What makes a chemical reaction faster or slower?
TEAS 7 outline: How conditions affect chemical reactions
Also covers HESI A2 Chemistry (Reaction rates and equilibrium)
- Particles must collide in the right orientation with enough energy.
- That minimum energy is called the activation energy.
- Higher temperature, higher concentration and more surface area speed most reactions.
- Catalysts and enzymes lower activation energy and are not used up.
- At equilibrium, forward and reverse rates are equal, not the amounts.
Key facts
- For a reaction to happen, particles must collide, in the right orientation, with at least the activation energy.
- Higher temperature speeds most reactions: particles move faster and a larger fraction have enough energy to react.
- Higher concentration (more particles in the same volume, for example by compressing a gas) means more collisions, so a faster rate.
Show all 7 key factsShow fewer
- Smaller pieces have more surface area, so powdered iron reacts with acid faster than one large chunk.
- A catalyst lowers the activation energy by providing a different pathway; it is not consumed and gives the same products.
- Enzymes are biological catalysts, usually proteins, with an active site that fits a specific substrate; high heat or extreme pH can denature them.
- At equilibrium, the forward and reverse reactions continue at equal rates, so concentrations stay constant but are usually not equal.
Quick check1 question · 2 more at the end
Which statement best defines activation energy?
Correct answer: B.
Activation energy is the energy barrier, or hill, that colliding reactant particles must get over before they can form products. It is separate from the overall energy change of the reaction (ΔH), which compares the energy of the reactants with the energy of the products. Catalysts speed reactions by lowering this barrier, not by adding energy.
A Heat released to the surroundings describes an exothermic reaction's energy change, not the barrier particles must overcome.
B Correct: activation energy is the minimum collision energy needed for reactants to turn into products.
C The energy difference between reactants and products is the enthalpy change (ΔH), a different quantity from the activation energy.
D A catalyst does not add energy; it provides a pathway with a lower activation energy.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Reactions happen when particles collide with enough energy, the activation energy, and the right orientation. Raising temperature, increasing concentration, and increasing surface area all make useful collisions more frequent. Catalysts, including enzymes, speed reactions by offering a path with lower activation energy and are not used up. At equilibrium, forward and reverse rates are equal.
1 What has to happen for a reaction to occur?Chemists explain reaction speed with collision theory. 1 min
Chemists explain reaction speed with collision theory. Reactant particles have to bump into each other. A collision only works if two things are true: the particles hit in an orientation that lets the right atoms touch, and they hit hard enough.
That minimum energy is the activation energy. Picture it as a hill between the reactants and the products. During a successful collision, the particles briefly form an unstable, short-lived arrangement at the top of the hill. It is called the activated complex or transition state. Then the particles fall down the other side as products.
So anything that makes collisions more frequent, or gives more of them enough energy, speeds a reaction up. Anything that does the opposite slows it down.
2 How do temperature, concentration, and surface area change the rate?Some everyday examples: for many reactions, a rise of 10 °C roughly doubles the rate. 1 min
| Change | Effect on rate | Why |
|---|---|---|
| Raise the temperature | Faster | Particles move faster, collide more, and a bigger fraction have at least the activation energy |
| Increase concentration (or compress a gas into less space) | Faster | More particles in the same space means more collisions |
| Increase surface area (grind or cut smaller) | Faster | More of the solid is exposed to the other reactant |
| Add a catalyst | Faster | A new pathway with a lower activation energy |
| Lower the temperature or dilute a reactant | Slower | Fewer collisions, and fewer with enough energy |
Some everyday examples: for many reactions, a rise of 10 °C roughly doubles the rate. Phosphorus burns much faster in pure oxygen than in air, because the oxygen is more concentrated. Large pieces of iron react with acid more slowly than iron powder, and a log smolders while sawdust can burn explosively. The nature of the reactants matters too: sodium reacts with air overnight, while iron barely changes.
3 How do catalysts and enzymes work?A catalyst is a substance that speeds up a reaction without being used up. 1 min
A catalyst is a substance that speeds up a reaction without being used up. It does not push the particles harder; it provides a different reaction pathway with a lower activation energy, so more collisions can succeed at the same temperature. The starting materials and the final products are the same, and so is the overall energy difference between them.
Hydrogen peroxide (2H₂O₂ → 2H₂O + O₂) breaks down very slowly in the bottle. Pour it on an open wound and it foams, because substances in the tissue speed up the breakdown and release oxygen gas quickly.
Enzymes are the body's catalysts, usually proteins. Each enzyme has an active site whose shape fits a specific substrate, so an enzyme usually speeds up just one particular reaction.
In the induced fit model, the enzyme shifts its shape slightly as the substrate binds. Enzymes work best within an optimal temperature and pH range. Very high temperatures or extreme pH can denature an enzyme. That changes the shape of its active site so it can no longer do its job, and the reaction slows.
4 What is chemical equilibrium?Many reactions are reversible: products can react to re-form reactants. 1 min
Many reactions are reversible: products can react to re-form reactants. A double arrow (⇌) shows this. As reactants are used up, the forward reaction slows; as products build up, the reverse reaction speeds up. Equilibrium is reached when the forward and reverse rates are equal.
At equilibrium, the reaction has not stopped. Both directions keep going, but because they cancel out, the concentrations of reactants and products stay constant. Constant does not mean equal: an equilibrium mixture may be mostly products or mostly reactants.
5 What happens when you disturb an equilibrium?Le Châtelier's principle says a system at equilibrium shifts to counteract a stress. 1 min
Le Châtelier's principle says a system at equilibrium shifts to counteract a stress. Add more reactant, or remove product, and the equilibrium shifts toward the products. Add product, or remove reactant, and it shifts toward the reactants. For an endothermic reaction, treat heat as a reactant, so heating shifts it toward the products. For an exothermic reaction, heat is a product, so heating shifts it toward the reactants.
A catalyst speeds up the forward and reverse reactions equally. It helps the system reach equilibrium sooner but does not shift it.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: A catalyst lowers activation energy; it does not add energy, heat the mixture, or get used up.
Watch out: Heating an enzyme far above its optimal temperature slows the reaction, because the enzyme denatures; the 'hotter is faster' rule fails here.
Watch out: Equilibrium means equal rates, not equal amounts of reactants and products.
Watch out: At equilibrium the reaction has not stopped; both directions keep going at the same rate.
Watch out: A catalyst does not shift an equilibrium; it only helps the system reach equilibrium sooner.
How to remember it
Memory hook: "More hits, harder hits, or a lower hill." Temperature, concentration, and surface area give more (and harder) collisions; a catalyst lowers the hill.
Self-check:
- Explain in one sentence why iron powder reacts with acid faster than a single iron nail.
- Say what a catalyst changes and what it does not change.
- Explain why heating an enzyme far past its optimal temperature makes its reaction slower, not faster.