K w, pH, and pOH

IONIZATION OF WATER Water is capable of reacting with itself in an ionization reaction H 2 O (l) + H 2 O (l) ⇌ H 3 O + (aq) + OH - (aq)

EQUILIBRIUM CONSTANT OF WATER H 2 O (l) + H 2 O (l) ⇌ H 3 O + (aq) + OH - (aq) Pure water has equal concentrations of H 3 O + and OH - [H 3 O + ] = [OH - ] = 1.0*10 -7 M

EQUILIBRIUM CONSTANT OF WATER H 2 O (l) + H 2 O (l) ⇌ H 3 O + (aq) + OH - (aq) As this is an equilibrium reaction, it has its own equilibrium constant. –We call this Kw – the equilibrium constant of water Kw = [H 3 O + ][OH - ] Kw = (1.0*10 -7 M)(1.0*10 -7 M) Kw = 1.0*10 -14

ADDING AN ACID HX (aq) + H 2 O (l) ⇌ H 3 O + (aq) + X - (aq) When an acid is placed in water, the [H 3 O + ] increases This affects the equilibrium of the water ionization reaction H 2 O (l) + H 2 O (l) ⇌ H 3 O + (aq) + OH - (aq) Equilibrium shifts to the left [OH - ] decreases

ADDING A BASE M (aq) + H 2 O (l) ⇌ MH + (aq) + OH - (aq) When a base is placed in water, the [OH - ] increases This affects the equilibrium of the water ionization reaction H 2 O (l) + H 2 O (l) ⇌ H 3 O + (aq) + OH - (aq) Equilibrium shifts to the left [H 3 O + ] decreases

USING KW – CALCULATING UP THE “OPPOSITE” ION. There will be times when you need to calculate up the [OH-] in an ACID or the [H 3 O+] in a BASE In an acidic or basic environment, Kw stays the same 1.0* For any acid or base you can calculate both [H 3 O + ] and [OH - ] –Acids First determine [H 3 O + ] then use K w to calculate [OH - ] –Bases First determine [OH - ] then use K w to calculate [H 3 O + ]

EXAMPLES 1.What is the [H 3 O + ] in a solution that has M Ca(OH) 2 ? 2.What is the [OH-] in a solution that has 0.050M HCl?

pH pH is a measurement of how much [H 3 O + ] is in the solution pH = -log [H 3 O + ] Substances with a pH < 7 are acidic Substances with a pH > 7 are basic Substances with a pH = are neutral Example A solution has 0.50M HCl. What is the pH of this solution?

pH and [H 3 O + ] If you know the pH, you can find the [H 3 O + ] [H 3 O + ] = 10 -pH Example We have a solution with a pH = 8.3. What is [H 3 O + ]?

pOH pOH is a measurement of how much [OH - ] is in the solution pOH = -log [OH - ] Example A solution has 0.25M NaOH. What is the pOH of this solution?

pOH and [OH - ] If you know the pOH, you can find the [OH - ] [OH - ] = 10 -pOH Example We have a solution with a pOH = 6.3 What is [OH - ]?

pH and pOH In any one solution, the following has to be true: pH + pOH = 14 This is because of our equilibrium in water H 2 O (l) + H 2 O (l) ⇌ H 3 O + (aq) + OH - (aq) As one concentration increases, the other lowers –As pH increases, poH decreases and vice versa