Ionization Constant of Water Chapter 12.2.2

Kw Electrolytes, such as acids and bases, dissociate in water to produce ions. These ions interact with water and equilibrium is established between the aqueous ions and the water molecules. Once the equilibrium is established, the pH (or acidity) of the solution can be determined. The equilibrium can be quantified using the equilibrium constant, Kw.

pH acidic solutions have a pH less than 7 neutral solutions have a pH equal to 7 basic solutions have a pH greater than 7

pH The pH, which stands for hydrogen potential, is a measure of the hydronium ions, H3O+, in solution (in mol/L). pH = - log [H3O+] or [H3O+] = 10-pH

pOH The pOH (or the potential of the solution to generate hydroxide ions) can also be measured. pOH = - log [OH-] or [OH-] = 10-pOH

Ionization of Water The ionization of water occurs when occurs when water molecules collide with one another and hydronium ions, H3O+, and hydroxide ions, OH-, are formed. Pure water has a pH of 7 when it is measured at 25C. This pH expression given earlier tells us that the concentration of hydronium ions is 10-7 mol/L. Since the pH is neutral, there must be an equivalent concentration of hydroxide ions in solution; therefore the concentration of hydroxide ions is also 10-7 mol/L.

2 H2O (l)  H3O+(aq) + OH-(aq) Ionization of Water The ionization of water is a reversible process that proceeds according to the following reaction, 2 H2O (l)  H3O+(aq) + OH-(aq) What would the equilibrium constant for water be? Kwater = [H3O+][OH-]

Ionization Constant of Water The ionization constant of water is dependent on temperature, just like any other equilibrium constant. At 25C, the ionization constant of water is equal to 10-14. If temperature increases, then Kw increases as well; this means that the ionization of water is endothermic. We can use the following expression at 25C, [H3O+][OH-] = 10-14 The pH and pOH values can be determined if we know one of the two. pH + pOH = 14

pH and [H3O+] and [OH-] Remember, according to Arrhenius: An acid is a substance that produces H+ ions when in solution; therefore the concentration of H3O+ ions increases, causing a decrease in pH. A base is a substance that produces OH- when in solution; therefore the concentration of OH- increases and thus the pH increases. The concentration of hydronium and hydroxide ions can be determined if the pH or pOH is known.