Exothermic and endothermic reactions

Energy is conserved in a reaction: it moves between the chemicals and their surroundings.

• Exothermic: energy is transferred to the surroundings, so the temperature of the surroundings rises. Combustion, respiration, neutralisation, most oxidation reactions. Uses: hand warmers, self-heating cans.
• Endothermic: energy is taken in from the surroundings, so the temperature falls. Thermal decomposition, photosynthesis, citric acid with sodium hydrogencarbonate. Uses: instant cold packs for sports injuries.

1. Start 20.0 °C, end 27.5 °C
2. change = 27.5 − 20.0 = +7.5 °C
3. a rise, so exothermic

Breaking bonds takes in energy.

Making bonds releases energy. If making the new bonds releases more than breaking the old ones took in, the reaction is exothermic overall.

A thermometer in the mixture reads the surroundings: the solution and the cup. That is why the reading rises in an exothermic reaction.