Chapter 19, Problem 1SAQ

Which reaction Is most likely to have a positive $\Delta S_{sys}$ ?

• $3{\text{ NO}}_2(g)+{\text{H}}_2\text{O(}l)\to 2{\text{HNO}}_3(l)+\text{NO(}g)$
• ${\text{SiO}}_2(s)+3\text{C(}s)\to \text{SiC}(s)+2\text{ CO(}g)$
• $\text{CO}(g)+{\text{Cl}}_2\text{(}g)\to {\text{COCl}}_2\text{(}g)$
• ${\text{6CO}}_2(g)+6{\text{H}}_2\text{O(}g)\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6(s)+6{\text{ O}}_2\text{(}g)$

Interpretation Introduction

Interpretation: Which reaction is most likely to have positive ${\text{ΔS}}_{\text{sys}}$ .

Concept introduction:

Entropy is defined as the measure of the randomness or disorder in the system.

Correct answer:

Correct answer is (b) option.

Explanation of Solution

Reason for correct option: Since, gas has greater entropy so; we have to calculate change in number of moles of gaseous reactants and products:

$\begin{array}{l}{\text{SiO}}_2\text{(s)+3C(s)}\to \text{SiC(s) + 2CO(g)}\\ \Delta \text{S = 2-0 = 2}\end{array}$

So, the most correct option is (b).

Reason for incorrect option:

  $\begin{array}{l}{\text{3NO}}_2{\text{(g)+H}}_2\text{O(l)}\to {\text{2HNO}}_3\text{(l) + NO(g)}\\ \Delta \text{S = 1-3 = -2}\end{array}$

  $\begin{array}{l}{\text{CO(g)+Cl}}_2\text{(g)}\to {\text{COCl}}_2\text{(g) }\\ \Delta \text{S = 1-2 = -1}\end{array}$

  $\begin{array}{l}{\text{6CO}}_2{\text{(g)+6H}}_2\text{O(g)}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6\text{(s) }\text{+}{\text{6O}}_2\text{(g)}\\ \Delta \text{S = 6-12 = -6}\end{array}$