A system is described as follows - HSC - SSCE Chemistry - Question 8 - 2022 - Paper 1

To derive the equilibrium expression for the reaction, we need to use the general form of the equilibrium constant (K_eq), which is given by the concentration of the products raised to the power of their stoichiometric coefficients divided by the concentration of the reactants raised to their stoichiometric coefficients.

In this case, the balanced chemical equation is:

$$2NaHCO₃(s) ⇌ Na₂CO₃(s) + CO₂(g) + H₂O(g)$$

The states of matter indicate that NaHCO₃ and Na₂CO₃ are solids, which do not appear in the equilibrium expression. Thus, we only consider products that are in the gas or aqueous phase.

The equilibrium expression is therefore:

$$K_{eq} = \frac{[CO₂][H₂O]}{1} = [CO₂][H₂O]$$

Thus, the correct answer, based on the options provided, is B: K_eq = [CO₂][H₂O].