The Question is from neet-chemistry , and it was created by the author CHANDRAKANTH

Question

When ammonia is added to green aqueous solution of nickel(II) sulphate, the colour of the solution changes to blue violet. This is caused by:

CHANDRAKANTH · Accepted Answer

## CORE INFORMATION
✓ **Answer**: B
## CONCEPT FRAMEWORK
**Primary Concept**: Ligand Exchange
**Related Topics**: Coordination Complexes, Transition Metal Chemistry
**Prerequisites**: Basic knowledge of coordination compounds and ligands.
## SOLUTION PATHWAY
**Given Information**:
  - Nickel(II) sulphate solution is green.
  - Adding ammonia changes the color to blue-violet.
  - We need to determine the reason for the color change.
**Method & Steps**:
1.  **Understanding the initial state:** In an aqueous solution, nickel(II) ions exist as a complex with water molecules, [Ni(H₂O)₆]²⁺, which is responsible for the green color.
   Reasoning: Transition metal ions in aqueous solutions usually form complexes with water molecules.
   Formula/Rule: None
2.  **Understanding the effect of ammonia:** Ammonia is a stronger ligand than water. When ammonia is added, it replaces the water molecules around the nickel(II) ion, forming a new complex, [Ni(NH₃)₆]²⁺.
   Reasoning: Ligand exchange occurs due to differences in the strength of the interaction with the metal center.
   Working:  [Ni(H₂O)₆]²⁺ + 6NH₃  -> [Ni(NH₃)₆]²⁺ + 6H₂O
   Key Point: The change in ligand changes the electronic transitions and therefore the color.
3. **Relating to the color change:** The change in ligand from water to ammonia results in a different electronic environment around the nickel ion, thus the change in color.
   Result: The color change from green to blue-violet is due to the formation of a new nickel complex with ammonia ligands.
   Verification: This is a well-known reaction in coordination chemistry.
 ## OPTION ANALYSIS
[A] :x: Nickel does not change oxidation state. It remains +2.
[B] ✓: This is the correct explanation. Ammonia replaces water molecules.
[C] :x: The coordination number of nickel remains 6 in both complexes.
[D] :x: While the pH may change, this is not the primary reason for the color change.
 ## LEARNING AIDS
**Common Mistakes**:
1.  Mistaking oxidation state change for color change: The oxidation state of the metal ion does not change in this case.
     How to avoid: Be careful in understanding the role of each species.
2. Not realizing the stronger ligand replaces weaker one: Always consider the ligand exchange principle.
     How to avoid: Learn the ligand strength series.
**Quick Tips**:
1.  Color changes in transition metal complexes are often due to changes in ligands.
2. Ammonia is a common ligand that forms strong complexes with many transition metal ions.

CHANDRAKANTH · Answer

Nickel undergoing a change in oxidation state

CHANDRAKANTH · Answer

The correct Option is "Ammonia molecules replacing water molecules surrounding nickel"

CHANDRAKANTH · Answer

Change in coordination number of nickel

CHANDRAKANTH · Answer

Change in ${\rm{pH}}$ value of the solution

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