In which of the following reactions, will the equilibrium mixture contain an appreciable concentration of both reactants and products.

Options

A. `Cl_(2)(g) Leftrightarrow 2Cl(g), K_(c)=6.4 xx 10^(-39)`
B. `Cl_(2) (g)+2NO (g) Leftrightarrow 2NOCl(g) K_(c)=3.7 xx 10^(8)`
C. `Cl_(2)(g)+2NO_(2) (g) Leftrightarrow 2NO_(2) Cl(g), K_(C)=1.8`
D. `H_(2)(g)+S(s) Leftrightarrow H_(2)S(g), K=7.8 xx 10^(5)`

Answer

To determine in which of the given reactions the equilibrium mixture will contain an appreciable concentration of both reactants and products, we need to analyze the equilibrium constant (Kc) values provided for each reaction. The Kc value indicates the ratio of the concentrations of products to reactants at equilibrium and helps us understand the position of equilibrium.

### Step-by-Step Solution:

1. **Understand the Concept of Kc**:
- The equilibrium constant (Kc) is a numerical value that reflects the ratio of the concentrations of products to reactants at equilibrium.
- A high Kc value (much greater than 1) indicates that the products are favored, while a low Kc value (much less than 1) indicates that the reactants are favored.

2. **Analyze Each Kc Value**:
- **Reaction 1**: Kc = 6.4 x 10^-39
- This value is very small, indicating that the equilibrium will favor the reactants. Thus, the concentration of products will be negligible.
- **Reaction 2**: Kc = 3.7 x 10^8
- This value is very high, indicating that the equilibrium will favor the products. The concentration of reactants will be negligible.
- **Reaction 3**: Kc = 1.8
- This value is close to 1, suggesting that both reactants and products will be present in significant amounts at equilibrium.
- **Reaction 4**: Kc = 7.8 x 10^5
- This value is also high, indicating that the equilibrium will favor the products, and the concentration of reactants will be negligible.

3. **Conclusion**:
- From the analysis, only **Reaction 3** (Kc = 1.8) has a Kc value around 1, which indicates that both reactants and products will be present in appreciable concentrations at equilibrium.

### Final Answer:
The equilibrium mixture will contain appreciable concentrations of both reactants and products in **Reaction 3**.