Examples of exothermic processes include burning of coal, rust formation and dissolution of quick lime in water. On the other hand, some endothermic processes include dissolution of Ammonium Chloride in water and nitric oxide formation.
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¢ÛÊÛª Carbon dioxide + Heat
C(s) + O2 (g) Ì¢ÛÊÛª CO2 (g) + Heat
Rusting of Iron Iron metal reacts with oxygen and water to form rust Ì¢âÂÛÏThe 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¢ÛÊÛª Hydrated iron oxide + Heat
Fe(s) + O2 (g) + H2O(l) Ì¢ÛÊÛª 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¢ÛÊÛª Calcium hydroxide + Heat
CaO(s) + H2O(l) Ì¢ÛÊÛª 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¢ÛÊÛª Zinc Chloride + Hydrogen gas + Heat
Zn(s)+ 2HCl(aq) Ì¢ÛÊÛª 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¢ÛÊÛª Ammonium Chloride solution Ì¢âÂÛÏ Heat
NH4Cl(s) + H2O(l) Ì¢ÛÊÛª NH4Cl(aq) Ì¢âÂÛÏ 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¢ÛÊÛªSodium Thiosulphate solution - Heat
Na2S2O3.5H2O + H2O Ì¢ÛÊÛª 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¢ÛÊÛª Nitric Oxide gas - Heat
N2(g) + O2(g) + Ì¢ÛÊÛª 2NO(g) - Heat
