ABSTRACT
P2.1. Basic Principles and Definitions P2.1.1. Two Approaches to Studying Matter ◮ Thermodynamic approach. The thermodynamic approach of studying substances consists of establishing links and relations between experimentally determined (phenomenological) parameters (called thermodynamic parameters) based on several postulates (laws of thermodynmics). ◮ Statistical approach. The statistical approach relies on kinetic-molecular postulates on the structure of substance (fundamentals of kinetic-molecular theory). These postulates include the following:
1. All bodies consist of a huge number of tiny particles-atoms and molecules. 2. These molecules are in constant random motion. 3. The molecules interact with one another: they experience attractive forces at large
distances and repulsive forces at small enough distances. The thermodynamic parameters are calculated within the framework of a specific model
of internal structure of substance (i.e., the model of motion and interaction of atoms and molecules) by averaging over a huge number of states of the system. Statistical physics employs methods of probability theory and mathematical statistics. The classical theory relies on the classical laws of molecular motion, while quantum statistics relies on the laws of quantum mechanics. ◮ Amount of substance. Mole. The amount of substance in a system-or the number of constituting structural units, atoms and molecules-is measured in moles. One mole of any substance contains a certain number of molecules, called Avogadro’s number (or the Avogadro constant) and equal to the number of atoms in 12 g of carbon-12. Avogadro’s number equals NA ≈ 6.02 × 1023 mol-1. The number of moles in the system is expressed as
ν = N
NA =
m
µ , (P2.1.1.1)
where N is the number of molecules in the system, m = m0N is the mass of the system, m0 being the mass of a single molecule, and µ = m0NA is the molar mass of the substance.
P2.1.2. Equation of State ◮ Equilibrium states. A thermodynamic system that is kept under unchanged external conditions comes to an equilibrium state, where there are no fluxes of any kind (e.g., mass or energy fluxes). The thermodynamic parameters of the equilibrium state (pressure p, temperature T , volume V , density ρ, molar mass µ, etc.) are related by an equation of state. For example, the ideal gas law (also known as the Clapeyron-Mendeleev equation), which is the equation of state of an ideal gas, has the form
