Spontaneity of Reactions

Spontaneous chemical reactions are those that occur without continuous outside help. This does not necessarily mean that they happen quickly. For example: 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s) (the formation of rust) A sample of iron can rust without much help (therefore this is spontaneous), but it may take months or years to complete.

Answer the following question. You don’t have to include it in your notes, but do write down any questions you might have about the answers.

Spontaneity is determined by two factors: 1.Most spontaneous reactions/processes are exothermic. (-∆H) 2.Most spontaneous reactions/processes result in an increase in entropy of the system. (+∆S) Entropy is most easily described as the randomness/disorder of the system. Things tend to become more disordered over time. Entropy increases when… a liquid turns to a gas (either by a phase change or a reaction). a solid turns to a liquid or gas (by phase change or reaction). A reaction occurs which increases the number of moles from reactants to products. Ex) 2NH 3  N 2 + 3H 2 (2 moles are becoming 4 moles)