Thanks to the second law, entropy is the concept by wich the arrow of time can be expressed. Irreversibility is defined as internal production of heat. Work by compression of a gas is analysed for both reversible and irreversible processes, making obvious the difference between the internal pressure, which is the conjugate of the volume, and the external pressure associated with the force exerted on a piston. Chemical potentials are defined as the conjugate of the number of moles of substances contained in the system. Chemical reactions are readily seen as source of entropy production. Simple systems are defined, playing a role analogous to point masses in mechanics. Evolution of simple systems is worked out when they are subjected to heat and mechanical action.