MCQs on Entropy

Entropy is a measure of randomness or disorder in the system. Entropy is a thermodynamic function and is denoted by S. The higher the entropy more the disorder in the isolated system. A change in entropy in a chemical reaction is related to the rearrangement of atoms from reactants to products. Entropy increases if the structure of the product is more disordered. Added heat in the system increases randomness and thus entropy also increases. Change in entropy (ΔS) is more at a lower temperature than higher temperature with the addition of the same amount of heat.

1. For spontaneous adsorption of gas

(a) ΔS is negative so ΔH should be highly positive

(b) ΔS is negative so ΔH should be highly negative

(c) ΔS is positive so ΔH should also be highly positive

(d) ΔS is positive so ΔH should be negative

Answer: (b)

2. For which of the following conditions a reaction will be spontaneous at all the temperature?

(a) ΔH < 0 and ΔS < 0

(b) ΔH < 0 and ΔS = 0

(c) ΔH < 0 and ΔS > 0

(d) both (b) and (c)

Answer: (d)

3. At which of the following temperatures the reaction C_(graphite) + CO_{2 (g)} → 2CO_(g) will occur spontaneously if ΔH = 170 kJ and ΔS = 170 J K^-1?

(a) 710 K

(b) 1110 K

(c) 910 K

(d) 510 K

Answer: (b)

4. At which of the following temperatures the reaction Br_{2 (l)} + Cl_{2 (g)} → 2BrCl_(g) will be at equilibrium if ΔH = 30 kJ mol^-1 and ΔS = 170 J K^-1 mol^-1?

(a) 450 K

(b) 300 K

(c) 273 K

(d) 285.7 K

Answer: (d)

5. Which of the following is the correct criteria for a spontaneous process?

(a) ΔS_system – ΔS_surroundings > 0

(b) ΔS_surroundings > 0 only

(c) ΔS_system + ΔS_surroundings > 0

(d) ΔS_system > 0 only

Answer: (c)

6. What is the unit of entropy?

(a) J mol^-1

(b) J K mol^-1

(c) J^-1 K^-1 mol^-1

(d) J K^-1 mol^-1

Answer: (d)

7. For a spontaneous reaction

(a) ΔS must be negative

(b) (ΔH – TΔS) must be negative

(c) ΔH must be negative

(d) (ΔH + TΔS) must be negative

Answer: (b)

8. Which of the following is true for the entropy of an isolated system?

(a) It can never increase

(b) It can never decrease

(c) It can never be zero

(d) None of the above

Answer: (b)

9. Which of the following is correct for the net entropy change in an irreversible process?

(a) It is positive

(b) It is negative

(c) It is zero

(d) All of the above

Answer: (a)

10. Calculate ΔG for the reaction X₂O_{4 (l)} → 2XO_{2 (g)} if ΔU = 2.1 kcal and ΔS = 20 cal K^-1 at 300°K.

(a) – 9.3 kcal

(b) – 2.7 kcal

(c) 2.7 kcal

(d) 9.3 kcal

Answer: (b)