A process in which a system absorbs heat from the environment, is called endothermic. Notice in the top diagram that the products of the reaction have MORE energy than the reactants. A process in which a system loses heat is to the environment is called exothermic. Notice in the bottom diagram that the products of the reaction have LESS energy than the reactants. Activation energy, (Ea in the diagrams), is the energy that reactant molecules must overcome before a reaction can occur. Notice that for an endothermic reaction, there must be an extra input of energy above the energy level of the products in order for a reaction to occur.

Bond Enthalpies and the Enthalpies of Reactions:

Average bond enthalpies (click for a chart) can be used to estimate whether a reaction will be endothermic (ΔH > 0) or exothermic (ΔH < 0).

ΔH_rxn = Σ (enthalpies of broken bonds) − Σ (enthalpies of bonds formed)

Consider the reaction:     CH_{4 (g)} + Cl_{2 (g)} → CH₃Cl _(g) + HCl _(g)

On the reactant side, one atom of hydrogen is removed from the methane molecule and the chlorine molecule is broken apart.

• Bonds Broken -   1 mole C - H   and   1 mole Cl - Cl

On the product side, one chlorine atom bonds with the carbon atom and one chlorine atom bonds with the hydrogen.

• Bonds Formed -   1 mole C - Cl   and   1 mole H - Cl

Solving for the equation above:

ΔH_rxn = [moles(C - H) + moles(Cl - Cl)] − [moles(C - Cl) + moles(H - Cl)]
          = [1mole(413 kJ/mole) + 1mole(242 kJ/mole)] &minus [1mole(328 kJ/mole) + 1mole(431 kJ/mole]
          = −104 kJ   (The reaction is exothermic)