Exothermic and endothermic reactions
Warmer or cooler
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.
- Start 20.0 °C, end 27.5 °C
- change = 27.5 − 20.0 = +7.5 °C
- a rise, so exothermic
Bonds are the reason
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.
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