Question

Which of the following changes will cause the free energy of a cell reaction to decrease ? $Zn|ZnS{O}_4\left(aq\right)\left(x_{1}M\right)||HCl\left(aq\right)\left(x_{2}M\right)/H_2\left(g\right),Pt$

• A. Increase in the volume of HCl solution from 100 mL to 200 mL
• B. Increase in the pressure of hydrogen from 1 atm to 2 atm
• C. Increase in molarity $x_2$ from 0.1 to 1 M
• D. Decrease in molarity $x_1$ from 1 M to 0.1 M

Answer 1

Answer: (C), (D)

The correct options are
C Increase in molarity $x_2$ from 0.1 to 1 M
D Decrease in molarity $x_1$ from 1 M to 0.1 M

Free energy to decrease $\Rightarrow$ cell is more spontaneous $(E_{cell}\uparrow )$

$Zn+2H^{\oplus }\left(x_{2}M\right)\to Z{n}^{2+}\left(x_{1}M\right)+H_2\left(g\right)$

$\Rightarrow E_{cell}=E_{cell}^{\oplus }-\frac{0.059}{2}log\frac{\left[Z{n}^{2+}\right]p{H}_2}{[H^{\oplus }{]}^2}$

$\Delta G=-ve$ or $E_{cell}=+ve$

This happens when

c. When $x_2$ (0.1 M) changes to 1 M.
d. When $x_1$ (1 M) changes 0.1 M.

Hence, options C and D are correct