Energy changes & rates
Exothermic and endothermic reactions and reaction profiles, rates of reaction and collision theory, and reversible reactions.
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Every point below, worked through in one video, and each point links straight to its own part.
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Energy changes & rates, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Distinguish exothermic and endothermic reactions
An exothermic reaction gives out energy, usually as heat, so the surroundings warm up - combustion and neutralisation are examples, and hand warmers use the effect. An endothermic reaction takes energy in, so the surroundings cool - as in some sports cold packs. Measuring the temperature change tells you which is happening.
Interpret reaction profiles and activation energy
A reaction profile plots energy through a reaction. Reactants must first climb an energy barrier - the activation energy - before products form. If the products sit lower than the reactants the reaction is exothermic; if higher, endothermic. The height of the barrier explains why some reactions need a spark to get going.
Describe rate of reaction and how it is measured
Rate of reaction is how fast reactants turn into products. You can measure it by timing how quickly a product forms - the volume of gas given off, or how long a cross takes to disappear as a precipitate forms - or how fast a reactant is used up. Rate = amount changed ÷ time, and it usually slows as reactants run out.
Use collision theory to explain what changes the rate
Reactions happen when particles collide with enough energy. Raising the temperature makes particles move faster and collide harder; a higher concentration or pressure packs particles closer for more collisions; breaking a solid into powder increases its surface area. Each of these speeds the reaction up because collisions become more frequent or more energetic.
Explain how catalysts speed up reactions
A catalyst speeds up a reaction by providing a different pathway with a lower activation energy, so more collisions succeed. It is not used up, so a small amount works again and again. Enzymes are biological catalysts, and catalysts are vital in industry because they cut the energy - and cost - needed.
Describe reversible reactions and equilibrium
Some reactions can go both ways, shown by ⇌. In a closed container the forward and backward reactions eventually happen at the same rate - this is equilibrium, where the amounts of reactants and products stay constant (though both reactions continue). If the forward reaction is exothermic, the backward one is endothermic by the same amount.
Energy changes & rates key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Exothermic reaction
- A reaction that transfers energy to the surroundings, so the temperature rises. Combustion and neutralisation are exothermic.
- Endothermic reaction
- A reaction that takes in energy from the surroundings, so the temperature falls. Thermal decomposition is endothermic.
- Activation energy
- The minimum energy that colliding particles must have in order to react.
- Catalyst
- A substance that speeds up a reaction without being used up, by providing an alternative reaction pathway with a lower activation energy.
- Collision theory
- Reactions happen when particles collide with enough energy. Anything that makes collisions more frequent or more energetic increases the rate.
- Reversible reaction
- A reaction whose products can react to re-form the reactants, written with the ⇌ symbol.
- Equilibrium
- The state of a reversible reaction in a closed system where the forward and reverse reactions happen at the same rate, so the amounts of each substance stay constant.
Common mistakes in Energy changes & rates
The slips that cost marks year after year, and what to do instead.
Saying that breaking bonds releases energy.Higher only
Breaking bonds takes energy in, and making bonds gives energy out. A reaction is exothermic when making the new bonds releases more energy than breaking the old ones took in.
Writing "more collisions" to explain a faster rate, without saying they are more frequent.
Rate depends on how often particles collide with enough energy, so write "more frequent collisions". For a temperature rise, also say the particles move faster, so more collisions have at least the activation energy.
Saying a catalyst gives the particles more energy, or gets used up.
A catalyst provides a different pathway with a lower activation energy, so more collisions are successful. It is not used up, and it does not change how much energy the particles have.
Saying the reaction stops at equilibrium, or that the amounts of reactants and products become equal.
At equilibrium both reactions keep going at exactly the same rate, so the amounts stay constant but are usually not equal. It can only happen in a closed system, where nothing escapes.
Drawing the activation energy arrow from the products, or from the bottom of the energy axis.
Activation energy runs from the reactants' energy level up to the peak of the curve. The overall energy change is the gap between reactants and products, with products lower for an exothermic reaction.
Practice
Try a Energy changes & rates question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
72 cm³ of hydrogen is collected from a reaction in 1 minute 20 seconds. Work out the mean rate of reaction, in cm³/s.
- 0.72
- 1.2
- 5760
- 0.9
Show the answer and the working
Answer: 0.9
1 minute 20 seconds is 80 s, so mean rate = 72 ÷ 80 = 0.9 cm³/s.
- First convert the time: 1 minute 20 seconds = 60 + 20 = 80 seconds.
- Mean rate = volume of gas ÷ time taken.
- Mean rate = 72 ÷ 80 = 0.9 cm³/s.