Write chemical equations and corresponding equilibrium expressions for each of the three ionization steps of phosphoric acid.

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Identify the chemical formula for phosphoric acid, which is \( \text{H}_3\text{PO}_4 \).

Write the chemical equation for the first ionization step: \( \text{H}_3\text{PO}_4 (aq) \rightleftharpoons \text{H}^+ (aq) + \text{H}_2\text{PO}_4^- (aq) \).

Write the equilibrium expression for the first ionization step: \( K_{a1} = \frac{[\text{H}^+][\text{H}_2\text{PO}_4^-]}{[\text{H}_3\text{PO}_4]} \).

Write the chemical equation for the second ionization step: \( \text{H}_2\text{PO}_4^- (aq) \rightleftharpoons \text{H}^+ (aq) + \text{HPO}_4^{2-} (aq) \).

Write the equilibrium expression for the second ionization step: \( K_{a2} = \frac{[\text{H}^+][\text{HPO}_4^{2-}]}{[\text{H}_2\text{PO}_4^-]} \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionization of Acids

Ionization of acids refers to the process by which an acid donates protons (H⁺ ions) to water, resulting in the formation of hydronium ions (H₃O⁺) and the corresponding anions. For polyprotic acids like phosphoric acid (H₃PO₄), this process occurs in multiple steps, each involving the release of one proton at a time.

Equilibrium Expressions

Equilibrium expressions are mathematical representations of the relationship between the concentrations of reactants and products at equilibrium for a given chemical reaction. For ionization reactions, the equilibrium constant (K) is defined as the ratio of the concentration of products to the concentration of reactants, each raised to the power of their coefficients in the balanced equation.

Phosphoric Acid Ionization Steps

Phosphoric acid undergoes three ionization steps, each producing a different anion: H₃PO₄ → H₂PO₄⁻ + H⁺, H₂PO₄⁻ → HPO₄²⁻ + H⁺, and HPO₄²⁻ → PO₄³⁻ + H⁺. Each step has its own equilibrium expression, reflecting the concentrations of the species involved at equilibrium, which are essential for understanding the acid's behavior in solution.