Important Questions and Answers: Chemical thermodynamics

1. What is the physical concept of entropy? Write down its unit.

Ans: Entropy is the measure of randomness of the system. The molecules in a system always tend to have greater entropy with respect to time. Entropy increases with increase in temperature. So, the body with low temperature has low entropy. The entropy of system is zero at -273oC. At this temperature, molecules are in perfect order. The increasing order of entropy is Vapour  > Liquid > Solid.

The unit of entropy is JK-1mole-1.

2. How is free energy change of a reaction related to enthalpy change or entropy change ?

Ans: Free energy change of a reaction is related to enthalpy change (ΔH)

or entropy change (ΔS) as follows.


Here, T is the absolute temperature.

3. What is standard free energy of a reaction?

Ans: The free energy change that takes place during chemical reaction when the reactants in their standard state are changed into products in their standard state as given by the chemical equation.

Note: Here, standard state means the substance at 298 Kelvin and 1 atmospheric pressure.

4. What do you mean by spontaneous and non-spontaneous process?

Ans: The process which occurs on its own without the help of any external agencies is called spontaneous process. All the natural processes like rusting of iron, burning of fuels, rolling of ball from top to bottom are spontaneous process.  

The process which occurs with the help of external agencies under different conditions is called non-spontaneous process. Example: Water lift up into the tank on applying force, heat flows from low temperature to high temperature on applying different conditions.

5. What is thermodynamic efficiency of heat engine?

Ans: The conversion of heat into work by the engine is called thermodynamic efficiency of heat engine. It is denoted by η.

Mathematically, it is the ratio of work done (W) to quantity of heat (Q).

$$\text{Efficiency} ( \eta ) =  \frac{\text{Work done (W)}}{\text{Quantity of heat (Q)}}$$

 6. What are the characteristics of spontaneous process?

Ans: The characteristics of spontaneous process are as follows:

  • It is unidirectional and irreversible in nature.
  • It occurs until the equilibrium state is attained.
  • No any external agent is needed for the work done.
  • It increases the entropy of a system.

7. State second law of thermodynamics.

Ans: Second law of thermodynamics state that:

  • The entropy of the system is constantly increasing for every spontaneous process.
  • Heat energy cannot be transferred from colder to hotter bodies.
  • It is impossible to convert all of its energy into work without losing heat.

8. What is criterion of spontaneity of reaction?

Ans: The criterion of spontaneity of reaction is decrease in free energy (i.e., ΔG = -ve)

For spontaneous reaction: ΔG = -ve

For non-spontaneous reaction: ΔG = +ve

For a reaction at equilibrium, ΔG = 0

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