Q37 / 125 · BITSAT 2009 · Chemistry

Ammonia forms the complex ion $\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}$ with copper ions in alkaline solutions but not in acidic solutions. What is the reason for it?

  • Correct answer
    A
    In acidic solutions protons coordinate with ammonia molecules forming $\mathrm{NH}_{4}^{+}$ ions and $\mathrm{NH}_{3}$ molecules are not available.
  • B
    In alkaline solutions insoluble $\mathrm{Cu}(\mathrm{OH})_{2}$ is precipitated which is soluble in excess of any alkali.
  • C
    Copper hydroxide is an amphoteric substance.
  • D
    In acidic solutions hydration protects copper ions.

Solution

$\quad \mathrm{N} \mathrm{H}_{3}+\mathrm{H}^{+}$ (acidic medium) $\rightleftharpoons \mathrm{NH}_{4}^{+}$

Hence, the correct answer is option: 'A'.

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