Exothermic Reactions


A reaction that gives out heat to the surroundings is called an exothermic reaction.

The surroundings consist of the reaction container, the laboratory room and anything else other than the reactants and the products (the system). The temperature of the surroundings increases when an exothermic reaction takes place; the reaction container becomes hot. The total energy content of the products is lower than the total energy content of the reactants.

Bond forming occurs during a chemical reaction. It is the formation of bonds that causes energy changes in chemical reactions; forming a bond is hence exothermic.

ΔH is given a negative sign for exothermic reactions.
The net energy change when the energy needed to break bonds in the reactants is less than the energy released when new bonds are formed in the products.

Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules and provide a 'different pathway' for the reaction. This effectively means the activation energy is reduced, irrespective of whether its an exothermic or endothermic reaction.

Examples of exothermic reactions are:
  • A burning or combustion reaction usually means a very fast exothermic reaction where a flame is observed.
    • Combustion of hydrogen
    • Combustion of ethanol
  • Respiration
    • Combustion of carbohydrates in the body
  • Neutralization (Reaction between an acid and an alkali)
    • Aqueous Sodium Hydroxide with Hydrochloric acid
    • Aqueous Potassium Hydroxide with Nitric acid
  • Converting anhydrous salt to hydrated salt
    • Adding water - White anhydrous Copper (II) Sulfate to blue hydrated Copper (II) Sulfate
  • Haber Process
    • Ammonia is manufactured from Nitrogen and Hydrogen, where heat energy is given off
  • Contact Process
    • Sulfur Dioxide is oxidized to Sulfur Trioxide
  • Formation of Hydrogen Chloride from Hydrogen and Chloride

The following pictures illustrate this clearly:

external image Image30.gifexternal image Energy-Diagram-Exothermic.gifexternal image Image34.gif

And here is a Youtube video further explaining this reaction: