Hydrogen peroxide keeps for months. Add a pinch of powder and it fizzes at once.
A bottle of hydrogen peroxide keeps for months. Drop in a pinch of manganese dioxide and the liquid fizzes at once. When the fizzing stops, the manganese dioxide is still there. Catalytic converters, fertilizer plants and the cells in your body all use this trick. A catalyst lowers the energy barrier of a reaction. It leaves the energy released unchanged.
Core Claims
- It lowers the barrier. A catalyst speeds up a reaction by giving it a path with a lower activation energy (Ea).
- It is not used up. The catalyst comes out of the reaction unchanged, so it works again and again.
- It leaves the energy released unchanged. The reactants and the products have the same energy with or without a catalyst, so ΔH stays the same.
- Enzymes. An enzyme is a protein that acts as a catalyst in living things.
With and without a catalyst
Retrieval Checklist
- Compare the energy profile of a reaction with and without a catalyst.
- Explain why a catalyst does not change ΔH.
- Say what an enzyme is and what it does.
Lowering the hurdle without changing the start or finish line.
In Lesson 4-1a you saw that only collisions with at least the activation energy (Ea) can react. If Ea is high, only a tiny share of the collisions can react. The rate is too slow to notice.
A catalyst speeds up a reaction by giving it a different path with a lower activation energy. At the same temperature, more collisions have enough energy to react. A catalyst does three things.
- It lowers the barrier. The peak of the energy profile is lower, so more collisions get over it.
- It is not used up. The catalyst comes out of the reaction unchanged, so it works again and again.
- It leaves the energy released unchanged. The reactants and the products sit at the same energies, so ΔH stays the same.
Thinking a catalyst adds energy to the reactants or heats the mixture. A catalyst does not speed up the particles. It gives them an easier path, so a collision needs less energy to react.
EnrichmentMetal surfaces and the final yield
Extra depth. The rest of the lesson works without it.
On a metal surface. Solid metals such as platinum, palladium and iron are catalysts. Reactant molecules stick to the metal surface. The surface weakens their bonds and holds them in a position that favors reaction.
At equilibrium. A catalyst lowers the barrier for the reverse reaction by the same amount, so it speeds up both directions equally. A reaction reaches equilibrium sooner, but the final yield is the same.
Your cells run on catalysts too.
A metal catalyst in a factory works at hundreds of degrees. A living cell has to work in water at about 37 °C. Its catalysts are enzymes. Each enzyme is a large protein molecule shaped to speed up one reaction. An enzyme lowers the activation energy just as any catalyst does, and it comes out of the reaction unchanged.
Thinking an enzyme is used up, like a reactant. An enzyme is a catalyst. It comes out of the reaction unchanged and works again.
How enzymes work: a pocket, a shape, and a top speed.
EnrichmentThe active site, damage from heat and pH, and top speed
Extra depth. The rest of the lesson works without it.
The active site
An enzyme is a protein folded into a particular three-dimensional shape. It has a pocket called the active site. The molecule that reacts, the substrate, fits into this pocket.
One early model, lock and key, pictured the substrate fitting the pocket like a key in a lock. The modern model, induced fit, says the pocket changes shape a little when the substrate arrives. The pocket then holds the substrate snugly and strains its bonds toward the transition state. That strain lowers Ea.
- Binding. The substrate enters the active site.
- Strain. The side chains of the amino acids in the pocket strain the substrate toward the transition state.
- Reaction. Bonds break and new bonds form.
- Release. The products have a different shape and leave. The pocket is ready for the next substrate.
Damage from heat and pH
An enzyme works only while it keeps its shape. From 0 °C to 37 °C the rate rises, because faster molecules collide more often and more of the collisions have enough energy. Above the best temperature (about 40 to 45 °C for many human enzymes), the particles shake the folded protein apart. The pocket unravels and the enzyme stops working.
A large change in pH does the same thing. The amino acids in the pocket (aspartate, glutamate, lysine) have side chains that gain or lose H+. Extra H+ or OH− changes their charges, so the pocket pushes the substrate away or unfolds. The permanent loss of shape is called denaturation.
Top speed
Double the reactant in an uncatalyzed reaction and the rate doubles, because collisions happen twice as often. An enzyme reaction behaves differently. At first the rate rises with the amount of substrate. Then it levels off at a top speed.
Think of enzymes as cashiers at a busy grocery store. With 10 open registers and 2 shoppers in line, more shoppers means more checkouts. With 200 shoppers, every cashier is already scanning as fast as possible. Adding 50 more shoppers does not speed anything up. The only way to raise the speed is to hire more cashiers. In chemistry, that means adding more enzyme.
