CHEM 108b
Recognizing Bronsted Acids and Bases
Strong Acids: HCl, HNO3, H2SO4 (first proton only), HBr, HI, HClO4 (perchloric acid), HClO3
(chloric acid)
Strong Bases: Group I hydroxides, Ca(OH)2, Sr(OH)2, Ba(OH)2
Weak acids: CH3COOH, H2CO3, HSO4-, HCO3-,
Carboxylic acid
Alcohol (usually weakly acidic)
(R is almost any group of atoms, usually with lots of carbons)
Weak bases: NH3, CN-, amines RNH2, R2NH (many important biological bases are amines)
O
R
C
O
+ HA
N
H
H
H
N
R
R
R
OH
R
C
+ H+
OH
+
N
H
H
H
+
H
+ HA
N
R
R
R
Chemical reactivity of Bronsted Acids and Bases
Rules:
1. strong acids and strong bases dissociate completely. Calculating their effect on pH is easy
–simply determine [H+] or [OH-] by determining moles of acid or base per L and recalling that
every strong acid and strong base will completely dissociate.
What will the pH of a solution be if 1 ml of 1 M HCl is added to 50 mL H2O?
What will the pH of a solution be if 2 g of NaOH is added to 50 mL of H2O? (assume total
volume is 50 mL).
2. Weak acids and weak bases dissociate or associate only partially. The degree to which they
dissociate (for acids) or associate (for bases) is determine by the equilibrium constant for their
characteristic acid or base reaction (Ka or Kb). We have done a lot of these problems in class.
Recall that the pKa of an acid represents the pH at which it will be half dissociated and half
associated.
3. Strong acids and strong bases when put together in solution strive to neutralize each other.
(H+ + OH-  H2O K = 1 x 1014). To determine the final pH when a strong acid and strong base
are added together, one must figure out if either H+ or OH- is limiting and then determine how
much OH- or H+ is left over after the neutralization has occurred and calculate the pH from that.
CHEM 108b
What will the pH be if 10 mL 1 M HCl is added to 10 mL 2 M NaOH(aq)?
What will the pH be if 10 ml 1 M HNO3 is added to 10 mL 1 M KOH(aq)?
What will the pH be if 10 ml 3 M HBr is added to 10 mL 1 M Ca(OH)2(aq)?
4. A weak acid (WA) and a strong base (SB) together in solution react to generate the conjugate
base of the weak acid. The final pH will depend on the relative amount of weak acid and strong
base. Three scenarios exist when [WA]>[SB], [WA] = [SB], [WA]<[SB].
5. A weak base (WB) and strong acid (SA) together in solution react to generate the conjugate
acid of the weak base. The final pH will depend on the relative amount of weak base and strong
acid. Three scenarios exist when [WB]>[SA], [WB] = [SA], [WB]<[SA].
6. A weak acid and the corresponding weak conjugate base together in solution generate a
buffer. Buffers are very important in the environment and biology because the weak acid and
conjugate weak base work together to BUFFER the effect on pH from the addition of acid or
base. The pH of a buffer solution is easy to determine.
pH = pKa + log ([A-]/[HA]) where [A-] = concentration of base and [HA] = concentration
of conjugate acid. pKa is the negative log of the acid dissociation constant.
The first scenario in item 4 and 5 above are also buffers because the strong base converts some
of the weak acid to its conjugate base (item 4) and the strong acid converts some of the weak
base to its conjugate acid (item 5).
On Wednesday, we will explore reactivity of acids and bases in more detail but you can look
over the relevant section in your textbook to prepare and try the problems given above.