Acid-Base Equilibrium According to Arrhenius theory, substances were acids if they contained hydrogen ion (H + ), and bases if they contained hydroxide ion (OH - ). However, some substances, like the base ammonia (NH 3 ), could not be classified using this definition. Bronsted-Lowry theory defined acids as proton donors (give away an H + ), and bases as proton acceptors (gain an H + ).

Example 1: HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) donates acceptsconjugate partners proton protonacidbase acid base Example 2: NH 3(aq) + H 2 O (l)  NH 4 + (aq) + OH - (aq) accepts donatesconjugate partners proton protonacidbase base acid

Water can behave as either an acid or a base depending on the reaction it is in. It is called amphoteric. Example 3: H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) donates acceptsconjugate partners proton protonacidbase acid base

Example 4: HCO 3 - (aq) + H 2 O (l)  H 2 CO 3(aq) + OH - (aq) accepts donatesconjugate partners proton protonacid base base acid H 2 CO 3(aq) is amphiprotic (can donate more than 1 proton) and also amphoteric.

Equilibrium Constants (for acids, bases and water) An equilibrium expression and constant can be determined for acids and bases. Given the example of water acting as both an acid and as a base, the equilibrium constant for water can be determined. H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) K = [products]or K w = [H 3 O + (aq) ][OH - (aq) ] [reactants] = 1.0 X

pH Scale (power of hydrogen ion) The pH scale is a measure of the acidity of a solution. It covers a range of values from 0-14 (based on the K w value). If [H 3 O + ] >[OH - ] then solution is acidic, pH<7 If [H 3 O + ] 7 If [H 3 O + ]=[OH - ] then solution is neutral, pH=7

To calculate pH or acid concentration use: pH = -log[H 3 O + ][H 3 O + ]= 10 -pH Similar formulas are used to calculate pOH and base concentration: pOH = -log[OH - ][OH - ]= 10 -pOH The relationship between pH and pOH is: pH + pOH = 14

The only numbers that are counted as significant digits in pH and pOH values are the numbers that are after the decimal place. Example: pH = 5.24  2 sig digs pOH =  4 sig digs pH and pOH values can be calculated directly from the concentrations of strong acids and bases only (any substance with a large K a or K b value)