Key Stage 4 science · Chemistry – Energy changes in chemistry
GCSE chemistry: energy changes and reaction profiles
Every chemical reaction involves energy moving between the chemicals and their surroundings. In Key Stage 4, students measure these changes, usually as a rise or fall in temperature, and explain them in terms of bonds being broken and made. They also learn to read and sketch reaction profiles, the energy diagrams that show activation energy and whether a reaction releases or absorbs energy overall. At this level the treatment is qualitative.
Energy changes and reaction profiles, explained
In an exothermic reaction, energy is transferred to the surroundings, so the temperature of the mixture rises. Burning fuels, respiration and neutralisation are all exothermic, and so are the chemicals in some hand warmers. In an endothermic reaction, energy is taken in from the surroundings, so the temperature falls. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are endothermic. Students are asked to measure the temperature change in a simple experiment and decide which type a reaction is from the sign of the change.
The measuring is usually done in a polystyrene cup with a lid, because polystyrene is a good insulator and keeps heat loss to the air small. Students record the starting temperature, stir, and record the highest or lowest temperature reached. The statutory requirement is qualitative, so the key skill is interpretation and fair testing, such as using the same volumes, measuring temperature at the same points, repeating the experiment and explaining why some energy is still lost. The statutory wording is qualitative, though many courses go on to calculate overall energy changes from bond energies, so ask how far your child’s school takes it.
The reason for the energy change lies in the bonds. Breaking the bonds in the reactants takes in energy from the surroundings, so bond breaking is endothermic. Making new bonds in the products releases energy to the surroundings, so bond making is exothermic. If more energy is released making bonds than was needed to break them, the reaction is exothermic overall. If it takes more to break bonds than is given out in making them, it is endothermic overall.
A reaction profile is a graph with the energy of the chemicals on the vertical axis and the progress of the reaction on the horizontal axis. The reactants are drawn at one level and the products at another. For an exothermic reaction the products sit lower, with the gap labelled as the overall energy change. For an endothermic reaction the products sit higher. In both cases the curve rises to a peak between them. The height from the reactants to that peak is the activation energy, the minimum energy particles need when they collide for the reaction to happen. That is why even exothermic reactions such as burning paper need a spark or a flame to begin.
Words your child will hear
- Exothermic
- A reaction that transfers energy to the surroundings, so the temperature rises.
- Endothermic
- A reaction that takes in energy from the surroundings, so the temperature falls.
- Surroundings
- Everything around the reacting chemicals, including the solvent, container and air.
- Activation energy
- The minimum energy colliding particles need for a reaction to take place.
- Reaction profile
- An energy diagram showing the energy of reactants and products and the activation energy hump between them.
- Bond energy
- The energy needed to break a particular chemical bond; the same amount is released when it forms.
- Insulator
- A material that reduces energy transfer, such as the polystyrene cup and lid used to limit heat loss.
What the National Curriculum says
The programme of study for Key Stage 4 says:
- Students should be taught about the measurement of energy changes in chemical reactions (qualitative)
- Students should be taught about bond breaking, bond making, activation energy and reaction profiles (qualitative)
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: interpreting a temperature experiment
A student mixes citric acid solution with sodium hydrogencarbonate in a polystyrene cup. The temperature falls from 21 °C to 14 °C. What type of reaction is this, why does the temperature change, and how would its reaction profile look?
- Find the temperature change: the final temperature minus the starting temperature.
14 °C − 21 °C = −7 °C
- The temperature of the mixture fell, so energy was taken in from the surroundings. This is an endothermic reaction.
- Explain in terms of bonds. More energy is needed to break the bonds in the reactants than is released when the new bonds in the products form. The difference is taken from the surroundings, which is why the cup feels cold.
- Describe the profile. Draw the energy axis, put the reactants at a lower level and the products at a higher level, and join them with a curve that rises to a peak above both. Label the gap from the reactants to the peak as the activation energy, and the gap between the two levels as the overall energy change.
- Suggest an improvement. Use a lid and stir gently, take readings at fixed times, and repeat the experiment. Because heat from the room flows in, the lowest temperature reached is slightly higher than it would be in a perfectly insulated cup.
It is an endothermic reaction. Bond breaking takes in more energy than bond making releases, so the products are at a higher energy than the reactants on the profile, with an activation energy peak between them.
Common mistakes and how to help
Believing that breaking bonds releases energy
Everyday language says energy is “released” when something breaks, and a snapping twig seems to give out energy.
What helps: Pulling two magnets apart takes effort, and letting them snap together releases it. Bond breaking works like pulling the magnets apart, so it needs energy. Bond making is the snap together, which releases it.
Thinking exothermic reactions do not need any energy to start
If a reaction gives out energy overall, it seems it should just happen.
What helps: Every reaction has an activation energy. A match gives the first push for burning, and the energy released then keeps it going. On a profile, even a downhill reaction has a hump to climb first.
Saying a catalyst changes the overall energy change
Catalysts are described as making reactions “easier”, which sounds like changing the energy.
What helps: A catalyst gives a different route with a lower activation energy, so the hump is lower. The reactants and products stay at the same levels, so the overall energy change is unchanged.
Thinking energy is used up or created in a reaction
A cold pack or a burning fire can look as though energy appears from nowhere or vanishes.
What helps: Energy is always conserved. In an exothermic reaction it moves from the chemicals to the surroundings, and in an endothermic reaction it moves the other way. Ask “where did the energy come from and where did it go?” each time.
How to help at home
- Look for hot and cold packs. Single-use hand warmers get warm as iron reacts with oxygen in the air, which is exothermic. Instant cold packs for injuries use a dissolving process that takes in energy. Ask your child to name each one and say which direction the energy moves.
- Measure a reaction that gets colder. If you have a kitchen thermometer, pour a few tablespoons of vinegar into a cup, record the temperature, stir in a teaspoon of bicarbonate of soda and record the lowest temperature reached once the fizzing dies down. The fall of a few degrees shows an endothermic reaction. Do it on a tray, repeat it to see whether the result is the same, and do not taste anything.
- Sketch reaction profiles for everyday reactions. Pick a fire, a hand warmer and the vinegar and bicarbonate reaction, and ask your child to sketch the profile for each, labelling reactants, products and activation energy. A ball rolling over a small hill and ending lower down is a helpful picture for an exothermic reaction.
- Ask why the cup has a lid. Whenever you see an insulated mug, ask how it works and which way the energy would move if the lid were off. This links to fair testing, since the same ideas explain why chemistry experiments use polystyrene cups with lids.
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