Entropy JEE Mains and Advanced Solved Questions

1. The process with negative entropy change is

1. dissolution of iodine in water
2. sublimation of dry ice
3. synthesis of ammonia from N₂ and H₂
4. dissociation of CaSO_4(s) to CaO_(s) and SO_3(g)

Solution:

N_2(g) + 3H₂ → 2NH_3(g)

∆s = 2 – 4 = – 2 < 0

So, entropy is negative.

Hence, option (3) is the answer.

2. Identify the correct statement regarding a spontaneous process

1. For a spontaneous process in an isolated system, the change in entropy is positive
2. Endothermic processes are never spontaneous
3. Exothermic processes are always spontaneous
4. Lowering of energy in the reaction process is the only criterion for spontaneity

Solution:

The change in entropy is positive for a spontaneous process in an isolated sys­tem.

Hence, option (1) is the answer.

3. For a particular reversible reaction at temperature T, ∆H and ∆S were found to be both +ve. If T_e is the temperature at equilibrium, the reaction would be spontaneous when

1. T_e > T
2. T > T_e
3. T_e is 5 times T
4. T_e = T

Solution:

At equilibrium, ∆G = 0.

∆G = ∆H – T∆S

For a reaction to be spontaneous ∆G should be negative.

So T > T_e

Hence, option (2) is the answer.

4. The entropy change involved in the isothermal reversible expansion of 2 moles of an ideal gas from a volume of 10 dm³ to a volume of 100 dm³ at 27°C is

1. 32.3 J mol^-1K^-1
2. 42.3 J mol^-1K^-1
3. 38.3 J mol^-1K^-1
4. 35.8 J mol^-1K^-1

Solution:

Given V₂ = 100

V₁ = 10

n = 2

∆S = 2.303nR log (V₁/V₂)

= 2.303 × 2 × 8.314 × log (100/10)

= 2.303 × 2 × 8.314 × log 10

= 38.29 J mol^-1 K^-1

Hence, option (3) is the answer.

5. An ideal gas undergoes isothermal expansion at constant pressure. During the process

1. enthalpy increases, but entropy decreases
2. enthalpy remains constant, but entropy increases
3. enthalpy decreases, but entropy increases
4. both enthalpy and entropy remain constant.

Solution:

During isothermal expansion at constant pressure, ∆H = nC_p ∆T = 0

∆S = nRln (V_f/V_i) > 0

Hence, option (2) is the answer.

6. The entropy of an isolated system can never __.

1. increase
2. decrease
3. be zero
4. none of the mentioned

Answer: 2

Explanation: The entropy of an isolated system always increases and remains constant only when the process is reversible.

7. Clausius summarised the first and second laws of thermodynamics as ___.

1. the energy of the world is constant
2. the entropy of the world tends towards a maximum
3. both of the above
4. none of the above

Solution:

Answer: 3

Explanation: Clausius gave these two statements.

Entropy means the amount of disorder or randomness of a system. It is a measure of thermal energy per unit of the system which is unavailable for doing work. The concept of entropy can be applied in various contexts and stages, including cosmology, economics, and thermodynamics. Entropy is a concept that essentially discusses the spontaneous changes that take place in ordinary phenomena or the Universe’s inclination towards disorder.

In this article, we will learn what is meaning of Entropy, the entropy change formula, and how it is associated with the laws of thermodynamics.