Created By : Rina Nayak

Reviewed By : Rajashekhar Valipishetty

Last Updated : May 02, 2023

This free online devise entropy calculator is helpful to find the change in entropy for a reaction, change in Gibbs free energy and change in entropy of isothermal ideal gas. Just you have to do is enter the values as the input field of the entropy, then hit the calculate button to avail the output as fast as possible

Total Entropy Of Products
Total Entropy Of Reactants
Entropy Change of a reaction

Gibbs free energy ΔG = ΔH - T*ΔS

Change in enthalpy
Change in entropy
Change in Gibbs free energy

Isothermal entropy change of an ideal gas

Base variable
Amount of moles
Initial Pressure
Final Pressure
Change in entropy

What is Entropy?

Entropy is defined as a measure of the randomness or disorder of a system. This concept was introduced by a German physicist named Rudolf Clausius in the year 1850. The two definitions of entropy are the thermodynamic definition and the statistical definition.

In Thermodynamic point of view, Entropy is used to describe the behaviour of a system in terms of thermodynamic properties such as temperature, pressure, entropy, and heat capacity. This thermodynamic description took into consideration the state of equilibrium of the systems.

The statistical definition is developed, based on thermodynamic properties. Entropy is a measure of the molecular disorder.

  • A spontaneous process is thermodynamically irreversible.
  • The irreversible process will attain equilibrium after some time.

Properties of Entropy

  • It is a thermodynamic function.
  • It is a state function. It depends on the state of the system
  • It is represented by S°.

SI unit of Entropy is J/Kmol whereas the CGS unit is cal/Kmol.

Entropy Formulas

Entropy is a state function that it is dependent on the entropies of the initial and final states of a system, For a chemical reaction,the change in entropy formula is given as, ΔSreaction = ΔSproducts - ΔSreactants

Entropy (S) is normally measured in J/K, but standard entropy (So) is also used. The above equation is can also written in standard entropy is (ΔSo)reaction = (ΔSo)products - (ΔSo)reactants

  • Where as
  • S is entropy
  • S° is standard entropy
  • Δ is change in the reactions
  • Some substances of standard entropy values


Standard Entropy

H2 (gas)


O2 (gas)


H2O (gas)


H2O (liquid)


C (diamond)


By using the formula Gibbs Free Energy equation we can calculate entropy. Gibbs free energy is the energy in a system available to do a work on its surroundings at constant pressure and temperature. The formula is given as ΔG = ΔH - (T * ΔS)

  • ΔG is the change in gibbs free energy
  • ΔH is the change in enthalpy
  • T is the temperature in kelvin
  • ΔS is the change in entropy

Isothermal Change in Entropy of an Ideal Gas - Change in Enthalpy Formula

For the isothermal process of an ideal gas, entropy for pressure and volume can be as follows:

ΔS = n * R * ln(V₂/V₁) = - n * R * ln(P₂/P₁)


  • ΔS is the change in entropy
  • n is the number of moles
  • R is the universal gas constant, 8.3145 J/Kmol
  • V₁ is the initial volume
  • V₂ is the final volume
  • P₁ is the initial pressure 
  • P₂ is the final pressure

Entropy Examples

1. Find the entropy change for the following reaction using the table of entropy values?

H2(g) + F2(g) = 2HFg


So J/Kmol.







Given that,

Values are given in a table

ΔSproducts is 2HFg= 2(173.5)

ΔSreactants is H2(g) + F2(g) = 130.6 + 202.7

Substitute the values in formula

For a chemical reaction,the change in entropy formula is given as ,

ΔSreaction = ΔSproducts - ΔSreactants

2(173.5) - 130.6 + 202.7 = 13.7 j/kmol

ΔSreaction enthalpy of the reaction is 13.7j/kmol

FAQs on Entropy Calculator

1. What is meant by entropy?

Entropy is a measure of randomness or disorder of a system. It can be explained in thermodynamics and statistics. The second law of thermodynamics states that the disorder of a system is always increasing.

2. What are the properties of entropy?

Entropy is dependant on the following properties 

  1. It is a thermodynamic function.
  2. It is a state function. It depends on the state of the system
  3. Entropy is an extensive property which means that it scales with the size or extent of a system.

3. What are the units of entropy?

The SI unit of entropy is J/Kmol. The CGS unit of entropy is cal/Kmol.

4. What is the formula of entropy ?

For a chemical reaction, the change in entropy formula is given as, ΔSreaction = ΔSproducts - ΔSreactants