Step 1: Relationship Between \( K_p \) and \( K_c \)
The equilibrium constant \( K_p \) is expressed in terms of partial pressures, and \( K_c \) is expressed in terms of concentrations. They are related by the following equation:
$$ K_p = K_c \left( RT \right)^{\Delta n} $$
Step 2: Condition for \( K_p \) and \( K_c \) to be Equal
For \( K_p \) and \( K_c \) to be equal, \( \Delta n \) must be zero. This means that the number of moles of gas on both sides of the reaction must be the same.
Step 3: Analyze Each Option
Option 1: \( H_2(g) + I_2(g) \rightleftharpoons 2HI(g) \)
Option 2: \( CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g) \)
Option 3: \( 2BrCl(g) \rightleftharpoons Br_2(g) + Cl_2(g) \)
Option 4: \( PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) \)
Step 4: Conclusion
The correct answer is: Option (1): \( PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) \), as \( K_p \neq K_c \).