Chapter 9: Unit 5. Dissociation of Weak Acids and Bases
Dissociation of Weak Acids and Bases
The dissociation of mono, di and triprotic weak acids has introduced the acid dissociation constant K a . K a is the ratio of the concentrations of the products over the concentrations of reactants.
Acid Dissociation ExpressionKa
Phosphoric Acid Dissociation Constants at 25 o C:
H3PO4 (aq) + H2O(l)⇔ H3O + (aq) + H2PO4-(aq) K a1 = 7.5 x 10-3
H2PO 4-(aq) + H2O(l) ⇔ H3O + (aq) + HPO4 2-(aq) K a2 = 6.2 x 10-8
HPO4 2-(aq) + H2O(l) ⇔ H3O + (aq) + PO4 3-(aq) K a3 = 4.2 x 10
K a1 = 7.5 x 10-3 = {[H3O + (aq)] x [H2PO4-(aq)]} / [H3PO 4 (aq)]
K a2 = 6.2 x 10–8 = {[H3O + (aq)] x [HPO4 2–(aq)]} / [H2PO4–(aq)]
K a3 = 4.2 x 10–13 = {[H3O + (aq)] x [PO4 3–(aq)]} / [HPO4 2–(aq)]
[H3PO4 (aq)], [H2PO 4–(aq)], [HPO4 2–(aq)] and [PO4 3–(aq)] are molarities of the corresponding compounds and anions.
The general rule is: The large K ais, the Stronger the Acidis.
A table of Acids –Bases Strength and K aValuesis given below:
https://depts.washington.edu/eooptic/links/acidstrength.html
In this table
- a. Strong acids are listed at the top left hand corner of the table and have K a values > 1
- b. Acid with values less than one are considered weak.
- c. The strong bases are listed at the bottom right of the table and get weaker as we move to the top of the table.
Acids –Bases Strength and K aValues
| K a | Acid | Base | ||
| Name | Formula | Formula | Name | |
| Large | Perchloric acid | HClO 4 | ClO 4 – | Perchlorate ion |
| 3.2 * 10 9 | Hydroiodic acid | HI | I– | Iodide |
| 1.0 * 10 9 | Hydrobromic acid | HBr | Br– | Bromide |
| 1.3 * 10 6 | Hydrochloric acid | HCl | Cl– | Chloride |
| 1.0 * 10 3 | Sulfuric acid | H2SO 4 | HSO4 – | Hydrogen sulfate ion |
| 2.4 * 10 1 | Nitric acid | HNO3 | NO3 – | Nitrate ion |
| ——– | Hydronium ion | H3O+ | H2O | Water |
| 5.4 * 10–2 | Oxalic acid | HO2C2O2H | –HO 2 C 2 O 2 | Hydrogen oxalate ion |
| 1.3 * 10–2 | Sulfurous acid | H2SO3 | HSO3 – | Hydrogen sulfite ion |
| 1.0 * 10–2 | Hydrogen sulfate ion | HSO4 – | SO4 2– | Sulfate ion |
| 7.1 * 10–3 | Phosphoric acid | H3PO4 | H2PO 4 – | Dihydrogen phosphate ion |
| 7.2 * 10–4 | Nitrous acid | HNO2 | NO3 – | Nitrite ion |
| 6.6 * 10–4 | Hydrofluoric acid | HF | F – | Fluoride ion |
| 1.8 * 10–4 | Methanoic acid | HCO2H | HCO2 – | Methanoate ion |
| 6.3 * 10–5 | Benzoic acid | C6 H5COOH | C6 H5COO– | Benzoate ion |
| 5.4 * 10–5 | Hydrogen oxalate ion | HO2C2O2– | O2C2O2 2– | Oxalate ion |
| 1.8 * 10–5 | Ethanoic acid | CH3COOH | CH3COO | Ethanoate (acetate)ion |
| 4.4 * 10–7 | Carbonic acid | CO3 2– | HCO 3 – | Hydrogen carbonateion |
| 1.1 * 10–7 | Hydrosulfuric acid | H2S | HS– | Hydrogen sulfide ion |
| 6.3 * 10–8 | Dihydrogen phosphateion | H2PO 4 – | HPO4 2– | Hydrogen phosphateion |
| 6.2 * 10–8 | Hydrogen sulfite ion | HS– | S 2– | Sulfite ion |
| 2.9 * 10–8 | Hypochlorous acid | HClO | ClO– | Hypochlorite ion |
| 6.2 * 10–10 | Hydrocyanic acid | HCN | CN– | Cyanide ion |
| 5.8 * 10–10 | Ammonium ion | NH 4 + | NH 3 | Ammonia |
| 5.8 * 10–10 | Boric acid | H3 BO3 | H2BO 3 – | Dihydrogen carbonate ion |
| 4.7 * 10–11 | Hydrogen carbonateion | HCO3 – | CO3 2– | Carbonate ion |
| 4.2 * 10–13 | Hydrogen phosphate ion | HPO4 2– | PO4 3– | Phosphate ion |
| 1.8 * 10–13 | Dihydrogen borate ion | –H2BO3 | HBO3 2– | Hydrogen borate ion |
| 1.3 * 10–13 | Hydrogen sulfide ion | HS– | S2– | Sulfide ion |
| 1.6 * 10–14 | Hydrogen borate ion | HBO3 2– | BO3 3– | Borate ion |
| ——— | water | H2O | OH– | Hydroxide |
A table of Base Dissociation Constant K bvaluesis given below:
https://www.ck12.org/book/CK–12–Chemistry–Concepts–Intermediate/section/21.13/
In this table:
- a. Conjugate base of a weaker acid is a stronger base
- b. Conjugate base of a stronger acid is a weaker base
Base Dissociation Constant K bValues
A video of You Tube illustrates the dissociation constant expression calculations:
The dissociation of acids is illustrated in the video below: