The following reaction is at equilibrium ,
`underset("Yellow")(Fe_((aq))^(3+))+ underset("Colourless")(SCN_((aq)))rArrunderset("Deep red") ([Fe(SNC)]_((aq))^(2+))`
`K_(c)=([Fe(SCN)])/([Fe^(3+)][SCN])`
In the above reaction , colour intensity of red colour can be increased by :- (A) addition of KSCN (B) addition of oxalic acid which reacts with F e 3 + ions (C) addition of H g 2 + ions which react with S C N − ions (D)red colour intensity cannot be changed

Options

A. addition of KSCN
B. addition of oxalic acid which reacts with `Fe^(3+)` ions
C. addition of `Hg^(2+)` ions which react with `SCN^(-)` ions
D. red colour intensity cannot be changed.

Explanation

To solve the question regarding the equilibrium reaction between \( \text{Fe}^{3+} \) and \( \text{SCN}^- \) ions, we need to analyze how the color intensity of the resulting complex \( \text{[Fe(SCN)]}^{2+} \) can be increased.

### Step-by-Step Solution:

1. **Understanding the Reaction:**
The reaction is:
\[ \text{Fe}^{3+} (aq) + \text{SCN}^- (aq) \rightleftharpoons \text{[Fe(SCN)]}^{2+} (aq) \]
The \( \text{Fe}^{3+} \) ions are yellow, \( \text{SCN}^- \) ions are colorless, and the complex \( \text{[Fe(SCN)]}^{2+} \) is deep red.

2. **Equilibrium Constant Expression:**
The equilibrium constant \( K_c \) for this reaction is given by:
\[ K_c = \frac{[\text{Fe(SCN)}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]} \]
This indicates that the concentration of the red complex is directly related to the concentrations of the reactants.

3. **Le Chatelier's Principle:**
According to Le Chatelier's Principle, if we increase the concentration of either reactant, the equilibrium will shift to the right to form more products, which in this case is the red complex.

4. **Evaluating the Options:**
- **(A) Addition of KSCN:**
- Adding \( \text{KSCN} \) increases the concentration of \( \text{SCN}^- \). This will shift the equilibrium to the right, resulting in more \( \text{[Fe(SCN)]}^{2+} \) being formed, thus increasing the intensity of the red color.
- **(B) Addition of oxalic acid:**
- Oxalic acid reacts with \( \text{Fe}^{3+} \) ions, reducing their concentration. This will shift the equilibrium to the left, decreasing the formation of the red complex. Hence, this option is incorrect.
- **(C) Addition of Hg\(^{2+}\) ions:**
- Mercury ions react with \( \text{SCN}^- \), decreasing its concentration. This will also shift the equilibrium to the left, reducing the red color intensity. Therefore, this option is incorrect.
- **(D) Red color intensity cannot be changed:**
- This statement is false as we can change the intensity by altering the concentrations of the reactants.

5. **Conclusion:**
The correct answer is **(A) addition of KSCN**, as it increases the concentration of \( \text{SCN}^- \), shifting the equilibrium to the right and enhancing the red color intensity.