The value of K_a for nitrous acid (HNO₂) at 25 °C is given in Appendix D. (a) Write the chemical equation for the equilibrium that corresponds to K_a.

Verified step by step guidance

1. Identify the chemical formula for nitrous acid, which is HNO2.
2. Recognize that the dissociation of an acid in water typically results in the formation of hydrogen ions (H+) and the conjugate base of the acid.
3. Write the chemical equation for the dissociation of nitrous acid in water: HNO2(aq) ⇌ H+(aq) + NO2-(aq).
4. Understand that Ka, the acid dissociation constant, quantifies the strength of the acid in solution, indicating how completely the acid dissociates in water.
5. The equilibrium expression corresponding to this reaction is Ka = [H+][NO2-] / [HNO2], where the concentrations are those at equilibrium.

Key Concepts

Acid-Base Equilibrium

Acid-base equilibrium refers to the state in which the concentrations of the reactants and products in an acid-base reaction remain constant over time. For weak acids like nitrous acid (HNO2), this equilibrium involves the dissociation of the acid into its ions, which is characterized by the acid dissociation constant (Ka). Understanding this concept is crucial for writing the correct equilibrium expression.

Acid Dissociation Constant (Ka)

The acid dissociation constant (Ka) quantifies the strength of an acid in solution. It is defined as the ratio of the concentration of the products (the dissociated ions) to the concentration of the undissociated acid at equilibrium. A higher Ka value indicates a stronger acid that dissociates more completely in solution, which is essential for determining the behavior of HNO2 in aqueous solutions.

Chemical Equations and Equilibrium Expressions

Chemical equations represent the reactants and products involved in a chemical reaction, while equilibrium expressions mathematically describe the relationship between the concentrations of these species at equilibrium. For HNO2, the equilibrium equation would show the dissociation into H+ and NO2-, allowing for the calculation of Ka. Writing these equations correctly is fundamental for analyzing acid-base reactions.