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Question

When ammonia is added to the solution, $$pH$$ is raised to $$11$$. Which half-cell reaction is affected by $$pH$$ and by how much? 


A
Eoxd will increase by a factor of 0.65 from Eooxd
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B
Eoxd will decrease by a factor of 0.65 from Eooxd
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C
Ered will increase by a factor of 0.65 from Eored
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D
Ered will decrease by a factor of 0.65 from Eored
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Solution

The correct option is A $$\mathrm{E}_{\mathrm{o}\mathrm{x}\mathrm{d}}$$ will increase by a factor of 0.65 from $$\mathrm{E^o}_{{\mathrm{o}}\mathrm{x}\mathrm{d}}$$
On addition of Ammonia $$H^+$$ concentration will decrease as the following reaction will take place.:
$$NH_3+H^+\rightarrow NH_4^+$$
On addition of ammonia $$pH$$ is raised to $$11$$. Therefore, $$[H^+]=10^{-11}$$
Oxidation reaction: $$H_2O\rightarrow 2H^++2e^-$$

$$E_{ox}=E^0_{ox}-\dfrac{0.059}{2}log\dfrac{[H^+]^2}{1}$$

$$E_{ox}=-0.05-22\times\dfrac{0.059}{2}log10$$
$$=(-0.05-0.65)V$$
Therfore, $$E_{ox}$$ will increase by a factor of $$0.65V$$.

Chemistry

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