Spontaneity of Chemical Reaction: Spontaneous and Non-Spontaneous Process (For CBSE, ICSE, IAS, NET, NRA 2022)

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Spontaneous Process

  • A spontaneous process is a process that occurs in a system by itself; once started, no action from outside the system (outside agency) is necessary to make the process continue.
  • Cooling down of hot water is an example of a spontaneous process.
  • The rusting of iron is a slow process, but it always takes place in the same direction and it is also a spontaneous process.

Non- Spontaneous Process

  • A non-spontaneous process will not take place unless some external action is continuously applied.
  • Heating of water (at room temperature) is an example of a non-spontaneous process.
  • The reverse of the rusting of iron is non-spontaneous process.

Points About Spontaneous and Non-Spontaneous Process

  • If a process is spontaneous, the reverse process is non-spontaneous .
  • All the spontaneous processes and most of the non-spontaneous processes are possible.
  • Spontaneous processes occur naturally whereas non-spontaneous processes require the help of an outside agency to occur.

Entropy

  • The entropy is the measure of disorder or randomness in a system.
  • The greater the disorder in a system, the greater is the entropy of the system.

To understand entropy, consider the following fig.

The Entropy is the Measure of Disorder
  • In fig (a) the bulb ‘I’ contains 1 mol of an ideal gas ‘A’ at a pressure of 1 bar and the bulb II contains 1 mol of another ideal gas ‘B’ at 1 bar.
  • The two bulbs are joined together through a valve.
  • When the valve between the two bulbs is opened as in Fig (b) , the two gases mix spontaneously.
The Entropy is the Measure of Disorder
  • The mixing of gases continues until the partial pressure of each gas becomes equal to 0.5 bar in each bulb i.e.. the equilibrium is attained.
  • The process cannot be reversed spontaneously, i.e.. the gases do not unmix on their own.
  • The internal energy (U) and enthalpy (H) of an ideal gas depends only upon the temperature of the gas and not upon its pressure or volume.
  • There are no intermolecular forces in ideal gases, when ideal gases mix at constant temperature.
  • Thus, energy change is not driving force behind the spontaneous mixing of ideal gases.
  • The driving forces is the tendency of the molecules of the two gases to achieve maximum state of mixing, i.e.. disorder.
  • This thermodynamic property related to the disorder of the system is called entropy. It is denoted by the symbol S.
  • Entropy of any substance increases on heating.
  • In a chemical reaction, entropy increases if there is an increase in number of moles of gases during the reaction and vice-versa.

When a system changes from one state to another, the change of entropy S is given by

is the heat supplied reversibly at a constant temperature T.

Points About Entropy for a Given Substance

  • The crystalline state is the most ordered state; hence its entropy is the lowest.
  • The gaseous state is the most disordered state; hence its entropy is the maximum.
  • The disorder in the liquid state is intermediate between the solid and the gaseous state.

Criteria for Spontaneous Change

The Second Law of Thermodynamics

  • Most of the processes which occur spontaneously are exothermic, but water kept in a container at room temperature evaporates spontaneously and it is endothermic process.
  • Thus , the enthalpy Change be used as a criteria for spontaneous change.

To take entropy as a criteria for the spontaneous process we must consider simultaneously two entropy changes:

  • The entropy change of the system itself
  • The entropy change of the surroundings

  • According to the second law of thermodynamics all spontaneous or natural processes produce an increase in entropy of the universe.
  • For a spontaneous process when a system is at equilibrium, the entropy is maximum, and the change in entropy is zero.

(at equilibrium)

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