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8. Reactions will occur if the products are more stable than the reactants and the energy barrier is low enough. Reactions will be thermodynamically favorable if the products are of lower Gibbs free energy than the starting materials, for example, if stronger bonds are made than are broken (enthalpy change is favorable), if a weaker acid or base is formed in the product (enthalpy change is favorable) or if more molecules are created than consumed (entropy change is favorable).  Even if thermodynamically favorable, a reaction will only occur if the energy barrier (Gibbs free energy of activation) has no step in the mechanism containing a species of such high energy that it cannot be formed at the temperature being used.  Note that steric interactions (unreactive atoms bumping into each other) can prevent otherwise favorable reactions by keeping the reacting atoms away from each other.