Thermodynamics

The study of the transformation of energy is called thermodynamics. Actually, it deals with energy in its various forms, which include thermal, chemical, electrical, and mechanical, with the restrictions on the transformation of one type of energy in to the other types and with the relation of energy changes to physical and chemical changes.

Terminology of Thermodynamics :

System and Surroundings :
System is part of the universe which is arbitrarily set off from the rest of the universe by definite boundaries for the purpose of experimental or theoretical studies. The remainder of the universe is then, in fact the surroundings of the system. The space separating the system from its surrounding is termed as boundary.

Types of Systems :

a) Real system In experimental work, the system is called Real.

b) Ideal system In pencil and paper work, the system treated is called ideal. An ideal system is always considered to simplify the thermodynamic problems.

c) Isolated system A system is said to be isolated when it can neither exchange energy nor matter with its surroundings.

d) Closed system A system is said to be closed when it permits passage of energy but not mass, across the boundary.

e) Open system A system which can exchange both energy and matter with its surroundings. State of System (State variables)

The quantities whose value serve to describe the system completely are called the thermodynamic properties of the system. Once the properties of the system are completely specified, one says that the state of the system is specified. Thus, the defining properties are sometimes called state variables or state properties. Examples of state properties are pressure, volume, temperature and composition. The question now arises as how many variables must be determined to define the system completely. The answer to this question can be obtained by considering the following example. A homogeneous system consists of a single substance and hence the composition is fixed automatically. The state of a homogeneous system can, therefore, be defined by only three variables:

i) Pressure

ii) Volume and

iii) Temperature

For a homogenous system of definite mass, these three properties are related to one another by a mathematical equation PV = RT, called equation of state. With the help of this equation (PV = RT) the values of any one of these properties can be determined knowing the values of the other two properties. Therefore, state of a simple homogeneous system may be completely defined by specifying only two of the three variables (i.e.) pressure, temperature and volume. The two variables generally specified are temperature and pressure. These are termed as independent variables. The third variable, generally volume is called a dependent variable because its value depends upon the values of pressure and temperature.

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 homogeneous system having more than one substance, one must consider and describe each of the phases of the system. For each phase, one must specify the content (i.e.) the amount of each substance present, and two other independent variables.

In order to define a system completely, the state variables are generally (T), Pressure (P) Volume (V) and concentration (n). Besides these there are two more variables. Work (W) and heat (q), which are not state properties. These six variables play an important role in defining chemical systems completely.

Properties of a System: The observable properties of system are of two types: a) Extensive Properties: There are some properties called Extensive properties whose values are proportional to the mass of the portion of the system or one can say that extensive properties are dependent upon size of the system.

b) Intensive Properties: There are some properties of a system called intensive properties, whose values are independent of the quantity of matter contained in the system.

Extensive Property      Intensive Property

Volume                        Molar volume

No. of moles               Density

Mass                            Refractive index

Free Energy                  Surface tension

Entropy                         Viscosity

Enthalpy                   Free energy per mole, specific heat

Heat capacity                 Pressure, Temperature, Boiling Point, Freezing Point

Internal Energy :

It is the energy associated with a system by virtue of its molecular constitution and the motion of its molecules. The contribution of energy due to molecular constitution is known as cinternal potential energy and the contribution of energy due to the motion of molecules is called internal kinetic energy. Internal energy of a system is given by the sum of two types of energies.

Determination of ΔE: When a reaction is carried out in such a manner that the temperature and volume of the reacting system remain constant, then the internal energy change (ΔE) of the reaction is equal to the heat exchanged with the surrounding.

Enthalpy :

When we deal certain process in open vessels (at constant pressure). It becomes essential to introduce in place of internal energy, a new themrodynamic function called heat enthalpy. This new function is denoted by H.

H = E + PV

The change in enthalpy of a given system is given as follows,

ΔH = H2 − H1

or ΔH = (E2 + P2V2) − (E1 + P1V1) = (E2 − E1) + (P2V2 − P1V1)

or ΔH = ΔE + ΔPΔV

If P is maintained constant

ΔH = ΔE + PΔV

or ΔH = Q

Hence the change in enthalpy of the system ΔH may be defined as the amount of heat absorbed at constant pressure.

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