Chemical changes include exothermic reactions, which release heat, and endothermic reactions, which absorb heat. Many exothermic and endothermic reactions are used in our daily lives.
Basic Knowledge
Exothermic reaction: A reaction that releases heat to the surroundings as a chemical change occurs. The temperature rises after the reaction.
Examples:
・Combustion (combustion of natural gas, charcoal, alcohol, and so on)
・Oxidation of iron: 4Fe + 3O2 → 2Fe2O3 (rusting and disposable hand warmers)
・Neutralization reaction (heat is produced when an acid reacts with a base)
・Neutralization of hydrochloric acid and sodium hydroxide: the reaction vessel becomes warm
Endothermic reaction: A reaction that absorbs heat from the surroundings as a chemical change occurs. The temperature falls after the reaction.
Examples:
・Reaction between sodium hydrogen carbonate and citric acid (cooling packs)
・Reaction between barium hydroxide and ammonium chloride: Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH3 + 2H2O (the temperature falls greatly)
・Photosynthesis (organic substances are produced from carbon dioxide and water)
Exothermic reaction (heat is released by a chemical change. The temperature of the surroundings rises after the reaction)
Endothermic reaction (heat is absorbed by a chemical change. The temperature of the surroundings falls after the reaction)
Chemical energy (the chemical energy possessed by a substance. It is converted into heat, light, and so on during a reaction)
Disposable hand warmer (an application of an exothermic reaction that uses heat produced by the oxidation of iron)
Cooling pack (uses heat absorption caused by the dissolution of substances such as ammonium nitrate)
Activation energy (the minimum energy required to start a chemical change)
Further Study
Exothermic and endothermic reactions are determined by the direction of energy transfer. If the chemical energy possessed by substances becomes lower after a reaction, heat is released (exothermic); if it becomes higher, heat is absorbed (endothermic).
Disposable hand warmers contain iron powder, salt water, activated carbon, and water, and are an application of an exothermic reaction using the oxidation of iron. Fine iron powder is used to make it easier to take in oxygen.
Classification of familiar exothermic and endothermic reactions:
Exothermic: combustion of fuels, hand warmers, neutralization reactions, reactions between metals and acids
Endothermic: cooling packs, photosynthesis, citric acid + baking soda, production of ammonia
The law of conservation of mass holds for both types of reaction. Only energy enters or leaves; the total number of atoms does not change.
- Exothermic reaction = releases heat (the temperature of the surroundings rises)
- Endothermic reaction = absorbs heat (the temperature of the surroundings falls)
- Combustion, neutralization, and oxidation of iron are exothermic reactions
- Photosynthesis, cooling packs, and baking soda + citric acid are endothermic reactions
- Disposable hand warmer = heat produced by oxidation of iron
- The law of conservation of mass holds for both exothermic and endothermic reactions
- Activation energy = energy required to start a reaction
Points to Note
① The temperature of the surroundings rises in an exothermic reaction (because heat leaves the system), while it falls in an endothermic reaction (because heat is absorbed). Be careful, since these are easy to reverse. ② Photosynthesis is an endothermic reaction that absorbs light energy and synthesizes organic substances. ③ A neutralization reaction releases heat whenever an acid reacts with a base. ④ Even an endothermic reaction may require initial energy (activation energy) before it starts.
Practice
- State the type of chemical change used in a disposable hand warmer and the substances involved.
- Classify the following chemical changes as exothermic or endothermic: ① combustion of carbon ② photosynthesis ③ rusting of iron
- What happens to the temperature of the surroundings when an endothermic reaction occurs? Explain why.