Thermo. You can not create work in a closed cycle with a single heat source. It takes at least two sources at temperatures T1 and T2 to produce power. The theoretical maximum efficiency is independent while temperatures at which heat is exchanged, the principle of Carnot implies the relationship: Q1 / Q1 + Q2 / Q2 <= 0 with equality for the reversible transformation, Q1 (resp. Q2) designating the amount of heat (measured in algebraic) is provided by the source 1 (resp. to the source 2). 

This is the first statement of the second law of thermodynamics.
Reversible transformation Means a process ideal for two families of real transformation, which can be regarded as a sequence of states of equilibrium far neighbors. Ant. irreversible. 

Chim. Is said, by abuse of language, a reaction likely to evolve in both directions. It should normally speak overwhelming response (Duhem, 1912).