Thermodynamics

Thermodynamics is the subject dealing with quantitative relation between heat energy and other forms of energy in physico-chemical processes. These results are formulated in to four laws - zero, first, second and third laws of thermodynamics.

Thermodynamics deals heat changes occurring between system and surroundings. 

System: A small part of universe that is under thermodynamic study.

Surrounding: The remaining part of the universe. 

Universe = System + Surroundings

Systems are classified on the basis of their interaction with the surroundings as:

1. Open System: The system where matter and energy are exchanged with surroundings. Boundary is not sealed and not insulated.
2. Closed System: The system where only the energy but not the matter is exchanged with the surroundings. Boundary is sealed but not insulated.
3. Isolated System: The system which does not exchange either the matter or energy with the surroundings. Boundary is sealed and insulated.

Thermodynamic Properties

State of System: The system is said to be in a certain state, when its macroscopic properties have definite values. It is defined in terms of its state functions such as P, V, T. If any one of the state functions is changed, the state of that system is said to be changed.

Extensive Property: It is the property of a substance that depends on its mass. For example: volume of a gas, internal energy, enthalpy, entropy, heat capacity, Gibbs energy, heat content.

Intensive Property: It is the property of a substance that does not depend on its mass. For example: density, molar properties (molar volume, molar entropy, molar heat capacity), surface tension, viscosity, specific heat, refractive index, pressure, temperature, boiling point, freezing point, vapour pressure.

Work, Heat and Energy

These are important thermodynamically useful concepts. There are algebraic quantities hence these can be positive or negative.

Work: Work is said to be done when an unbalanced force causes some displacement in its own direction. The displacement of an object through a distance (d) against a force (F) is called work.

W = F x d

Work is calculated as the product of external pressure and change in volume.

W = - P ΔV

W is positive when work is done on the system. W is negative when work is done by the system. Work is a path function.

Heat (Q): It is the form of energy which flows between a system and surroundings by virtue of temperature difference.

Calorie: The heat required to raise the temperature of 1 gram of water by 1°C is known as calorie.

Energy: It is defined as the capacity to do work. The property that is obtained through work or property that can be converted into work is known as energy.

First Law of Thermodynamics

It is another form of law of conservation of energy. It can be stated as energy is neither created nor destroyed but it may be transformed from one form to another form. The energy of an isolated system is constant whatever changes take place in it.

The net energy change in a closed system is equal to heat absorbed plus the work done by the system.

ΔE = ΔQ + W

Enthalpy (H): The total heat content of a system at constant pressure is called its enthalpy. It is a state function and an extensive property.

ΔH = Q_P

ΔH = E + PΔV

Heat capacity (C) of a substance in the amount of heat required to raise its temperature through one degree.

Specific heat capacity is the heat required to raise the temperature of one gram of a substance through 1°C.