Energy changes & rates
Exothermic and endothermic reactions and reaction profiles, rates of reaction and collision theory, and reversible reactions.
Learn
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.
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.