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JEE Main levelSingle correct optionDifficulty 2 of 3ec21

Excess aqueous ammonia is added to the $Cu X^{2 +}$ compartment of a working Daniell cell. The cell EMF

Aincreases, because $[Cu (NH X_{3}) X_{4}] X^{2 +}$ is a stronger oxidant
Bdecreases, because free $[Cu X^{2 +}]$ falls
Cremains unchanged, because $E^{\circ}$ is fixed
Dbecomes zero immediately

Answer: B

Complexation

Cu X^{2 +} + 4 NH X_{3} [Cu (NH X_{3}) X_{4}] X^{2 +}

lowers

[Cu X^{2 +}]

by many orders of magnitude. Nernst:

E = E^{\circ} - \frac{0.0591}{2} lo g \frac{[ Zn X ^{2 +} ]}{[ Cu X ^{2 +} ]}

, so smaller

[Cu X^{2 +}]

lowers

E

. It does not reach zero while any driving force remains.

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