Q11 / 160 · TS EAMCET 2020 (14 Sep Shift 1) · Chemistry

In which of the processes, the entropy will decrease

  • A
    $\mathrm{H}_2(g) \longrightarrow 2 \mathrm{H}(g)$
  • Correct answer
    B
    Liquid water crystallises to ice
  • C
    Temperature of ice is raised from 10 K to 115 K
  • D
    $2 \mathrm{NaHCO}_3(\mathrm{~s}) \longrightarrow \mathrm{Na}_2 \mathrm{CO}_3(\mathrm{~s})+\mathrm{CO}_2(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}(\mathrm{s})$

Solution

(a) $\mathrm{H}_2(g) \longrightarrow 2 \mathrm{H}(g) \Rightarrow \Delta n_g>0 \Rightarrow \Delta S>0$ (b) $\mathrm{H}_2 \mathrm{O}(l) \longrightarrow \mathrm{H}_2 \mathrm{O}(s) \Rightarrow$ degree of randomness Water Ice of $\mathrm{H}_2 \mathrm{O}$ molecules decreases, when freezing of water into ice takes place, $\Rightarrow \quad \Delta S < 0$ (c) With increase in temperature, $\mathrm{KE}$ of $\mathrm{H}_2 \mathrm{O}$ molecules of ice also increase. So, $\Delta S>0$. (d) $2 \mathrm{NaHCO}_3(s) \longrightarrow \mathrm{Na}_2 \mathrm{CO}_3(s)+\mathrm{CO}_2(g)+\mathrm{H}_2 \mathrm{O}(s)$ Here, $\Delta n_g>0 \Rightarrow \Delta S>0$. So, entropy decreases when liquid water crystallises into ice.

Hence, the correct answer is option: 'B'.

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