Why do some reactions give out heat and others take it in, and how do we show this on an energy profile?
Exothermic and endothermic reactions and everyday examples, temperature changes in reactions, reaction profile diagrams, activation energy, and the energy change in terms of breaking and making bonds.
A focused answer to the OCR Gateway GCSE Combined Science A topic C3 on energetics, covering exothermic and endothermic reactions and examples, temperature changes, reaction profile diagrams, activation energy, and the energy change explained by breaking and making bonds.
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What this topic is asking
OCR wants you to distinguish exothermic and endothermic reactions, give examples and uses, interpret reaction profile diagrams including activation energy, and explain the overall energy change in terms of breaking and making bonds.
Exothermic and endothermic reactions
You can tell them apart by measuring the temperature change with a thermometer. Exothermic reactions include combustion (burning fuels), neutralisation (acid and alkali), and many oxidation reactions; everyday uses include hand warmers and self-heating cans. Endothermic reactions include thermal decomposition (such as heating a metal carbonate) and the reaction of citric acid with sodium hydrogen carbonate; an everyday use is a sports injury cold pack, which gets cold when the chemicals inside react. In the required practical you measure the temperature change of reactions such as neutralisation or displacement to classify them.
Reaction profiles and activation energy
A reaction profile (energy level diagram) shows how the energy changes as a reaction proceeds.
The activation energy is the energy barrier that must be overcome to start a reaction, which is why some reactions need heat or a spark to begin. A catalyst provides an alternative pathway with a lower activation energy, so more particles have enough energy to react and the reaction speeds up, without the catalyst being used up.
Bond breaking and bond making
The overall energy change comes from the bonds. Breaking the bonds in the reactants takes in energy (this part is endothermic). Making the new bonds in the products releases energy (this part is exothermic). The net energy change depends on the balance:
- If more energy is released making bonds than is taken in breaking them, the reaction is exothermic overall (and the temperature rises).
- If less energy is released making bonds than is taken in breaking them, the reaction is endothermic overall (and the temperature falls).
This bond-energy idea explains why the same reaction always has the same energy change, and it links to calculating energy changes from bond energies on Higher tier.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksExplain the difference between an exothermic and an endothermic reaction in terms of energy and temperature, and give one everyday use of each.Show worked answer →
A Chemistry Paper 3 structured question on energetics. Reward: an exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings rises (for example combustion, neutralisation, and many oxidation reactions; a use is a self-heating can or a hand warmer). An endothermic reaction takes in energy from the surroundings, so the temperature falls (for example thermal decomposition and the reaction of citric acid with sodium hydrogen carbonate; a use is a sports injury cold pack). Markers credit energy released versus taken in, the temperature rising versus falling, and a valid everyday use of each.
OCR 20214 marksUsing the idea of breaking and making bonds, explain why a reaction is exothermic. Sketch is not required, but refer to the energy of bond breaking and bond making.Show worked answer →
A C3 question on bond energy. Reward: breaking the bonds in the reactants takes in energy (it is endothermic), and making the new bonds in the products releases energy (it is exothermic). In an exothermic reaction, more energy is released when the new bonds form than was taken in to break the old bonds, so there is a net release of energy to the surroundings and the temperature rises. (In an endothermic reaction the opposite is true: more energy is needed to break bonds than is released making them.) Markers credit bond breaking taking in energy, bond making releasing energy, and the comparison that determines whether the overall reaction releases or absorbs energy.
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