Why do some reactions warm their surroundings while others cool them, and how do we show this on an energy diagram?
Describe exothermic and endothermic reactions in terms of energy transfer, interpret simple energy level diagrams, and link energy change to bond breaking and bond making
A focused answer to the N(A) Chemistry outcome on energy changes in reactions. Exothermic versus endothermic, reading simple energy level diagrams, and how bond breaking takes in energy while bond making gives it out.
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What this dot point is asking
The syllabus wants you to describe exothermic and endothermic reactions in terms of energy transfer with the surroundings, to read a simple energy level diagram, and to link the overall energy change to bond breaking and bond making. The one idea that ties it all together is that breaking bonds takes in energy and making bonds gives out energy, and the balance between the two decides whether a reaction warms or cools its surroundings.
The answer
Exothermic reactions
An exothermic reaction gives out energy to the surroundings, usually as heat. You can tell because the temperature rises. Burning fuels, neutralising an acid with an alkali, and most displacement reactions are exothermic. Because energy leaves the chemicals, the products store less energy than the reactants.
Endothermic reactions
An endothermic reaction takes in energy from the surroundings. You can tell because the temperature falls (the mixture feels cold). Dissolving some salts in water, and thermal decomposition (heating a compound to break it down), are endothermic. Because energy enters the chemicals, the products store more energy than the reactants.
Energy level diagrams
An energy level diagram shows the energy of the reactants and products as two horizontal lines:
- in an exothermic reaction the products line is lower than the reactants line, and the drop is the energy given out,
- in an endothermic reaction the products line is higher than the reactants line, and the rise is the energy taken in.
The difference in height between the two lines is the overall energy change of the reaction.
Bond breaking and bond making
Every reaction breaks the bonds in the reactants and makes new bonds in the products:
- breaking bonds takes in energy (you must pull atoms apart),
- making bonds gives out energy (atoms snap together and release energy).
If more energy is given out making bonds than is taken in breaking them, the reaction is exothermic. If more energy is taken in breaking bonds than is given out making them, the reaction is endothermic.
Examples in context
Example 1. Instant cold packs. A sports cold pack works by dissolving a salt such as ammonium nitrate in water, an endothermic change that takes in heat from its surroundings and makes the pack feel cold. This is a direct everyday use of an endothermic process to treat an injury.
Example 2. Hand warmers and fuels. A chemical hand warmer and a gas stove both rely on exothermic reactions that give out heat. Knowing a reaction is exothermic lets engineers design devices that release energy in a useful, controlled way, from warming hands to heating homes.
Try this
Q1. A reaction causes the temperature of a solution to fall. State whether it is exothermic or endothermic. [1 mark]
- Cue. The temperature falls, so energy is taken in from the surroundings; the reaction is endothermic.
Q2. State whether breaking bonds takes in or gives out energy, and do the same for making bonds. [2 marks]
- Cue. Breaking bonds takes in energy; making bonds gives out energy.
Q3. On an energy level diagram for an exothermic reaction, state whether the products are higher or lower than the reactants, and why. [2 marks]
- Cue. The products are lower than the reactants, because energy has been given out to the surroundings during the reaction.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksWhen a student adds magnesium to dilute hydrochloric acid, the temperature of the mixture rises from to . (a) State whether the reaction is exothermic or endothermic. (b) Explain your answer in terms of energy. (c) Draw what the energy of the products looks like compared with the reactants.Show worked answer →
(a) The reaction is exothermic.
(b) The temperature rises, so energy is released to the surroundings. The reaction gives out heat, which is what exothermic means.
(c) On an energy level diagram, the products are at a lower energy than the reactants, because energy has been given out to the surroundings.
What markers reward: stating exothermic, linking the temperature rise to energy released to the surroundings, and products lower than reactants on the diagram.
Original3 marksPhotosynthesis takes in light energy to make glucose. (a) State whether this is an exothermic or endothermic change. (b) Explain why, in terms of energy taken in or given out.Show worked answer →
(a) Photosynthesis is endothermic.
(b) It takes in energy (light energy) from the surroundings to make the products. A reaction that takes in energy from the surroundings is endothermic, so the products store more energy than the reactants.
What markers reward: stating endothermic, and explaining that energy is taken in from the surroundings so the products store more energy.
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