Reactions and temperature changes

Energy is conserved in chemical reactions. The total amount of energy in the universe never changes. When a chemical reaction occurs, energy is transferred to or from the reaction’s immediate surroundings.

  • in endothermic reactions, energy enters.
  • in exothermic reactions, energy exits.

Exothermic reactions

An exothermic reaction transfers energy to its surroundings. The temperature of its surroundings increases.

Exothermic reactions include:

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You see exothermic reactions in burning candles, your metabolism, setting cement, even rusting.

Endothermic reactions

An endothermic reaction takes in energy from its surroundings. The temperature of its surroundings decreases.

Endothermic reactions include:

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You see endothermic reactions at work in photosynthesis, cooking, water evaporation and ice melting.

An exothermic reaction between dilute sodium hydroxide and hydrochloric acid.

Sodium hydroxide solution is poured into a beaker of hydrochloric acid which contains a thermometer showing room temperature

1. Sodium hydroxide solution is poured into a beaker of hydrochloric acid. Inside is a thermometer showing room temperature.