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Entropy is defined as the measure of the disorder or randomness of a system. It is a Greek word which stands for ‘trope’ meaning change and a prefix ‘en’ is written to identify it as thermodynamic property which belongs to the family of energy and enthalpy. It is the ratio of heat and temperature. Since heat is expressed as joule (J) and temperature is expressed in Kelvin (K), therefore the entropy and change in entropy is expressed as Joules per Kelvin (J.K-1). It is denoted by symbol ‘S’.

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Example of Entropy

When ice melts into water, it can easily be observed the disorder of the system. Ice is made up of water molecules which are bonded to each other in a crystal lattice. Molecules get more energy when ice melts into water because water molecules spread further apart and break the structure to form liquid. Similarly, a phase change from liquid to gas, where water changes into steam and increase the energy of the system. We can take another example as liquid water freezes into ice, in this situation energy of the system decreases due to restriction of the movement of the water molecules and formation of crystal lattice.


Entropy (S) can never be measured but change in entropy ∆S can be measured.

∆S = Sf – Si

Where Sf is final entropy and Si is initial entropy.

It is a state function. Change in entropy does not depend upon the path through which change occurs. The change in entropy only depends upon initial and final points.

∆S = Sf – Si

It is an extensive property. Extensive property depends upon mass or moles or amount of matter. Greater the amount of the substance greater will be the entropy.

Irreversible Process 

The reaction proceeds in one direction in an irreversible process. If the value of ∆S is +ve than it suggests entropy has increased and the process is spontaneous. If ∆S is -ve than we can say entropy has decreased and the process is non-spontaneous.


For spontaneous process randomness will increase so disorder of the system will also increase. The change entropy will be positive and greater than zero.

∆S ˃ 0

For non-spontaneous process randomness will decrease so disorder of the system will also decrease. The change in entropy will be negative and lesser than zero.

∆S ˂ 0

Reversible Process 

In reversible process, reaction proceeds in both the direction and spontaneous process takes place, so both the sides have same entropy and change in it would be zero.


Formula of Entropy

The following formulas can be used to calculate the change in the entropy of the system.


Where ∆Q is change in heat.

If temperature is constant and system gains heat (Q), it has +ve value so the change in it would be positive, and if system loose heat (Q), it has –ve value, in this case change in it would be negative.

If temperature is not constant, the following equation can be used to calculate the change in entropy.

Isolated System 

An isolated system is covered with insulating material and no transfer of heat energy takes place and there is no any effect of surrounding on the system. We take an example of two gases present in the two compartments which are separated by a valve. When we remove the valve between two compartments than gases will mix together through spontaneous process and the randomness will increase. Due to the isolated system ∆Q = 0 but it does not mean that entropy will also be zero. If spontaneous process takes place in an isolated system than randomness increases (∆S ˃ 0) even though there is no change in ∆Q.

Non-isolated system 

A system which has interaction with surroundings is known as non-isolated system. The entropy system and surroundings will be considered simultaneously.

∆S total = ∆S system + ∆S surroundings

If spontaneous process takes place in non-spontaneous process; ∆S total ˃ 0. ∆S total is also known as ∆S universe.

∆S universe ˃ 0

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