Chemical energetics

6.1 Energetics of a reaction

Exothermic reaction: gives out energy to the surroundings. Endothermic reaction: takes in energy from the surroundings. Bond breaking is endothermic

Bond making is exothermic

If you subtract the energy stored in the bonds of the reactants from the product you get the change in enthalpy or ΔH. A positive ΔH occurs in an endothermic reaction (since products have more energy stored in their bonds)

A negative ΔH occurs in an exothermic reaction (since reactants have more energy stored in their bonds)

The bond energy is the amount of energy needed to break bonds, or released when the bonds form in kJ/mole for example the bond energy of a H-H is 436kJ/mol so it takes that much energy to break that bond

if energy in – energy out > 0 the reaction is endothermic if energy in – energy out < 0 the reaction is endothermic