Entropy is energy flow from order to disorder. Entropy is quantitative measure of disorder or randomness or chaos in a system. A highly ordered system has low entropy.

Only change in entropy is of importance in any process which is expressed as


In a cyclic process it is presented by an integral from the starting stage to the final stage.

alt=”\int {\frac {\delta Q}{T}}\geq 0″ class=mwe-math-fallback-image-inline aria-hidden=true v:shapes=”_x0000_i1025″> where dQ = heat energy transferred reversibly to the system from the surroundings

T = the absolute temperature at which the transfer occurs

The entropy can be decreased by lowering of temperature and decrease in volume. Such decrease will take place in a refrigerator where cooling is taking place. However this decrease in entropy possible at the expense of an entropy increase in the surroundings due to the heat addition.

Increase of entropy is decrease of availability or increase of entropy is decrease of available useful work.

Entropy and disorder is related to equilibrium. Perfect internal disorder is equilibrium and random disorder is non equilibrium.

The SI units of entropy are J/K (joules/degrees Kelvin).

In a closed system, the entropy of the system will either remain constant or increase.

For an irreversible process, the combined entropy of the system and its environment always increases

In order to decrease entropy or to become more orderly, energy must be transferred from somewhere outside the system into the system.

  1. If heat is added at constant temperature as in a phase change , the change of entropy will be ds = dq/T                                                         Where T is the absolute temperature
  2. When a gas is heated in any manner, the change in entropy is given by      ds = s2–s1 = R ln (v2/v1) + Cvln (T2/T1) = Cp ln (v2/v1) + Cvln (p2/p1)              = Cp ln (T2/T1) — R ln (p2/p1)
  3. Change in entropy when a gas is heated under constant volume condition                                                                                                                       ds = S2–S1 = m Cvln (T2/T1)
  4. Change in entropy when a gas is heated under constant pressure condition                                                                                                                     ds = S2–S1 =m Cp ln (T2/T1)
  5. Change of entropy during a reversible adiabatic process will be zero.
  6. Entropy increases with the addition of heat and entropy decreases with the extraction of heat from a system.

Calculation of changes in entropy of a system and its surroundings from the heat of reaction

reaction at constant pressure and temperature can be expressed by the formula. ΔS = -ΔH/T

where ΔS is the change in entropy of the surroundings

-ΔH is heat of reaction

T = Absolute Temperature (Kelvin)

If the reaction is exothermic, the entropy of the surroundings will increase.

If the reaction is endothermic, the entropy of the surroundings will decrease.