Distinguish exothermic and endothermic reactions by energy change, draw and interpret energy profile diagrams, and explain energy change in terms of bond breaking and bond forming

What this dot point is asking

SEAB wants you to distinguish exothermic and endothermic reactions by whether they give out or take in heat, draw and read energy profile diagrams, and explain the overall energy change in terms of the energy needed to break bonds and the energy released when bonds form. This is the energetics core of the module, examined both qualitatively and with simple profile diagrams.

The answer

Exothermic and endothermic reactions

Reactions transfer energy to or from their surroundings:

• An exothermic reaction gives out heat energy to the surroundings, so the temperature of the surroundings rises. Combustion, neutralisation and most displacement reactions are exothermic.
• An endothermic reaction takes in heat energy from the surroundings, so the temperature of the surroundings falls. Examples include thermal decomposition and the dissolving of some salts such as ammonium nitrate.

A simple temperature measurement before and after tells you which a reaction is: a rise means exothermic, a fall means endothermic.

Energy profile diagrams

An energy profile plots the energy of the chemicals as the reaction proceeds (reactants on the left, products on the right):

• Exothermic: the products are at a lower energy than the reactants, so energy is released. The downward step from reactants to products is the energy given out.
• Endothermic: the products are at a higher energy than the reactants, so energy is absorbed.

In both cases there is an energy "hump" between reactants and products. The height of the hump above the reactants is the activation energy, the minimum energy needed for the reaction to start (to begin breaking bonds).

Bond breaking and bond forming

The overall energy change comes from two opposite processes:

• Breaking bonds takes in energy (it is endothermic).
• Forming bonds gives out energy (it is exothermic).

Whether a reaction is exothermic or endothermic overall depends on the balance:

• If more energy is released forming the product bonds than is taken in breaking the reactant bonds, the reaction is exothermic.
• If more energy is taken in breaking bonds than is released forming them, the reaction is endothermic.

This is the explanation examiners want whenever they ask "in terms of bonds".

Examples in context

Example 1. Hand warmers and cold packs. A reusable hand warmer uses an exothermic process to release heat and warm your hands, while an instant cold pack uses an endothermic process (a salt dissolving) that absorbs heat and feels cold. Both are everyday devices built directly on the difference between exothermic and endothermic changes.

Example 2. Respiration and photosynthesis. Respiration releases energy from glucose (exothermic), which is why living things are warm, while photosynthesis takes in light energy to build glucose (endothermic). The pair shows the two energy directions operating in biology, linking chemistry to living systems.

Try this

Q1. State whether a reaction that makes its surroundings colder is exothermic or endothermic. [1 mark]

• Cue. Endothermic (it takes in heat from the surroundings).

Q2. On an energy profile, state where the products lie relative to the reactants for an exothermic reaction. [1 mark]

• Cue. The products are lower in energy than the reactants.

Q3. Explain, in terms of bonds, why a reaction is exothermic. [2 marks]

• Cue. More energy is released when the product bonds form than is taken in to break the reactant bonds, so energy is given out overall.