Weak acids and bases II - Lecture 6

Question 1

What is a weak acid?

Answer 1

Undergoes incomplete dissocitation i.e it reaches an ⇌

Question 2

Where does ka usually lie for weak acids?

Answer 2

Ka is usually very small so it lies completely to the left

Question 3

Equation for a weak acid

Answer 3

HA(aq) + H2O(l) ⇌ H3O+ (aq) + A-

Question 4

Acid dissociation constant

Answer 4

Ka = ([H3O+]e [A-]e)/[HA]e

Question 5

pKa =

Answer 5

-log[Ka]

Question 6

What is a weak base

Answer 6

Incomplete dissociation into Conjugate acid and OH- ions i.e they also reach an ⇌

Question 7

Kb =

Answer 7

([BH+]e [OH-]e)/[B]e

Question 8

pKb =

Answer 8

-log[Kb]

Question 9

If pKa/pKb&raquo_space; OR Ka/Kb «

Answer 9

Then the acid/base is weak

Question 10

Relationship between Ka and Kb

Answer 10

[Ka][Kb] = Kw

pKa + pKb = pKw

Kw = 14

Question 11

Conjugate strengths

Answer 11

If pKa + pKb = pKw, then the stronger the weak acid (i.e the more it dissociates), the weaker the conjugate base as conc. of the [H3O+] and the [OH-] need to balance out.

Question 12

Generalisation made with water

Answer 12

Water doesn’t dissociate because a lot of the H3O+ ions come from the weak acid.

Question 13

Other generalisation

Answer 13

If acid is very weak i.e if Ka is very small, it doesn’t dissociate a lot. So we can make the assumption that the initial amount + conc. of the weak acid = the final amount of the weak weak acid.

Question 14

Equation for calculating the pH of a weak acid

Answer 14

[H3O+] = √Ka x [HA] initial

Question 15

Transition metal compounds

Answer 15

Transition metal complex called ligand (lewis base) bonds to a transition metal cation (lewis acid). When ligand is H2O, the resulting complex is a weak acid.

*It forms genuine cov bonds with less dynamic i.e slow exchange of H2O molecules