Exothermic vs endothermic
When a reaction happens, bonds in the reactants break (which takes in energy) and new bonds in the products form (which releases energy). The overall energy change depends on which of those wins.
- Exothermic reaction
- A reaction that releases energy to the surroundings, so the temperature rises. Examples: combustion, neutralisation, most oxidation.
- Endothermic reaction
- A reaction that takes in energy from the surroundings, so the temperature falls. Examples: thermal decomposition, and dissolving some salts.
A quick test: if the reaction mixture or beaker gets warmer, it is exothermic; if it gets colder, it is endothermic.
Energy profile diagrams
An energy profile (reaction pathway diagram) plots the energy of the particles as the reaction proceeds. Two features matter.
First, the overall energy change. In an exothermic reaction the products sit lower than the reactants, because energy has been released. In an endothermic reaction the products sit higher, because energy has been taken in.
Second, the activation energy, drawn as a hump between reactants and products.
- Activation energy (\(E_\text{a}\))
- The minimum energy that colliding particles must have for a reaction to occur. It is the height of the hump in an energy profile.
Even an exothermic reaction needs this initial push: a match will not light itself, but a small spark supplies enough energy to get it over the hump, after which it releases far more.
A reaction profile shows the reactants at 250 kJ, a peak at 400 kJ, and the products at 150 kJ. Is the reaction exothermic or endothermic, and what is the activation energy?
Common slip
Activation energy is measured from the reactants up to the peak, not from zero and not from the products. Read the hump height relative to where the reactants start.
Collision theory
Reactions happen when particles collide. But not every collision works. Collision theory says a collision only leads to a reaction if two conditions are met:
- the particles collide with enough energy (at least the activation energy), and
- they collide in the correct orientation.
A collision that meets these conditions is called a successful (or effective) collision. So the rate of a reaction depends on two things: how often particles collide, and what fraction of those collisions are successful. Every rate factor below works by changing one or both of these.
Factors that change rate
Here are the four factors you must be able to explain, each in terms of collisions.
| Factor | Effect on rate | Explanation (collisions) |
|---|---|---|
| Higher temperature | Faster | Particles move faster, so they collide more often and a greater fraction have the activation energy, giving more successful collisions. |
| Higher concentration (or pressure for gases) | Faster | More particles in the same volume, so collisions happen more frequently. |
| Larger surface area (smaller pieces) | Faster | More of the solid is exposed, so more collisions with it can happen per second. |
| Adding a catalyst | Faster | Provides an alternative pathway with a lower activation energy, so a greater fraction of collisions are successful. The catalyst is not used up. |
Temperature is the strongest, because it does two jobs at once: it speeds up collisions and it raises the fraction that are energetic enough. A catalyst is special because it does not change the amount of product, only the speed, and it can be recovered unchanged at the end.
Answer template
For any rate question, use two phrases: "more frequent collisions" and, where relevant, "a greater proportion of collisions have the activation energy". Naming the mechanism, not just the outcome, is what earns the mark.
Where this is assessed
Explaining rate using collision theory is Criterion A. Designing a fair test to investigate one factor, controlling the others, is core Criterion B (Inquiring and designing), and plotting the results is Criterion C.
Check yourself
1. A beaker gets cold during a reaction. Exothermic or endothermic? +
The surroundings lose energy to the reaction, so the temperature falls. The reaction is endothermic.
2. Why does powdered marble react faster with acid than a single lump of the same mass? +
Powder has a much larger surface area exposed to the acid, so there are more frequent collisions between acid particles and the marble each second, giving a faster rate.
3. Explain, using collisions, why a reaction speeds up when heated. +
Heating gives the particles more kinetic energy, so they move faster and collide more often. Crucially, a greater fraction of collisions now have at least the activation energy, so more collisions are successful and the rate rises.
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