This statement is restricted to isolated systems to avoid having to worry about whether the reaction is exothermic or endothermic. By definition, neither heat nor work can be transferred between an isolated system and its surroundings. We can apply the second law of thermodynamics to chemical reactions by noting that the entropy of a system is a state function that is directly proportional to the disorder of the system. For an isolated system, any process that leads to an increase in the disorder of the system will be spontaneous. The following generalizations can help us decide when a chemical reaction leads to an increase in the disorder of the system.
Solids have a much more regular structure than liquids.
Liquids are therefore more disordered than solids. The particles in a gas are in a state of constant, random motion. Gases are therefore more disordered than the corresponding liquids. Any process that increases the number of particles in the system increases the amount of disorder. Click here to check your answer to Practice Problem 2.