Final answer:
The second law of thermodynamics relates to entropy, which measures a system's disorder and unavailability of energy to do work. In chemical reactions, the reaction is spontaneous if the total entropy increases or remains constant, typically observed in exothermic reactions with spontaneous heat flow from hot to cold.
Step-by-step explanation:
The second law of thermodynamics can be applied to a chemical reaction by considering the concept of entropy. Entropy is a measure of disorder and the unavailability of a system's energy to do work. For a chemical reaction to be spontaneous, the total entropy - which includes both the system and the surroundings - must increase or remain unchanged.
According to the second law, energy will transfer spontaneously from a higher-temperature body to a lower-temperature body, as this results in an overall increase in entropy. During an exothermic chemical reaction, heat is released and transferred to the surroundings, often resulting in an increase in entropy. This spontaneous heat flow from hot to cold is a fundamental aspect of the second law.
However, the law also implies that processes which decrease the total entropy are non-spontaneous. This is why heat does not naturally flow from cold to hot bodies or why we cannot unmix substances without adding energy to the system.