Energy and Reactions, Chemistry, AQA Additional Science

Information on exothermic and endothermic reactions and energy and reversible reactions.

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Exothermic and Endothermic reactions

Energy is transferred when bonds are broken and made when a chemical reaction takes place. 'Exothermic' reactions transfer energy to the surroundings. The energy transferred often heats up up the surroundings and so the temperature increases.

Examples of exothermic reactions are: Combustion such as burning fuels and metals, respiration and neutralisation

Endothermic reactions transfer energy from the surroundings. Some cause a decrease in temperature and others require a supply of energy. When some solid compounds are mixed with water, the temperature decreases because endothermic reactions happen. Thermal decomposition reactions need a supply of heat to keep going. Photosynthesis is an important endothermic reaction that uses light energy.

Endothermic reactions take in heat.

Heat exits (transforms from) exothermic reactions.

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Energy and reversible reactions

In reversible reactions, the forward and reverse reactions involve equal but opposite energy transfers A reversible reaction that is exothermic in one direction must be endothermic in the other direction.

The amount of energy released by the exothermic reaction exactly equals the amount n taken in by the endothermic reaction.

Changing the temperature of ma reversible reaction in a closed system changes the amounts of the reactants and products. The amount of products from the endothermic reactions increase if the temperature increases.

If we decrease the temperature, the amount of products from the exothermic reaction increases.

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