Question: 59. In which of the following processes entropy increases?
A. A liquid evaporates to vapour
B. Temperature of a crystalline solid lowered from 130 K to 0K.
C. 2 \mathrm{NaHCO}_{3(\mathrm{~s})} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3(\mathrm{~s})}+\mathrm{CO}_{2(\mathrm{~g})}+\mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})}
D. \mathrm{Cl}_{2(\mathrm{~g})} \rightarrow 2 \mathrm{Cl}_{(\mathrm{g})}
Choose the correct answer from the options given below :
(1) A and C
(2) A, B and D
(3) A, C and D
(4) C and D
Answer: Option (3)
Explanation:
Entropy is a measure of randomness or disorder of a system.
Entropy increases when the disorder of the system increases.
In process A, a liquid evaporates to vapour. Vapour molecules have much greater freedom of movement compared to liquid molecules, so randomness increases and entropy increases.
In process B, the temperature of a crystalline solid is lowered from 130 K to 0 K. Lowering temperature reduces molecular motion and disorder.
At 0 K, entropy is minimum according to the third law of thermodynamics. Therefore, entropy decreases.
In process C, solid \mathrm{NaHCO_3} decomposes to give solid
\mathrm{Na_2CO_3} and gaseous \mathrm{CO_2}
and \mathrm{H_2O}.
Formation of gaseous products from solids greatly increases randomness, so entropy increases.
In process D, one mole of gaseous \mathrm{Cl_2} forms two moles of gaseous chlorine atoms.
Increase in number of gaseous particles increases disorder, so entropy increases.
Therefore, entropy increases in processes A, C and D,
and the correct answer is Option (3).