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A catalyst as a route with a lower barrier: what it changes, what it cannot change, and how it is regenerated.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to explain a catalyst as an alternative route with a lower activation energy, read catalysed and uncatalysed barriers and the enthalpy change off an energy profile, and use the energy distribution to explain why a larger share of collisions succeeds. You will be able to say what a catalyst leaves unchanged — the enthalpy change, the total product and its own mass — and why it speeds up the reverse reaction too.
Temperature changed the share of collisions with at least the activation energy by moving the particles' energies up towards a fixed barrier. There is another way to change that share: leave the particles alone and move the barrier down. From Chemistry 1 you have the energy profile, with the activation energy as the height of its hump; this lesson changes the hump.
A catalyst is a substance that increases the rate of a reaction and is chemically unchanged at the end of it. An alternative pathway is a different sequence of steps from the same reactants to the same products. A heterogeneous catalyst is in a different state from the reactants — usually a solid with gases or solutions reacting on its surface; a homogeneous catalyst is in the same state. An enzyme is a protein that acts as a catalyst in a living thing. Adsorption is molecules sticking to a surface; desorption is their leaving it.
A catalyst takes part in the reaction — it forms bonds with the reactants, or holds them on its surface — and then comes out again at the end, unchanged. Along the way it provides a different route from reactants to products, and that route has a lower activation energy.
On an energy profile the catalysed route is a lower hump between the same two ends. On the energy distribution the dashed activation-energy line moves to the left, and a much larger share of the particles now lies beyond it. The collision frequency is unchanged; the share of collisions that succeed goes up:
| collisions per second | share that succeed | rate | |
|---|---|---|---|
| catalyst added | unchanged | up | up |
Because only the route has changed, a catalyst cannot change anything that depends on the start and the finish:
One consequence is easy to miss. The lower hump is lower from both sides, so a catalyst speeds up the reverse reaction by the same factor as the forward one. That matters in the equilibrium unit: a catalyst gets a reversible reaction to equilibrium sooner and does not change where the equilibrium lies.
Another way: picture
Two towns separated by a mountain. The road over the top is the uncatalysed route; a tunnel through the mountain is the catalysed one. The towns are exactly where they were and so is the difference in their heights; far more traffic gets through each hour, in both directions, and the tunnel is still there afterwards.
Another way: steps
To explain a catalyst:
Most of the world's chemical industry runs on catalysts, because a catalyst lets a reaction go at a useful rate at a lower temperature, which saves energy and money.
| Process | Catalyst | Why it matters |
|---|---|---|
| making ammonia from nitrogen and hydrogen | iron | fertilisers that feed a large share of the world's population |
| oxidising sulfur dioxide to sulfur trioxide | vanadium(V) oxide | the step that makes sulfuric acid |
| cleaning car exhaust | platinum and rhodium on a honeycomb | turns carbon monoxide and nitrogen oxides into carbon dioxide and nitrogen |
| decomposing hydrogen peroxide | manganese(IV) oxide, or the enzyme catalase | the laboratory source of oxygen; catalase protects cells |
| digesting starch | amylase | begins in the mouth, which is why bread tastes sweeter if chewed for long |
A solid catalyst works on its surface: reactant molecules arrive, stick to it (adsorb), have their bonds weakened, react with each other there and leave (desorb), freeing the site for the next molecules. That is why industrial catalysts are made as fine powders, pellets or thin coatings on a honeycomb — the surface area lesson again — and why a substance that sticks to the surface and never leaves, such as lead in old petrol, poisons a catalyst by blocking its sites.
The definition makes a checkable prediction. Weigh 0.5 g of manganese(IV) oxide, add it to hydrogen peroxide, let the frothing finish, then filter the black solid off, wash it, dry it and weigh it again: 0.5 g. Add it to fresh hydrogen peroxide and it froths just as fast as before.
A catalyst can still take part: in some catalysed routes the catalyst is changed in one step and changed back in a later step. What matters is that, over the whole cycle, it comes out as it went in. That is also why a tiny amount of catalyst can convert an enormous amount of reactant — one molecule of the enzyme catalase can break down millions of hydrogen peroxide molecules every second.
A catalyst releases extra energy. The energy released is the difference between the reactants' level and the products' level, and a catalyst moves neither. It lowers the hump in between.
A catalyst gives more product. The amount of product is set by the amount of limiting reactant. A catalyst gets there sooner.
A catalyst is used up. It is regenerated; its mass at the end equals its mass at the start.
A catalyst speeds up only the forward reaction. The lowered hump is lower from both sides, so the reverse reaction speeds up by the same factor.
