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Ammonia can produce the complex ion [Cu(NH3)4]2+ with copper ions in alkaline medium, but this does not occur in acidic medium. What explains this behavior?

1. In acidic medium, H+ ions combine with ammonia to form NH4+, so free NH3 is not available
2. In alkaline medium, Cu(OH)2 first precipitates and then dissolves in excess alkali
3. Copper hydroxide shows amphoteric nature
4. In acidic medium, hydration shields the copper ions

Correct answer: In acidic medium, H+ ions combine with ammonia to form NH4+, so free NH3 is not available

Solution

In acidic medium, H+ reacts with ammonia to form NH4+, leaving no free NH3 available to coordinate to Cu2+, so the complex [Cu(NH3)4]2+ cannot form. In alkaline medium free NH3 is present and complexation occurs.

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