Thermodynamics | Introduction#

Thermodynamics is the branch of chemistry which deals with study of the interrelationships among heat, work, and the energy content of a system.

System

The part of universe which is taken into consideration for thermodynamics investigation is known as a system. There are three types of system: open system, closed system and isolated system.

Surrounding

The part of the universe except system is known as surrounding.

Boundary

Boundary is also known as wall. Boundary is defined as anything which separates system and surrounding. It can be real or imaginary. The point or place where the impact of system ends defines an imaginary boundary.
There are two types of boundary or wall: (a) Diathermic wall, which allows the passage of energy through it and (b) Adiabatic wall which does not allow the passage of energy through it.

Types of System#

Open System: An open system can exchange both matter and energy with its surrounding.
Closed System: A closed system can exchange only energy with its surrounding. Exchange of matter is not possible.
Isolated System: An isolated system does not allow exchange of energy and matter with its surrounding. It is very difficult to design an ideal isolated system in real world.

Image Credit: https://en.wikipedia.org/wiki/Isolated_system

Intensive and extensive properties#

Intensive Property: The property of a system which is independent of size of system and the amount of matter present in the system is known as intensive property. For example, if we take one glass of water from a container, then also temperature of water in both container and glass is same. Therefore, temperature is an intensive property. Other examples of intensive properties are temperature, molarity, molality, density, refractive index etc.
Extensive Property: The property of a system which depends on size of system and the amount of matter present in the system is known as extensive property. Volume, mass, moles, internal energy are some examples of extensive property.

Characteristics of intensive and extensive properties#

Intensive properties are non-additive while extensive properties are additive. For example, let's consider we have two 100 mL solutions of lemon juice of equal concentration. If we mix these two, then the concentration will remain same but volume of new solution will be 200 mL.
If E₁ and E₂ are two extensive properties, then E₁/E₂ becomes an intensive property. Example: density = mass/volume
Thus, an extensive property expressed per unit mole, per unit volume or per unit Kg becomes an intensive property.
Extensive property multiplied by intensive property yields an extensive property.

State variables and state of a system#

The variables which define the state of a system are known as state variables or state parameters. Examples: pressure, volume, temperature, number of moles.
When all the state parameters have a definite value, then the system is said to be in a definite state. If any one of the parameters changes, then the state of a system changes.

State Functions and Path Functions#

State Function: The function whose value depends on initial and final state of a system and is independent of the path followed to change the state of a system is called state function. Displacement is a state function and distance is a path function. Other examples of path function are enthalpy, internal energy, entropy, Gibbs Free energy etc.
Path Function: The function whose value depends on the path followed to change the state of the system is known as path function. Heat and work are path functions.