The given balanced chemical equation is:
[C_2H_4(g) + H_2(g) \rightarrow C_2H_6(g)]
We need to write the expression for the equilibrium constant ((K_{\text{eq}})) for this reaction. Remember that the equilibrium constant is defined as the ratio of the product concentrations to the reactant concentrations, each raised to their respective stoichiometric coefficients.
The expression for (K_{\text{eq}}) can be written as:
[K_{\text{eq}} = \frac{{[C_2H_6]}}{{[C_2H_4][H_2]}}]