Acids and Bases in Water


Self-ionization of Water

Liquid water is in equilibrium with solvated hydroxide and solvated protons. We typically use a value related to the equilibrium constant to define the concentration of these solvated ions in water. The water dissociation constant, Kw, is 1 x 10-14.



pH and pOH are important values for acidic and basic solutions. These are the log base 10 of the hydrogen ion concentration or of the hydroxide ion concentration, respectively.

pH = -log[H+] pOH = -log[HO-] 14 = pH + pOH


LeChateliers principle tells us that the addition of one of the products to a system at equilibrium will shift the equilibrium towards reactants. Acids and bases dissolve in water and, because they increase the concentration of one of the products of water self-ionization, either protons or hydroxide ions, they suppress water dissociation.

For any acid, Ka is the equilibrium constant for the acid dissociation reaction in water. See the tables page for a list of Ka values at room temperature. Note that both A- and H+ are surrounded and stabilized by water molecules. We could also write these as [A(H2O)x]-, [H(H2O)x]+ or A(aq)-, H(aq)+.



pH and pOH are frequently used to describe solutions of acids and bases in water. In pure water, the concentration of solvated protons equals the concentration of solvated hydroxide anions and the pH is 7. Acidic solutions have a lower pH while basic solutions have a higher one.




Strong Acids

Strong acids dissociate completely in aqueous solution and have negative values for Ka. We can assume that the [H+] in a solution of a strong acid is equal to the initial concentration of the acid. Strong acids are listed below.




Weak Acids

Weak acids are only partially dissociated in aqueous solution. The Ka values describe the equilibrium and the pKa = -log(Ka). Some of the weak acids are in the table below.

Acid Reaction pKa
hydrofluoric acid HF H+ + F- 3.17
carbonic acid H2CO3 H+ + HCO3- 6.37
bicarbonate HCO3- H+ + CO3-2 10.25
bisulfate HSO4- H+ + SO4-2 1.99
ammonium NH4+ H+ + NH3 9.24
hydrogen sulfide H2S H+ + HS- 7.0
water H2O H+ + HO- 15.74
ammonia NH3 H+ + NH2- 38



Strong Bases

Strong hydroxide bases are completely dissociated in aqueous solution. Other strong bases react completely with water. Strong bases include NaOH, LiOH, Ca(OH)2, NaNH2.





Weak Bases

Weak bases are in equilibrium with water. The Kb values describe the equilibrium and the pKb = -log(Kb).



Some weak bases are in the table below.

Base Reaction pKb
NH3 NH3 + H2O NH4+ + HO- 4.76
HS- HS- + H2O H2S + HO- 7
CO3-2 CO3-2 + H2O HCO3- + HO- 3.75
SO4-2 SO4-2 + H2O HSO3-2 + HO- 12.01



Buffers

A solution containing a weak acid and its conjugate base resists changes in pH if small amounts of acid or base are added. A solution that contains equal concentrations of both the weak acid and the conjugate base has a pH equal to the pKa of the acid. For example, the pH of a solution that is 0.1 M in HSO4-2 and 0.1 M in SO4-2 will be about 2.




Professor Patricia Shapley, University of Illinois, 2012