Interactive enzyme simulator.
EnrichmentSubstrate, temperature, pH and the top speed
Extra depth. The rest of the lesson works without it.
Change the substrate, the temperature and the pH. Watch the active sites fill, and watch the speed level off.
Why does adding more substrate stop speeding up the reaction?
The explanation appears after you predict.
- Substrate; filled in a pocket, it marks a busy active site
- Product, released when a site finishes
- Enzyme with a working pocket (an amber outline is strained, a dashed red outline is unfolded)
- Top speed, with every site busy all the time
- Run three amounts of substrate and press Record after each. Look for: 17, 30 and 33 reactions. The sites in use climb from 47% to 92%, and the dots level off under the top-speed line.
- Heat the enzyme. Load 70 °C, press Start run, then press Record. Look for: 0 reactions. The outline turns dashed and the pockets collapse, because heat unfolds the protein.
- Add strong acid. Load pH 2.4, press Start run, then press Record. Look for: 0 reactions. This enzyme works best near pH 7.4. Pepsin in your stomach has a different shape and works in acid. This model enzyme does not.
Use your evidence
Explain the top speed
Hydrogen peroxide breaks down: 2 H2O2(aq) → 2 H2O(l) + O2(g)
A student tests two catalysts at 25 °C. Catalase, an enzyme from liver, makes 14.2 mL of O2 per minute. Manganese dioxide (MnO2), a solid, makes 12.8 mL per minute. The student boils each catalyst at 100 °C for 15 minutes, cools it to 25 °C and adds it to fresh H2O2.
(a) Predict the result for the boiled catalase and for the boiled MnO2. Explain the difference between an enzyme and a solid catalyst. [2 marks]
(b) In a fourth trial the student raises the H2O2 to 12.0% with fresh catalase. The rate rises at first, then levels off. Explain why. [1 mark]
- Catalase (1 mark): You predict no oxygen, because boiling unfolds the enzyme and destroys its active site.
- MnO2 (1 mark): You predict the same rate, because a solid has no folded shape to lose.
- Top speed (1 mark): You say every active site is busy, so extra substrate cannot raise the rate.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. A catalyst speeds up a reaction by giving it a path with a lower .
2. A catalyst is not used up, so it comes out of the reaction .
3. A catalyst leaves the energy released by the reaction .
4. With a lower barrier, a larger share of the collisions has enough energy to .
5. The catalysts of living things are proteins called .
A student says: “A catalyst makes a reaction release more energy, because the reaction goes faster.”
Say whether the student is right. Explain what a catalyst changes and what it leaves alone.
Use the energy profile. Compare the height of the peak with the levels of the reactants and the products. Write your answer first, then compare it with the model answer.
The student is wrong. A catalyst changes only the path. It lowers the activation energy, so more collisions have enough energy to react and the rate goes up.
The reactants and the products sit at the same energies as before, so ΔH does not change. A faster reaction releases its energy sooner. It does not release more of it.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Hydrogen peroxide breaks down into water and oxygen:
At 25 °C the uncatalyzed reaction is too slow to notice, because its activation energy is about 75 kJ/mol. A student adds 0.5 g of solid manganese dioxide (MnO2) to 50 mL of 3.0% H2O2. The mixture makes 12.8 mL of O2 per minute.
(a) Explain at the particle level how MnO2 makes the reaction faster. [2 marks]
(b) State what happens to ΔH when MnO2 is added. Say why. [1 mark]
(c) At the end the student filters out the MnO2, dries it and weighs it. Predict its mass and explain why. [1 mark]
- Barrier (1 mark): You say MnO2 gives the reaction a path with a lower activation energy.
- Collisions (1 mark): You say more collisions have enough energy to react, so the rate rises.
- Energy released (1 mark): You say ΔH stays at −196 kJ/mol, because the reactants and products do not change.
- Not used up (1 mark): You say the mass is the same, because a catalyst comes out unchanged.
Many factories use harsh conditions: toxic metal catalysts, high pressure and temperatures above 400 °C. Engineered enzymes now replace some of them. An enzyme built for 30 °C water cuts the energy a factory uses and avoids toxic solvents. Some of these reactions reach 99.9% purity with no hazardous byproducts.
Adaptive Competency Practice: HS-PS1-5
Test your mastery of reaction rates and catalysis one question at a time. The adaptive engine covers alternative reaction pathways with lower activation energies, catalysts, enzyme denaturation, and experimental reaction progress curves.