A catalyst works by giving particles more energy. That is what heating does. A catalyst leaves the particles' energies alone and lowers the energy they need.
Reactants at 50 kJ, products at 20 kJ. The uncatalysed hump tops out at 170 kJ; the catalysed one at 110 kJ.
Three heights for the uncatalysed route, one new height for the catalysed one.
Activation energy without the catalyst: $170 - 50 = 120$ kJ. With it: $110 - 50 = 60$ kJ, so the catalyst lowered it by 60 kJ.
Both barriers are measured from the same reactants' level.
Enthalpy change: $20 - 50 = -30$ kJ either way; the reverse barrier with the catalyst is $110 - 20 = 90$ kJ, also 60 kJ lower than without it.
The ends did not move, so the enthalpy change did not change, and both directions got the same reduction.
Hydrogen peroxide decomposes very slowly at room temperature; with a pinch of manganese(IV) oxide it froths at once.
Same temperature, so the particles' energies have not changed.
The oxide provides a route with a much lower activation energy, so on the energy distribution the barrier line moves far to the left.
The catalyst moves the barrier, not the particles.
A far larger share of collisions now has enough energy, so far more succeed each second — and the oxide is recovered unchanged at the end.
Share of successful collisions up, frequency unchanged, catalyst regenerated.
Catalysed activation energy: $130 - 50 = 80$ kJ.
The catalyst lowers the barrier by the stated amount.
Enthalpy change: products minus reactants, $90 - 40 = \ldots$
…$+50$ kJ, the same with or without the catalyst, because the ends have not moved.
Without a catalyst, the reactants sit at $21$ kJ, the top of the hump is at $118$ kJ and the products at $64$ kJ. A catalyst lowers the top of the hump to $87$ kJ. Give the activation energy with the catalyst and the enthalpy change with the catalyst.
Two energy profiles on the same axes, with the same two ends and one hump lower than the other.
Activation energy with the catalyst, in kJ:
Enthalpy change with the catalyst, in kJ:
Balance the equation for the Haber process, catalysed by iron. The catalyst does not appear in the equation, because it is not used up.
This task has no paper form; do it on a device.
$0.8$ g of manganese(IV) oxide is added to $100$ mL of hydrogen peroxide solution, which froths as oxygen is given off. When the frothing stops, the black solid is filtered off, washed and dried. What mass is recovered, assuming none is lost in handling? Give the mass with its unit.
Answer: unit: g / kg / mg
Two identical flasks each hold $50$ mL of the same hydrogen peroxide solution. A little catalyst is added to one of them. Which statement about the two flasks is right?
Think about iron in the Haber process. For each quantity, say whether adding the catalyst increases it, decreases it, or makes no change.
| effect of adding the catalyst | |
|---|---|
| activation energy | |
| rate of the forward reaction | |
| rate of the reverse reaction | |
| enthalpy change | |
| total amount of product | |
| mass of catalyst at the end |
Match each process to the catalyst used for it.
| iron | vanadium(V) oxide | platinum and rhodium | amylase, an enzyme | |
|---|---|---|---|---|
| making ammonia from nitrogen and hydrogen | ||||
| oxidising sulfur dioxide to sulfur trioxide | ||||
| cleaning carbon monoxide and nitrogen oxides from car exhaust | ||||
| breaking starch down into sugars in the mouth |
A solid catalyst works at its surface. Put the steps of one reaction between nitrogen and hydrogen molecules on iron in order.
Number the steps in order (write the number in the box):
A pharmaceutical company tests a new catalyst for one step of a drug synthesis. At the working temperature, without the catalyst, one collision in $9000$ has at least the activation energy. With the catalyst's lower barrier, one collision in $3000$ has enough energy for the new route. The collision rate is unchanged. By what factor is the reaction faster?
Answer: times
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For a reaction, the reactants sit at $56$ kJ and the products at $26$ kJ. Without a catalyst the activation energy is $69$ kJ; a catalyst lowers it by $34$ kJ. What is the activation energy of the *reverse* reaction when the catalyst is present, in kJ?
Answer: kJ
You can explain a catalyst with the energy profile and the energy distribution, and you know what it cannot change. Say out loud why a catalyst that made a reaction release more energy would break conservation of energy. Next: rate evidence — how to tell which explanation a set of measurements actually supports.
10. Your turn: reactants at 40 kJ, products at 90 kJ, uncatalysed activation energy 130 kJ, and a catalyst lowers it by 50 kJ. What are the catalysed activation energy and the enthalpy change?, step 3