Properties and pH of Monoprotic Acids: Strong vs. Weak, Study notes of Chemistry

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MONOPROTIC ACIDS & BASES

CHEM 251 SDSU

ACIDS AND BASES

• (^) An acid is a proton donor - forms H 3 O

HA + H 2 O A

• H 3 O

• (^) A base is a proton acceptor - forms OH

B + H 2 O BH

• OH

• (^) Vinegar: CH 3 CO 2 H + H 2 O CH 3 CO 2 - + H 3 O +
• (^) Ethylamine: CH 3 CH 2 NH 2 +H 2 O CH 3 CH 2 NH 3 + + OH - 2

STRONG ACIDS AND BASES

Strong Acids:

HCl

HBr

HI

H 2 SO 4

HNO 3

HClO 4

Strong Bases:

LiOH

NaOH

KOH

RbOH

CsOH

R 4 NOH

“Not Weak” Bases:

Mg(OH) 2

Ca(OH) 2

Sr(OH) 2

Ba(OH) 2

Most acids/bases are considered weak - they do not

dissociate fully in water.

Only these select few dissociate fully and are strong

4

WEAK ACIDS AND BASES

• (^) All weak acids and bases have dissociation constants, strong acids and bases do not have tabulated values.
• (^) Weak acids/bases do not dissociate fully in solution - some HA or B remain.
• (^) The smaller the dissociation constant the weaker the acid/base - the closer the pH of the solution is to being 7. 5

KNOWING TO USE KA OR KB

• (^) Depending on what species you have in solution you may need to use Ka or Kb to determine the pH of a solution.
• (^) If the species you have produces H
  as a product you need to use Ka.

• (^) If the species you have produces OH

  as the product of the reaction you need to use Kb.

SAMPLE PROBLEM

What is the pH of a solution prepared by dissolving 0.458 g of aminobenzene into 200 mL of water? The following table provides p K a and K a values for selected weak acids. All values are from Martell, A. E.; Smith, R. M. Critical Stability Constants , Vols. 1–4. Plenum Press: New York, 1976. Unless otherwise stated, values are for 25 o C and zero ionic strength. Those values in brackets are considered less reliable. Weak acids are arranged alphabetically by the names of the neutral compounds from which they are derived. In some cases—such as acetic acid—the compound is the weak acid. In other cases—such as for the ammonium ion—the neutral compound is the conjugate base. Chemical formulas or structural formulas are shown for the fully protonated weak acid. Successive acid dissocation constants are provided for polyprotic weak acids; where there is ambiguity, the specific acidic proton is identified. To find the K b value for a conjugate weak base, recall that K a × K b = K w for a conjugate weak acid, HA, and its conjugate weak base, A–. Compound Conjugate Acid pKa Ka acetic acid CH 3 COOH 4.757 (^) 1.75× 10 – adipic acid HO OH O O

3.8× 10 – 3.8× 10

alanine +H 3 N^ CH^ C CH 3 OH O 2.348 (COOH) 9.867 (NH 3 ) 4.49× 10

1.36× 10

aminobenzene NH 3 +^ 4.601 (^) 2.51× 10 – 4-aminobenzene sulfonic acid NH 3 O + 3 S^ 3.232^ 5.86× 10

2-aminobenozic acid COOH NH 3 + 2.08 (COOH) 4.96 (NH 3 ) 8.3× 10

1.1× 10

OH