Problem 31 - Chemistry and Chemical Reactivity

Question

Write an equation that describes the equilibrium that exists when nitric acid dissolves in water. Identify each of the four species in solution as either Bronsted acids or Bronsted bases. Does the equilibrium favor the products or the reactants?

Step-by-Step Solution

Answer

The equilibrium favors the products since HNO₃ is a strong acid.

Step 1: Write the chemical equation

When nitric acid (HNO₃) dissolves in water (H₂O), it ionizes completely to form hydronium ions (H₃O⁺) and nitrate ions (NO₃⁻). The chemical equation for this process is:\[ \text{HNO}_3 (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_3\text{O}^+ (aq) + \text{NO}_3^- (aq) \]

Step 2: Identify the Bronsted acids and bases

In the equation, nitric acid (HNO₃) acts as a Bronsted acid because it donates a proton (H⁺) to water. Water (H₂O) acts as a Bronsted base because it accepts a proton to form hydronium (H₃O⁺). The hydronium ion (H₃O⁺) is a Bronsted acid because it can donate a proton, and the nitrate ion (NO₃⁻) is a Bronsted base because it can accept a proton.

Step 3: Determine the direction of equilibrium

Nitric acid is a strong acid, which means it ionizes completely in water. As a result, the equilibrium lies far to the right, favoring the formation of products, which are H₃O⁺ and NO₃⁻. This means that, in solution, almost all of the nitric acid exists in its dissociated form.

Key Concepts

Equilibrium Reactions

Equilibrium reactions occur when the forward and reverse reactions happen at the same rate, so the concentrations of the reactants and products remain constant over time. In the case of nitric acid (HNO₃) dissolving in water, we see the equilibrium reaction as:\[ \text{HNO}_3 (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{H}_3\text{O}^+ (aq) + \text{NO}_3^- (aq) \]In this equation, HNO₃ donates a proton to H₂O, forming the products H₃O⁺ and NO₃⁻.- **Direction of equilibrium:** Since HNO₃ is a strong acid, the equilibrium favors the formation of products, meaning the reaction goes almost entirely to the right.- **Balanced state:** The concept of equilibrium means that the reaction can proceed in both directions, but the presence of a very strong acid like HNO₃ ensures that the products predominate in the solution.Understanding equilibrium reactions is crucial in predicting the behavior of acids and bases in solution.

Strong Acids

Strong acids are substances that ionize completely in solution, meaning they donate all their available protons to the surrounding solvent, usually water. Examples of strong acids include nitric acid (HNO₃), hydrochloric acid (HCl), and sulfuric acid (H₂SO₄). - **Complete ionization:** When HNO₃ is added to water, it dissociates fully to form hydronium ions (H₃O⁺) and nitrate ions (NO₃⁻). - **Irreversible reaction perspective:** For strong acids, the ionization process is typically considered irreversible in practical terms due to their complete dissociation. - **pH implications:** The presence of strong acids results in a significantly lower pH of the solution because of the increased concentration of H₃O⁺ ions. The behavior of strong acids is contrasted with weak acids, which only partially ionize and have a more balanced equilibrium between their ionized and non-ionized forms.

Ionization in Water

Ionization in water is the process by which an acid or base transfers protons to or from water molecules, resulting in the formation of ions. When nitric acid (HNO₃) ionizes in water, it contributes to the formation of hydronium ions (H₃O⁺) and nitrate ions (NO₃⁻). - **Proton transfer:** In this ionization process, HNO₃ donates a proton to a water molecule, thus acting as a Bronsted-Lowry acid, and water acts as a Bronsted-Lowry base by accepting the proton. - **Hydronium and hydroxide balance:** Typically, in pure water, very few water molecules ionize to form H₃O⁺ and OH⁻ ions. However, the addition of acids increases the concentration of H₃O⁺ significantly. - **Driving force:** The complete ionization of strong acids in water is due to the stability of the resulting ions, such as the nitrate ion in this case. Knowing how ionization in water works helps in understanding acid-base reactions and predicting the resulting pH of acidic or basic solutions.