Some thermodyanamic terms

The term thermodynamics literally means flow of heat ( therm - heat and dynamics - movement or flow). Presently thermodynamics is considered to deal not only with the relationship between heat and work but also with the interconversion of all forms of energy. Theermodynamics may therefore be defined as " that branch of science which deals with the study of different forms of energy and the quantitative relationship between them." Thermodynamics is based on certain fundamental principles known as laws of thermodynamics. It deals with matter in bulk ( macroscopic systems), i.e, systems having many molecules and is independent of the atomic and molecular structure. But in statistical thermodynamics where the laws of mechanics are applied to the behaviour of individual molecules and then a suitable statistical average is taken. The results obtained from statistical and classical thermodynamics are however, complementary to each other.

Some Thermodynamic Terms

In order to study the laws of thermodynamics, it is essential to understand certain terms quite offenly used in thermodynamics. These terms have definite meanings. Some of the most commonly used terms are given below :

System, boundary and surroundings

The part of the universe under consideration is called a system, on which the effect of certain variables such as temperature and pressure is to be studied. The system is separated from rest of the space by a definite boundary. The real or imaginary surface separating the system from the surroundings is called the boundary.

Region outside the boundary of any system is termed surroundings. Thus, everything outside the system is considered as surroundings.

Homogeneous system and heterogeneous system

When a system is uniform throughout, it is called a homogeneous system. In other words, a system is said to be homogeneous system when it it has the same chemical composition throughout. For example, a mixture of gases, a pure single liquid or solid or a true soliution forms a homogenous system .

A heterogeneous system is one of which consists two or more phases. In other words, it is not uniform throughout. Examples of heterous systems are: ice in contact with water, ice in contact with vapours, etc. Here, ice, water and vapour consitute separate phases.

Types of System:

Depending on the nature of boundary, thermodynamic systems are classified into following three classes:

1.Open system:

A system which can exchange matter as well as energy with surroundings is called an open system. Some examples are given below:

i) Evaporation of water from a beaker represents an open system. Here, water vapours ( matter) move into atmosphere and heat ( energy) required is absorbed by water from the surroundings.

ii)In the photosynthesis, plants take up carbon dioxide, water ( matter) and sunlight ( energy) in presence of chlorophyll and prodduce carbohydrates. in this process, oxyzen matter is transferred to the surroundings. Hence, plants consitute an open system.

2.Closed system:

A system which may exchange energy but not matter with surroundings is called a closed system. For example, consider boiling water in a closed metallic vessel in which heat is transferred from he burner ( surroundings) to teh system; steam remains inside the vessel. Thus, matter is not exchanged.

3.Isolated system:

A system which cam=n neither exchange matter nor energy with the surrounding is called an isolated system. For example, some water is taken in asn insulated vessel and put a piecce of asodium metal on it. An exothermoic reaction takes place. Neither hydrogen gas( matter) nor heat (energy) is transferred to the surroundings.

State of the system and state functions

A thermodyanamic system is said to be in certain state when all its properties are fixed. The state of a system an be defined compleely by the following four properaties :

1. i) Mass and composition
2. ii) Temperature

iii) Pressure

1. iv) Volume

Any change in the magnitude of these properties alters the state of system, therefore, these properties are reffered to as state variables or state functions or thermodyanamic parameters. A change in any function of state is measured as difference between final state and initail state, because the value of these functions solely depends on the thermodynamic properties of a system in a given state and not on its spast history.

In actual practice, it is not neccesary to specify al lthe state variables because some of them are interdependent. For example, in the case of a dsingle gas, composition is fixed automatically as it remains always 100 percent. Thus, teh state of asystem can be defined by only three variables, pressure (P), temperature (T) and volume (V). Further, if the gas is ideal and 1 mole of gas is under examination, it obeys the equation PV = RT, where R is the gas constant. Therefore, if only two of three variables are known, the third can be easily calculated. The two variables generally specified are temperature and pressure. these are called independent state variables. The thirs state variable, generally volume, is said to be adependent state variable because its value depends upon the valuues of P and T. Thus, the state of a system containing definite amount of a single gas can be completely defined by specifying only two of those three variables.

When we are considering a closed system consisting of one or more components, mass is not a state variable. In order to define the state of a geterogeneous system having more than jone substance, we must consider and describe each of teh phases of the system. For, each phase we must specify the content, ie the amount of each substance present and other two imndependent state variables.

Extensive and Intensive Properties

The various measurable properties or the a state functions may be either extensive or intensive. An extensive property of a aystem is that which depends upon the ampunt of substance or substances present in the system, e.g., mass, volume, surface area, energy, number of moles, enthalpy, free energy, entropy, etc.

An intensive property of asystem is that which is indepenndent of amunt but depends upon the concentration of substance or subs tances present in the system, e.g., temperature, pressure, viscosity, refractive index, surface tension, specific heat capacity, freezing point, bpoiling point, etc.

Equilibrium and non-equilibrium states

A system is said to be in wquilibrium when temperature, pressure and conccentration of various species are the same at all points in the system and do not change with time. Let us consider we have a gas confined in cylinder that has a frictionless piston. If the piston is stationary, the atate of the gas can be specified by giving the values of pressure and volume. The system is then in a state of euilibrium.

Actually, the term thermodynamic equilibrium assumes the existence of three types of equilibria in system. They are :

i) thermal equlilibrium
ii) mechanical equilibrium

iii) chemical equilibrium.

i) Thermal equilibrium:

A system is in thermal equilibrium if the temperature remains same in all parts of teh system. This statement can also be called as zeroth law of thermodyanamics.

ii) Mechanical equilibrium :

It implies the uniformity of pressure throughout the whole system.

iii) Chemical equilibrium :

If the composition of system remains constant and uniform throughout, teh system is said to be in chemical equilibrium.

A sytem is said to be in non equilibrium state when temperature pressure and concentration have different values in different parts of the system.

Reference :

Kundu, N. and Jain, S.K.,Physical Chemistry, Ist Edition. Physical Chemistry. New Delhi: S. Chand and Company (Pvt) Ltd, 1996.

Maron, S.H. and Prutton C.F.,. Principles of Physical Chemistry. Oxford and IBH publication Company, 1992.

Moore, Walter J.,. Physical Chemistry. New Delhi: Orient Langman Ltd, 1999.