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Criterion For Spontaneity Of A Reaction

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Criterion For Spontaneity Of A Reaction - Lesson Summary

The first law of thermodynamics states that energy can neither be created nor destroyed although it may be converted from one form to another.

ΔU = Q - W

ΔU = change in internal energy
  Q = Heat added to the system
  W = Work done by the system

The transfer of energy usually occurs due to heat(q) or work (w). While the first law explains the relationship between the heat absorbed and work done but does not explains the direction of heat flow.


(i) If a cup of hot tea in a room then the law does not indicate whether the heat will flow from the hot cup to the cold surrounding or vice versa, but explains that if a heat transfer process has occurred then heat energy gained by one substance would be equal to heat energy lost by the other substance.

(ii) Water flows naturally from the tank to the ground floor while it requires using a motor to transfer water from ground level to the tower tank.

A spontaneous process is one that proceeds on its own without any external influence. If a process takes place only after energy is supplied to the system, is called non-spontaneous processes.

Enthalpy is the not only criterion for spontaneity. Change in the enthalpy is not only determining factor for spontaneity.

The tendency of a system to acquire maximum randomness is another factor that is responsible for the spontaneity of a process and it depends on two factors:

  • Tendency for decrease in enthalpy and
  • Tendency for maximum randomness.

Randomness or disorder of a system is expressed by a thermodynamic state function known as entropy, and is represented by the letter S.

The solid state will have the lowest entropy, while the gaseous state will have the highest entropy. The entropy of the liquid state will be in between the entropy values of solid and gaseous state. Greater the disorder in an isolated system, higher is its entropy.

The change in entropy is indicated by ΔS, obtained by subtracting the entropy of the initial state from the entropy of the final state.

When a system absorbs heat, the molecules within it start moving fast due to increased kinetic energy, causes a high amount of disorder in the system.

When the gases mix, there is an increase in randomness, resulting in an increase in entropy. So for spontaneous processes in an isolated system, the entropy is positive.

The entropy of a system at equilibrium is maximum, and there is no further change of entropy. Thus ΔS is equal to zero for a process in equilibrium. So, it can be concluded as

ΔStotal = ΔSsystem + ΔSsurr > 0


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