C2.5 Endothermic, Exothermic and Reversible Reactions

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Exothermic and Endothermic Reactions

Exothermic Reactions

  • increase temperature
  • gives out more energy
  • release energy - heats up surrounding

Endothermic Reactions

  • decreases temperature
  • absorb energy into chemicals from surroundings
  • surroundings = inside test tube

An exothermic reaction is one where energy is transferred from the reacting chemicals to the surroundings. Surroundings get hot.

An endothermic reaction is one where energy is transferred from the surroundings to the reacting chemicals. Surroundings get cold.

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Exothermic Reactions

Most chemical reactions are Exothermic. Exothermic Reactions can happen spontaneously. E.g. Combustion, Respiration, Neutralising acid with alkalis, Reacting metals with acids, The Thermit process

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Reversible Reactions

  • exothermic in one direction and endothermic in the other direction


Hydrated Copper Sulphate <--> Anhydrous Copper Sulphate + Water

CuSO4.5H2O <--> CuSO4 + 5H2O

Removing water from the hydrated copper sulphate is a chemical reaction and is different from evaporation.

The energy transferred in each direction is the same.

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