Multiple Choice
What are the products formed when water undergoes auto-ionization?
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17. Acid and Base Equilibrium
Auto-Ionization
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Which equilibrium expression represents the auto-ionization of water?
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Verified step by step guidance1
Identify the chemical equation for the auto-ionization of water: \( 2H_2O (l) \rightleftharpoons H_3O^+ (aq) + OH^- (aq) \). This shows water molecules reacting to form hydronium and hydroxide ions.
Recall that the equilibrium constant expression \( K_c \) is written as the concentration of products over reactants, each raised to the power of their coefficients in the balanced equation.
Since water is a pure liquid, its concentration is constant and typically omitted from the equilibrium expression. Therefore, only the aqueous ions appear in the expression.
Write the equilibrium expression as \( K = \frac{[H_3O^+][OH^-]}{[H_2O]^2} \), then simplify by removing the concentration of liquid water to get \( K_w = [H_3O^+][OH^-] \).
Recognize that the correct equilibrium expression for the auto-ionization of water is \( K_w = [H_3O^+][OH^-] \), which matches the option showing the product of hydronium and hydroxide ion concentrations without water in the denominator.
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