Question

The reaction in which standard reaction entropy change $\left(\right.\left(ΔS\right)^{o}\left.\right)$ is positive and standard Gibb's energy change $\left(\right.\left(ΔG\right)^{o}\left.\right)$ decreases sharply with increasing temperature is:

• A. $C$ (graphite) $+\frac{1}{2}\left(O_{2}\right)_{\left(\right. g \left.\right)}\overset{ }{ \rightarrow }\left(CO\right)_{\left(\right. g \left.\right)}$
• B. $\left(CO\right)_{g}+\frac{1}{2}\left(O_{2}\right)_{\left(\right. g \left.\right)}\overset{ }{ \rightarrow }\left(CO\right)_{\left(\right. g \left.\right)}$
• C. $\left(Mg\right)_{s}+\frac{1}{2}\left(O_{2}\right)_{\left(\right. g \left.\right)}\overset{ }{ \rightarrow }\frac{1}{2}\left(\left(CO\right)_{2}\right)_{\left(\right. g \left.\right)}$
• D. $\frac{1}{2}C\left(graphite\right)+\frac{1}{2}\left(O_{2}\right)_{\left(\right. g \left.\right)}\overset{ }{ \rightarrow }\frac{1}{2}\left(\left(CO\right)_{2}\right)_{\left(\right. g \left.\right)}$

Answer 1

Answer: (A)

$C_{g r a p h i t e}+\frac{1}{2}\left(O_{2}\right)_{\left(\right. g \left.\right)} \rightarrow CO_{g}$
$\Delta n_{g}=1-\frac{1}{2}=\frac{1}{2}$
As amount of gaseous substance is increasing in product, thus $\Delta S$ is positive for this reaction.
And we know that $\Delta G=\Delta H-T\Delta S$
As $\Delta S$ is positive, thus increase in temperature will make $T\Delta S$ more negative and $\Delta G$ will decrease.