CBSE 2025 · Region 2 · Set 1 · Q32 · 5 marks

(a)

Calculate the standard Gibbs energy $\displaystyle \left(\Delta_{\mathrm{r}} \mathrm{G}^{\circ}\right)$ of the following reaction at $\displaystyle 25$ °C : $\displaystyle 3$ + $\displaystyle 2$ $\displaystyle \mathrm{Au}(\mathrm{s})+\mathrm{Ca}^{2+}(1 \mathrm{M}) \rightarrow \mathrm{Au}^{3+}(1 \mathrm{M})+\mathrm{Ca}(\mathrm{s})$ $\displaystyle \mathrm{E}_{\mathrm{Au}^{3+} / \mathrm{Au}}^{\circ}=+1.5 \mathrm{~V}, \mathrm{E}_{\mathrm{Ca}^{2+} / \mathrm{Ca}}^{\circ}=-2.87 \mathrm{~V}$ Predict whether the reaction will be spontaneous or not at $\displaystyle 25$ $\displaystyle { }^{\circ} \mathrm{C}$. $\displaystyle \left[1 \mathrm{~F}=96500 \mathrm{C} \mathrm{mol}^{-1}\right]$

(b)

Tarnished silver contains $\displaystyle \mathrm{Ag}_{2} \mathrm{~S}$. Can this tarnish be removed by placing tarnished silverware in an aluminium pan containing an inert electrolytic solution such as NaCl ? The standard electrode potential for half reaction : $\displaystyle \mathrm{Ag}_{2} \mathrm{~S}(\mathrm{~s})+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Ag}(\mathrm{s})+\mathrm{S}^{2-}$ is -$\displaystyle 0.71$ V and for $\displaystyle \mathrm{A} l^{3+}+3 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Al}(\mathrm{s})$ is -$\displaystyle 1.66$ V

(b)

OR 32. (B) (a) Define the following : $\displaystyle 2+3$

(i)

Cell potential

(ii)

Fuel cell

(b)

Calculate emf of the following cell at $\displaystyle 25$ °C : \[\mathrm{Zn}(\mathrm{~s})\left|\mathrm{Zn}_{(0.1 \mathrm{M})}^{2+}\right|\left|\mathrm{Cd}_{(0.01 \mathrm{M})}^{2+}\right| \mathrm{Cd}(\mathrm{~s}) \] Given : $\displaystyle \mathrm{E}_{\mathrm{Cd}^{2+} / \mathrm{Cd}}^{\circ}=-0.40 \mathrm{~V}$ \[\mathrm{E}_{\mathrm{Zn}^{2+} / \mathrm{Zn}}^{\circ}=-0.76 \mathrm{~V} \] \[[\log 10=1] \]

CBSE Class 12 Chemistry past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.