Conjugate Acid-Base Pairs

Water as an Acid and Base

Water is amphoteric, meaning it can act as both an acid and a base. This is due to its ability to donate and accept protons ( $ H^+ $ ).

• Water as an acid: Water can donate a proton to a stronger base. $ H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq) $
  In this reaction, water acts as a Brønsted-Lowry acid, donating a proton to form the hydroxide ion ( $ OH^- $ ).

• Water as a base: Water can accept a proton from a stronger acid. $ H_2O(l) + H^+(aq) \rightleftharpoons H_3O^+(aq) $ Here, water acts as a Brønsted-Lowry base, accepting a proton to form the hydronium ion ( $ H_3O^+ $ ).

$$ K_w = [H^+]][OH^-]] = 1.0 \times 10^{-14} \text{ at 25}^\circ C $$

The ion product constant for water ( $ K_w $ ) describes the equilibrium constant for the autoionization of water. At 25°C, $ K_w $ is $ 1.0 \times 10^{-14} $ . This means that in pure water, $ [H^+]]=[OH^-]]=1.0 \times 10^{-7} $ M.

• pH and pOH:
  • $ pH = -\log[H^+]] $
  • $ pOH = -\log[OH^-]] $
  • $ pH + pOH = 14 $ (at 25°C)

Acids and Bases pH and pOH Calculations Acid-Base Equilibria

Further Notes:

• The value of $ K_w $ changes with temperature. At higher temperatures, $ K_w $ increases, indicating that the autoionization of water is endothermic.

• The autoionization of water is an important concept for understanding many acid-base reactions and calculations in aqueous solutions.