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41
(vi) Chemical energetics
Exothermic and Endothermic Reactions
• In this topic, we will be mainly focusing on two types of reactions: Exothermic reactions and
Endothermic reactions.
• Remember the mnemonic “Bendo and Mexo” (more energy absorbed in breaking of bonds in
endothermic reactions, and more energy released in making of bonds in exothermic reactions.
• The Enthalpy change (∆H) , also known as energy change, is the transfer of thermal energy
during a reaction.
• The Activation energy is the minimum amount of energy required for a reaction to take place. It
is always positive.
Exothermic Reactions
• Exothermic reactions are reactions in which high enthalpy reactants are converted into low
enthalpy products.
• On the basis of bond energy, exothermic reactions are those reactions in which the amount of
energy released during bond formation is greater than the amount of energy absorbed during the
breaking of bonds.
Exothermic Reactions
• In exothermic reactions, heat is released into the surroundings and the temperature of the
surroundings rises.
They are reactions where the energy released during bond making is greater than the energy
consumed during bond breaking
The enthalpy change of an exothermic reaction is negative.