Among the solubility rules previously discussed is the statement: Carbonates, phosphates, borates, and arsenates—except those of the ammonium ion and the alkali metals—are insoluble.
(a) Write the expression for the equilibrium constant for the reaction represented by the equation CaCO3(s) ⇌ Ca2+(aq) + CO3 −(aq). Is Kc > 1, < 1, or ≈ 1? Explain your answer.
To write the expression for the equilibrium constant for the given reaction CaCO3(s) ⇌ Ca2+(aq) + CO3^−(aq), we need to represent the concentration of the products and the reactant in the equation.
The expression for the equilibrium constant, Kc, is defined as the ratio of the concentrations of the products to the concentrations of the reactant, each raised to their stoichiometric coefficients.
In this case, since CaCO3 is a solid, its concentration remains constant and is not included in the expression for Kc. Therefore, the expression for Kc can be written as:
Kc = [Ca2+][CO3^−]
Now, considering the solubility rules provided, it states that carbonates (CO3^2−) are generally insoluble except for carbonates of the ammonium ion (NH4^+) and the alkali metals. This means that the concentration of CO3^2− would be very small as it will not significantly dissociate in water, resulting in a low concentration in the equilibrium expression.
On the other hand, since CaCO3 is a sparingly soluble salt, it will undergo some dissociation, resulting in a higher concentration of Ca2+ in the equilibrium expression.
Based on the concept of equilibrium constant and the given solubility rules, it can be inferred that the concentration of Ca2+ is relatively higher compared to the concentration of CO3^2−. As a result, the equilibrium constant Kc would be greater than 1 (Kc > 1).