Entropy.

According to the Second Law of of Thermodynamics, the degree of randomness increases in the universe.

Randomness can be:

-a change in state (solid to liquid/gas or liquid to gas)

-a change in energy form

-a change in number of particles

Examples:

-redox reactions

-formation of steam

*Energy goes downhill.

Whenever there is a spontaneous reaction, there must be an increase in entropy.

Exothermic reactions: heat is released-->entropy increases

Endothermic reactions: heat is absorbed (energy is required)-->entropy decreases