Equilibrium Constant Calculations
Water Autoionization
Water undergoes autoionization, meaning it reacts with itself to a small extent to produce hydronium ( $ H_3O^+ $ ) and hydroxide ( $ OH^- $ ) ions.
$$ K_w = [H_3O^+]][OH^-]] $$
This is the equilibrium constant expression for the autoionization of water, where $ K_w $ is the ion-product constant for water. At 25°C, $ K_w = 1.0 \times 10^{-14} $ .
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Relationship between $ [H_3O^+]] $ and $ [OH^-]] $ : In pure water, $ [H_3O^+]] = [OH^-]] = 1.0 \times 10^{-7} $ M. This leads to a neutral solution.
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pH and pOH:
- $ pH = -\log_{10}[H_3O^+]] $
- $ pOH = -\log_{10}[OH^-]] $
- $ pH + pOH = 14 $ (at 25°C)
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Acidic, Basic, and Neutral Solutions:
- Acidic: $ [H_3O^+]] > [OH^-]] $ ; $ pH < 7 $
- Basic: $ [H_3O^+]] < [OH^-]] $ ; $ pH > 7 $
- Neutral: $ [H_3O^+]] = [OH^-]] $ ; $ pH = 7 $
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Effect of Temperature: $ K_w $ increases with increasing temperature. This means that at higher temperatures, the autoionization of water is more significant, and the concentration of both $ H_3O^+ $ and $ OH^- $ ions increases.