Chemistry Form 5: Chapter 4 - Exothermic and Endothermic Reaction

Chemical energy is needed to transform a chemical substance into a new product through chemical reaction. Therefore, breaking or formation of chemical bond involves energy, which may be either absorbed or released from a chemical reaction.

• To break the chemical bond, energy from surrounding is absorbed resulting decrease of temperature of surrounding.
• To form the chemical bond, energy from reaction is released to the surrounding resulting increase of temperature of surrounding.

 

Exothermic Reaction	Endothermic Reaction
Chemical reaction that gives out heat to the surroundings.	Chemical reaction that absorbs heat from the surroundings.
Temperature of the surroundings increases.	Temperature of the surroundings decreases.
The total energy of reactants is more than total the energy of products.	The total energy of reactants is less than the total energy of products.
Bond formation releases more energy than is required in the bond breaking.	Bond breaking requires more energy than the energy that is released during bond formation
Examples: Combustion of fuel, burning of metal, neutralization, respiration, rusting of iron, reaction of a reactive metal with acid, reaction of alkaline metals with water, Haber process, freezing, condensation, dissolving alkali or concentrated acid in water, dissolving anhydrous salts in water.	Examples: Thermal decomposition of nitrate or carbonate salts, reaction of acid with hydrogen carbonate, photosynthesis, melting, boiling, sublimation, dissolving ammonium or potassium salts in water, heating of hydrated salt.

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