2.13
Solubility Equilibria: Ionic Product of Water
Pure water is a weak electrolyte; only a small amount ionizes into hydrogen and hydroxide ions. At any given temperature, the concentration of undisso…
Water is a weak electrolyte and undergoes a small amount of self-ionization.
At any given temperature, the concentration of undissociated water is considered constant, so the ionic product of water Kw is the product of the concentrations of hydrogen and hydroxide ions.
Kw increases with increasing temperature. At 25 °C, the Kw of pure water is 10−14.
Under these conditions, the hydrogen and hydroxide ion concentrations of pure water are equal, so the concentration of individual ions equals the square root of Kw.
A neutral solution consists of equal concentrations of these ions.
Suppose the hydrogen ion concentration is greater than 10−7. In that case, the solution is considered acidic, whereas it is alkaline or basic below this value.
So, the solution is basic if the hydroxide ion concentration is more than 10−7, whereas it is acidic below this value.
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Q1: What is the ionic product of water and why does it matter?
The ionic product of water, denoted Kw, is the product of hydrogen and hydroxide ion concentrations in aqueous solutions. Water is a weak electrolyte that undergoes self-ionization, producing these ions in equilibrium. At 25°C, Kw equals 10−14, providing a fundamental constant for determining solution acidity or basicity and understanding chemical equilibria in aqueous systems.
Q2: How does temperature affect the ionic product of water?
The ionic product of water increases with increasing temperature because higher thermal energy promotes water self-ionization. At 25°C, Kw is 1.0 × 10−14, but this value changes at different temperatures. This temperature dependence is critical when performing analytical work outside standard conditions, as it directly affects hydrogen and hydroxide ion concentrations and solution pH.
Q3: How do you calculate individual ion concentrations in pure water?
In pure water at 25°C, hydrogen and hydroxide ion concentrations are equal. Since their product equals Kw (10−14), each ion concentration is the square root of Kw, which equals 10−7 M. This equal concentration defines a neutral solution. In acidic or basic solutions, the concentrations differ, but their product still equals Kw, allowing you to calculate one ion concentration from the other.
Q4: What determines whether a solution is acidic, basic, or neutral?
Solution nature depends on hydrogen and hydroxide ion concentrations relative to 10−7 M at 25°C. A neutral solution has equal concentrations of both ions at 10−7 M. If hydrogen ion concentration exceeds 10−7 M, the solution is acidic; if hydroxide ion concentration exceeds 10−7 M, the solution is basic. These thresholds directly relate to the ionic product of water and enable classification of aqueous solutions.
Q5: How does Le Chatelier's principle apply to water ionization equilibrium?
Per Le Chatelier's principle, if the product of hydrogen and hydroxide ion concentrations exceeds Kw, excess ions combine to form water molecules until equilibrium is restored. Conversely, if the product falls below Kw, water ionizes to produce more ions. This self-correcting mechanism maintains the ionic product of water constant at a given temperature, ensuring equilibrium in aqueous solutions.
Q6: Why is water considered a weak electrolyte?
Water is a weak electrolyte because only a small amount of water molecules ionize into hydrogen and hydroxide ions at any given temperature. The concentration of undissociated water remains nearly constant, allowing Kw to be expressed simply as the product of ion concentrations. This limited ionization distinguishes water from strong electrolytes, which completely dissociate in solution.
Q7: How does Kw differ between acidic, basic, and neutral solutions?
Kw remains constant at a given temperature across all aqueous solutions, including acidic, basic, and neutral ones. In neutral solutions, hydrogen and hydroxide concentrations are equal at 10−7 M. In acidic or basic solutions, these concentrations are unequal, but their product still equals Kw. This constant product allows determination of one ion concentration from the other in any solution.