The stoichiometric coefficients and the extent of a reaction are defined. The affinity is defined in terms of chemical potentials and the corresponding Gibbs free energy and enthalpy of a reaction. The affinity can be expressed as a derivative in analogy with a force that is a derivative of a potential. Dissipation or internal entropy production occurs when chemical reactions take place. The extensivity of volume, entropy and enthalpy implies that molar volume, entropy and enthalpy can be defined for every substance present in a system. In view of the chemical potential dependence on partial pressure in a mixture of ideal gases, the chemical potential of an ideal mixture is defined. From it, the law of mass action is derived and osmosis can be understood. In electrochemistry, the Nernst potential potential can be obtained by considering the equilibrium conditions on the chemical potentials of electrons and ions at both electrodes. The working principle of a Daniell battery is analysed in detail. The second law implies phenomenological laws of transport in a thermogalvanic cell. The principle of an osmotic power plant can be understood by applying the van't Hoff estimate of an osmotic pressure.

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.

The definition of a thermodynamic system includes the characterisation of its enclosure. The system can be closed or open, adiabatic or diathermal, rigid or mobile. State variables may be extensive or intensive. State functions are functions of the state variables only. A system may be divided into subsystems separated by walls that can be impermeable or permeable, adiabatic or diathermal, fixed or mobile. The state of a system may be changed by mechanical processes or thermal processes, resulting in a thermal transfer, mass transfer or work. The first law is expressed in terms of the total energy that includes the kinetic energy, so that thermomechanical systems can be analysed, creating a conceptual link between classical mechanics and thermodynamics. By examining a damped harmonic oscillator in the framework of thermodynamics, the need for a non-mechanical state variable is revealed.

As internal energy is a function of entropy, volume and number of moles, its differential is given by the Gibbs relation, and temperature, pressure and chemical potentials are defined as conjugate variables. Extensivity implies the Euler relation. The Gibbs-Duhem relation will find applications later, in the analysis of phase transitions. Legendre transformations are introduced, leading to the definition of the thermodynamic potentials: free energy, enthalpy and Gibbs free energy. When a system is coupled to a thermal reservoir or heat bath, its equlibrium is characterised by a minimum of the free energy; when it is a work reservoir, the enthalpy is minimum, and when it is a work and heat reservoir, the Gibbs free energy is minimum. Maxwell relations establish relationships between quantities that would not immediately be associated. The cyclic chain rule links together the derivatives of one property function with respect to two others. It is conveniently applied to analyse the Joule expansion and Joule-Thomson effect.

Thermal response coefficients are thermal properties of matter that characterise the response of a system that is subjected to a heat transfer. Specific heat and latent heat are expressed in terms of entropy as a state function of temperature and pressure, or temperature and volume. A systematic approach is presented for the calculation of Mayer relations between thermal response coefficients. The Dulong-Petit law for the specific heat of solid is given. Expressions for the thermal response coefficients of an ideal gas are derived from its equation of state. The entropy variation of an ideal gas as a function of temperature, volume and pressure is calculated.