyst need replacing when the reaction is complete?

No. It isn't used up, so it's still there at the end.

Does a catalyst change the amount of product formed?

No. The same amount of product forms, just faster.

Define activation energy.

The minimum energy that reactant particles must have to react when they collide.

What does a catalyst do to the activation energy, and how?

Lowers it, by giving the reaction a different route with a lower peak.

Lower activation energy, faster reaction: what links them?

More particles have energy at least equal to the activation energy, so more collisions are successful and the frequency of successful collisions rises.

On a reaction profile, where do you measure the activation energy?

From the reactants' energy level up to the top of the curve.

Reaction profile with a catalyst: what stays the same, and what changes?

The reactants' and products' energy levels stay the same; the curve doesn't rise as high.

Name two catalysts for the decomposition of hydrogen peroxide.

Manganese(IV) oxide and potassium iodide.

Manganese dioxide gives rapid bubbling with hydrogen peroxide; copper oxide gives only a few bubbles. Conclusion?

Manganese dioxide is the more effective catalyst for that reaction.

Will a catalyst that works for one reaction work for every reaction?

Not necessarily: catalysts are specific to particular reactions.

What are enzymes, and why do cells need them?

Biological catalysts. Without them, many reactions in cells wouldn't happen quickly enough to keep cells alive.

What does the iron catalyst do in the Haber process?

It increases the rate at which ammonia is made from nitrogen and hydrogen.

What do platinum and rhodium do in a catalytic converter?

They catalyse the reaction of carbon monoxide with nitrogen monoxide to carbon dioxide and nitrogen, reducing pollution from cars.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What does a catalyst really do?

Why catalysts matter

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