What Are Some Examples of Exothermic and Endothermic Processes?

In chemical reactions, energy is required in the breaking up of atomic bonds. As the atoms rearrange to form new chemical bonds, energy is released in form of heat. In exothermic reactions, the total energy released is greater than the total energy spent in breaking up the bonds. However, for endothermic reactions, the total energy spent is greater than the total energy released from the formation of the new bonds. In this regard, exothermic processes cause a release of heat into the surrounding region while endothermic processes result in the absorption of heat from the surrounding environment.

Examples of Exothermic Reactions

Combustion of Coal
The combustion of coal results in the emission of large amounts of heat energy. This heat can be used for steam generation, cooking, and other industrial applications. The equation of combustion of coal can be represented as:

Carbon + Oxygen Ì¢‰Ûʉ۪ Carbon dioxide + Heat

C(s) + O2 (g) Ì¢‰Ûʉ۪ CO2 (g) + Heat

Rusting of Iron
Iron metal reacts with oxygen and water to form rust Ì¢‰â‰ÛÏThe red-orange-brown substance on corroded iron. This chemical reaction releases a small amount of heat energy hence an exothermic process. The equation of formation of rust can be represented as:

Iron + Oxygen + Water Ì¢‰Ûʉ۪ Hydrated iron oxide + Heat

Fe(s) + O2 (g) + H2O(l) Ì¢‰Ûʉ۪ Fe(OH)3(s) + Heat

Adding Water to Quick Lime
In the preparation of whitewash, the addition of water to quick lime results in temperature gain. This exothermic reaction can represented as the following equation:

Calcium Oxide + Water Ì¢‰Ûʉ۪ Calcium hydroxide + Heat

CaO(s) + H2O(l) Ì¢‰Ûʉ۪ Ca(OH)2(aq) + Heat

Reaction of Zinc and Hydrochloric Acid
The reaction of zinc with hydrochloric acid results in temperature gain of the aqueous solution. This equation can be written as:

Zinc + Hydrochloric acid Ì¢‰Ûʉ۪ Zinc Chloride + Hydrogen gas + Heat

Zn(s)+ 2HCl(aq) Ì¢‰Ûʉ۪ ZnCl2(aq) + H2(g) + Heat

Examples of Endothermic Reactions

Dissolution of Ammonium Chloride in Water
The dissolution of ammonium chloride in water results in absorption of heat energy from the surrounding environment hence a drop in temperature of the solution. This endothermic reaction can be represented as:

Ammonium Chloride + Water Ì¢‰Ûʉ۪ Ammonium Chloride solution Ì¢‰â‰ÛÏ Heat

NH4Cl(s) + H2O(l) Ì¢‰Ûʉ۪ NH4Cl(aq) Ì¢‰â‰ÛÏ Heat

Dissolution of Sodium Thiosulphate in Water
Crystals of sodium thiosulphate dissolve in water resulting in the formation of a lower temperature aqueous solution. This endothermic process can be represented using the following equation.

Sodium Thiosulphate + Water Ì¢‰Ûʉ۪Sodium Thiosulphate solution - Heat

Na2S2O3.5H2O + H2O Ì¢‰Ûʉ۪ Ns2SO3(aq) - Heat

Formation of Nitric Oxide
Oxygen and nitrogen do not react at ambient temperatures, however, at high temperatures; they undergo an endothermic reaction resulting in the formation of various oxides of nitrogen. Such a process can occur in internal combustion engines, fuel operated steam boilers and naturally during a lightning strike. The chemical reaction for the formation of nitric oxide can be written as:

Nitrogen gas + Oxygen gas Ì¢‰Ûʉ۪ Nitric Oxide gas - Heat

N2(g) + O2(g) + Ì¢‰Ûʉ۪ 2NO(g) - Heat