Energy change, bond energy and reaction profiles

Energy change, bond energy and reaction profiles

Energy is transferred in chemical reactions because energy is needed to break bonds and energy is released when new bonds form. Breaking bonds is endothermic; making bonds is exothermic. The overall enthalpy change depends on the balance: ΔH ≈ energy for bonds broken minus energy released when bonds form. A negative ΔH describes an exothermic reaction, in which the products have lower chemical potential energy than the reactants. A positive ΔH describes an endothermic reaction. A reaction profile represents relative energy, not a picture of molecules moving through space, and the activation-energy barrier is distinct from the overall energy change.

Work it through

Suppose 500 kJ mol⁻¹ is required to break relevant bonds and 650 kJ mol⁻¹ is released when new bonds form. ΔH ≈ 500 − 650 = −150 kJ mol⁻¹, so the reaction is exothermic. The negative sign is part of the conclusion: it tells you the system has transferred energy to the surroundings.

Mastery target

Use bond-energy information or a reaction profile to classify energy change and distinguish ΔH from activation energy.