The equilibrium HC2H3O2+H2O ⇄ H3O++C2H3O2-exists in a 1 M aqueous solution of HC₂H₃O₂ with an equilibrium constant K_eq = [H₃O⁺][C₂H₃O₂⁻]/[HC₂H₃O₂]. Which of the following expressions explains why K_eq does not include a term for [H₂O]?

Question

Quizplus · Accepted Answer

11ecba2d_121d_f3cc_a3bf_6dc3a1f96920_MD0008_00 The general reaction in which chemical species A and B react to form products C and D is written as 11ecba2d_121d_f3cd_a3bf_ef6ad5d06bcc_MD0008_00 where a, b, c, and d are the coefficients of the balanced reaction. For a reaction at equilibrium, the law of mass action states that an equilibrium constant K_eq can be expressed as a ratio of the molar concentrations of the products over the reactants, each raised to the power of its respective balanced reaction coefficient: 11ecba2d_121d_f3ce_a3bf_9f4cc6b1ae44_MD0008_00 However, for a reaction in which one of the reactants is also the solvent, the solvent species is omitted from the K_eq expression. For example, a 1 M aqueous solution of HC₂H₃O₂ with an equilibrium reaction of 11ecba2d_121e_1adf_a3bf_6dc283a2b256_MD0008_00 has an associated K_eq expression of 11ecba2d_121e_1ae0_a3bf_29557f161d0b_MD0008_00 In this reaction, the concentration of water is omitted from the K_eq expression because, although water is a reactant, it is also the solvent. As the solvent, the number of water molecules is significantly greater than the number of all other species in the solution; the HC₂H₃O₂ is dilute and dissociates only to a small extent. Therefore, in relative terms, the [H₂O] (55.5 M) is essentially constant and is omitted. (Choices A and C) Water is the solvent and the HC₂H₃O₂ concentration is low; therefore, the number of water molecules per unit volume is vastly greater than the number of molecules of HC₂H₃O₂ or the number of H₃O⁺ or C₂H₃O₂⁻ ions formed as dissociation products. (Choice D) As the solvent, water cannot have a concentration of zero. Educational objective: Applying the law of mass action, an equilibrium constant K_eq can be expressed for a reaction as a ratio of the molar concentrations of the products over the reactants, each raised to the power of its respective balanced reaction coefficient. Because the concentration of the solvent is much greater than the concentration of all other species in the solution, the solvent is relatively constant and is omitted from the K_eq expression.

The Equilibrium HC2H3O2+H2O ⇄ H3O++C2H3O2-Exists in a 1 M Aqueous Solution