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

1.1 Change of Entropy

Rather change in entropy is of importance in any process. In turn, it is expressed as

dS = dQ/T

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

Where dQ = heat energy transferred reversibly to the system from the surroundings

T = the absolute temperature at which the transfer occurs

1.2 Decrease in Entropy

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

1.3 Increase of Entropy

Increase of entropy is decrease of availability. In other words, increase of entropy is decrease of available useful work.

1.4 Entropy and Disorder

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.

Change in entropy

(a) Change in entropy during phase change   ds = dq/T

(b) Change in entropy for a unit mass

ds =s2–s1 = R ln (v2/v1) + Cvln (T2/T1)

ds= Cp ln (v2/v1) + Cv ln (p2/p1)

ds= Cp ln (T2/T1) — R ln (p2/p1)

(c) Change in entropy for mass m

  1.  When a gas is heated under constant volume condition                                                                                                                       dS = S2–S1 = m Cvln (T2/T1)
  2.  If a  gas is heated under constant pressure condition                                                                                                                     dS = S2–S1 =m Cp ln (T2/T1)
  3. Change of entropy during a reversible adiabatic process will be zero.
  4. Entropy increases with the addition of heat
  5. Entropy decreases with the extraction of heat from a system.

2. 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.