Ka, Kb Chapter 8.2

1. Ka, KbChapter 8.2

2. Review • Strong acids are acids that are completely or nearly 100% ionized in their solutions. • Ionization of a strong acid HA can be represented by: HA  H+ + A- • For a strong acid, [H+] = [A-] = concentration of acid if x is much greater than 1x10-7 • Strong bases are completely ionized in solution. • Relative to strong acids, there are fewer number of strong bases. Most strong bases are alkali hydroxides.

3. List of Strong Acids & Strong Bases

4. Ka – Acid Ionization constant • HA(aq) + H2O(l)  H3O+(aq) + A–(aq) • Ka = [H3O+][A–] / [HA] • Notice that H2O is ignored because it is liquid • HA cannot be ignored because it is aqueous • This is different than with Ksp. In Ksp, solids could only be in solution as ions • Generally Ka tells you about acid strength • Strong acids have high Ka values

5. Ka – Acid Ionization constant • A “weak” acid is an acid that doesn’t ionize completely. E.g. HF + H2O  H3O+ + F– • Note: don’t get confused between strength and concentration. 1 M HCN has a smaller [H+], thus a higher pH, than 0.001 M HCl • In general: Ka < 10 – 3 Weak acid 10 – 3 < Ka < 1 Moderate acid Ka > 1 Strong acid • Percent Ionization (p) of weak acid is the % ratio of the acid ionized [H +] to the concentration of acid solute [HA] or p = [H +]/[HA] x100 • Answer Q# 1 & 2 on page 554

6. List of Weak Acids and Weak Bases Weak ACIDS Weak BASES • CH3COOH     acetic acid • HCOOH         formic acid • HF                  hydrofluoric acid • HCN               hydrocyanic acid • HNO2             nitrous acid • HSO4-              hydrogen sulfate ion • NH3                ammonia • CH3NH2         methylamine • C5H5N            pyridine

7. Dissociation vs. Ionization • Ionization and dissociation indicate ions form • Dissociation: ions form when a chemical comes apart. E.g. NaCl melts to form Na+, Cl– • Ionization: ions form when two chemicals react. E.g. HCl(aq)+H2OH3O+(aq)+Cl–(aq) • Even though we write HCl  H+ + Cl– , this is just an abbreviation. In reality HCl reacts with H2O, thus it is an ionization not a dissociation • Note that NaCl can also dissociate in water. This is not an ionization, since water is only required to stabilize ions (it is not needed as a reactant involved in forming ions)

8. Kb – the last K (I promise) • Kb is called the base ionization constant • Kb is similar to Ka except b stands for base • The general reaction involving a base can be written as B(aq) + H2O  BH+(aq) + OH–(aq) • Thus Kb = [BH+] [OH–] / [B] • Higher Kb indicates stronger base. • Try Q# 9 (a-d), 10, 11 and 13 on page 579 of the textbook • Relationship between Ka, Kb and Kw: • Kw = Ka x Kb

9. Kw = Ka x Kb • This equation allows us to convert the Ka values of acids into the Kb values of their conjugate bases and vice versa at SATP. • Refer to table 4 on page 561 • Also read the BLUE section on page 562 • Answer Q# 6 (a – c) on page 563

10. Calculating the pH of a solution of a weak Monoprotic acid, HA(aq) given the value of Ka • Read the steps given on page 568 of the textbook • Look at examples given on page 563 -567 • Practice Q# 7 and 8 on page 568 • Practice Q# 9, 10 on page 570

11. Calculating the pH of a solution of a weak base, given the value of Kb • Read the steps given on page 574 of the textbook • Look at examples given on page 571 - 573 • Practice Q# 12 and 13 on page 574 Calculations involving a Polyprotic Acid • They ionize in a stepwise fashion, releasing ONE proton at a time • Each step has its own Ka value • Usually Ka1> Ka2> Ka3……