`{:(Lis - I, List -II),((A) H_(2) (g) + I_(2)(g) hArr 2HI(g),(P)"Unaffected by inert gas addition "),((B)CaCO_(3) (s) hArr CaO(s) + CO_(2) (g),(Q)" Forward shift by rise in pressure and backward shift by inert gas addition at constant pressure "),((C) N_(2) (g) + 3H_(2) + (g) hArr 2NH_(3) (g) , (R)" Unaffected by increase in pressure"),((D) PCl_(3) (g) hArr PCl_(3) (g) + Cl_(2) (g) ,(S)"ackward shift by rise in pressure and forward shift by inert gas addition at constant pressure "):}`

Answer

To solve the problem, we need to match the reactions from List-I with the corresponding effects on equilibrium from List-II based on the principles of chemical equilibrium, particularly Le Chatelier's principle.

### Step-by-Step Solution:

1. **Identify Reaction A**:
- Reaction: \( H_2(g) + I_2(g) \rightleftharpoons 2HI(g) \)
- Analysis: The number of moles of reactants (2) equals the number of moles of products (2). According to Le Chatelier's principle, changes in pressure do not affect the equilibrium position when the number of moles of gas on both sides is equal.
- **Match**: A matches with **R** ("Unaffected by increase in pressure").

2. **Identify Reaction B**:
- Reaction: \( CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g) \)
- Analysis: The number of moles of reactants is 1 (solid) and the number of moles of products is 2 (1 solid + 1 gas). An increase in pressure will shift the equilibrium to the side with fewer moles of gas, which is the reactant side.
- **Match**: B matches with **S** ("Backward shift by rise in pressure and forward shift by inert gas addition at constant pressure").

3. **Identify Reaction C**:
- Reaction: \( N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \)
- Analysis: The number of moles of reactants (4) is greater than the number of moles of products (2). An increase in pressure will favor the formation of products (forward shift).
- **Match**: C matches with **Q** ("Forward shift by rise in pressure and backward shift by inert gas addition at constant pressure").

4. **Identify Reaction D**:
- Reaction: \( PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) \)
- Analysis: The number of moles of reactants (1) is less than the number of moles of products (2). An increase in pressure will favor the side with fewer moles, but since this reaction involves gases, it is unaffected by inert gas addition.
- **Match**: D matches with **P** ("Unaffected by inert gas addition").

### Final Matches:
- A → R
- B → S
- C → Q
- D → P