Define exothermic and endothermic changes. Give two examples in each case.
Define exothermic and endothermic changes. Give two examples in each case.
When a chemical reaction occurs, energy is transferred to, or from, the surroundings - and there is often a temperature change. For example, when a bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer.
Exothermic reactions
These are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:
- burning
- Neutralization reactions between acids and alkalis
- the reaction between water and calcium oxide
Endothermic reactions
These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are:
- electrolysis
- the reaction between ethanoic acid and sodium carbonate
- the thermal decomposition of calcium carbonate in a blast furnace
The animation shows an exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.
Exothermic processes Endothermic processes making ice cubes melting ice cubes formation of snow in clouds conversion of frost to water vapor condensation of rain from water vapor evaporation of water a candle flame forming a cation from an atom in the gas phase mixing sodium sulfite and bleach baking bread rusting iron cooking an egg burning sugar producing sugar by photosynthesis forming ion pairs separating ion pairs Combining atoms to make a molecule in the gas phase splitting a gas molecule apart mixing water and strong acids mixing water and ammonium nitrate mixing water with an anhydrous salt making an anhydrous salt from a hydrate crystallizing liquid salts (as in sodium acetate in chemical handwarmers) melting solid salts nuclear fission reaction of barium hydroxide octahydrate crystals with dry ammonium chloride mixing water with calcium chloride reaction of thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate
When a chemical reaction occurs, energy is transferred to, or from, the surroundings - and there is often a temperature change. For example, when a bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer.
Exothermic reactions
These are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:
- burning
- Neutralization reactions between acids and alkalis
- the reaction between water and calcium oxide
Endothermic reactions
These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are:
- electrolysis
- the reaction between ethanoic acid and sodium carbonate
- the thermal decomposition of calcium carbonate in a blast furnace
The animation shows an exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.
| Exothermic processes | Endothermic processes |
| making ice cubes | melting ice cubes |
| formation of snow in clouds | conversion of frost to water vapor |
| condensation of rain from water vapor | evaporation of water |
| a candle flame | forming a cation from an atom in the gas phase |
| mixing sodium sulfite and bleach | baking bread |
| rusting iron | cooking an egg |
| burning sugar | producing sugar by photosynthesis |
| forming ion pairs | separating ion pairs |
| Combining atoms to make a molecule in the gas phase | splitting a gas molecule apart |
| mixing water and strong acids | mixing water and ammonium nitrate |
| mixing water with an anhydrous salt | making an anhydrous salt from a hydrate |
| crystallizing liquid salts (as in sodium acetate in chemical handwarmers) | melting solid salts |
| nuclear fission | reaction of barium hydroxide octahydrate crystals with dry ammonium chloride |
| mixing water with calcium chloride | reaction of thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate |
