Class 11 Chemistry


Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, radiation, and properties of matter. (Ref: Wikipedia)

System and Surroundings

The part of the universe in which observations are made is called the system. The remaining universe is called the surroundings.

The Universe = The System + The Surroundings

For all practical purposes, the surroundings are that portion of the remaining universe which can interact with the system.

Boundary: The system may be defined by physical boundaries. The wall that separates the system from the surroundings is called boundary. The wall can be real or imaginary.

Types of System

systems in thermodynamics

Fig Ref: Wikipedia

  1. Open System: There is exchange of energy and matter between the system and surroundings, in an open system. Example: Reactants in an open beaker.
  2. Closed System: When there is no exchange of matter but exchange of energy is possible between system and surroundings, it is called a closed system. Example: Reactants in a closed vessel.
  3. Isolated System: When there is no exchange of matter and energy between system and surroundings, it is called an isolated system. Example: Reactants in a thermos flask.

State of the System: The state of a thermodynamic system is described by its measurable (or macroscopic) or bulk properties. Thus, the state of a gas can be described by quoting its pressure (p), volume (v), temperature (T), and amount (n). These variables are called state variables or state functions because their values depend only on the state of the system and not on how it is reached.

Internal Energy as a State Function

The sum of all energies of a chemical system is called internal energy (U) of the system. The internal energy may change under following conditions.


Let us take an adiabatic system, i.e. a system which does not allow exchange of heat between the system and surroundings. The manner in which the state of such a system may be changed is called adiabatic process.

Let us do some work on the system to change the internal energy of the system.

Initial state of system = A

Initial temperature of system = TA

Internal energy in state A of system = UA

There can be many methods to change internal energy of system. It can be achieved by doing some mechanical work on the system, e.g. by rotating a stirrer. Alternately, it can be done by an immersion rod to do electrical work on the system. Let us assume that 1 kJ work is done on system in both cases. It can be illustrated as follows:

New state of system = B

Final temperature of system = TB

Internal energy in state of B of system = UB

It is found that TB > TA

Change in temperature &Detla;A = TB - TA

Change in internal energy ΔU = UB - UA

So, it can be said that the value of internal energy is characteristic of a system, whereby the adiabatic work (Wad) required to bring about a change of state is equal to the difference between the value of U in one state and that in another state. This can be given by following equation:

ΔU = U2 - U1 = wad

When work is done on the system, wad is positive. When work is done by the system, wad is negative.


Let us take water at temperature TA in a container having thermally conducting walls. Let us enclose this container in a huge reservoir at temperature TB. The heat absorbed by the system (water) is given as follows:


In this case, `ΔU=q`

When heat is transferred from the surroundings to the system then q is positive, but it is negative when heat is transferred in opposite direction.

General Case

Let us take a case in which internal energy is being changed by both, i.e. by doing work and by transfer of heat. In this case, change in internal energy can be given as follows:

`ΔU=q+w` …………….(1)

Equation (1) is the equation for the first law of thermodynamics, which is as follows:

“The energy of an isolated system is constant.”

This law is commonly stated as the law of conservation of energy.