Predict which of the following reactions will have an appreciable concentration of both reactants and products:
$(a)$ $Cl_{2(g)} \longleftrightarrow 2Cl_{(g)};$ $K_{c}=5 \times 10^{-39}$
$(b)$ $Cl_{2(g)}+2NO_{(g)} \longleftrightarrow 2NOCl_{(g)};$ $K_{c}=3.7 \times 10^{8}$
$(c)$ $Cl_{2(g)}+2NO_{2(g)} \longleftrightarrow 2NO_{2}Cl_{(g)};$ $K_{c}=1.8$

(C) For a chemical reaction to have an appreciable concentration of both reactants and products at equilibrium,the value of the equilibrium constant $K_{c}$ must lie in the range of $10^{-3}$ to $10^{3}$.
In reaction $(a)$,$K_{c} = 5 \times 10^{-39}$,which is much less than $10^{-3}$,indicating that the reaction barely proceeds.
In reaction $(b)$,$K_{c} = 3.7 \times 10^{8}$,which is much greater than $10^{3}$,indicating that the reaction proceeds almost to completion.
In reaction $(c)$,$K_{c} = 1.8$,which lies between $10^{-3}$ and $10^{3}$.
Therefore,the reaction given in $(c)$ will have an appreciable concentration of both reactants and products.