4.10 Relative Strengths of Acids and Bases Chemistry 12

4.10 In this lesson we will look at the relative strengths of some weak acids and use their Ka values to explain how they would behave when they are combined.

4.10 E.g. 1a) Write the Bronsted-Lowry acid base equilibria for the following reactants. H2PO4- + CO32- ↔ HPO4 2- + HCO3- amphiprotic Can only act as a base

4.10 E.g. 1b) What side is favoured? H2PO4- + CO32- ↔ HPO4 2- + HCO3- Larger Ka value means that H2PO4- is better at producing H+ products are favored Ka = 6.2 x10-8 Ka = 5.6 x10-11

CO32- is the stronger base 4.10 E.g. 1c) What is the stronger base? H2PO4- + CO32- ↔ HPO4 2- + HCO3- A stronger acid is ALWAYS on the same side as the stronger base CO32- is the stronger base

4.10 E.g. 2 a) Using 2 amphiprotic reactants HSO4- + H2PO4- ↔ HINT: Use your B-L table, as the reactant with the larger Ka will behave like an acid. SO42- + H3PO4

4.10 E.g. 2b) What side is favored? HSO4- + H2PO4- ↔ SO42- + H3PO4 the products are favoured Ka = 1.2 x10-2 Ka = 7.5 x10-3

4.10 E.g. 2c) Stronger base? HSO4- + H2PO4- ↔ SO42- + H3PO4 Stronger acid  Stronger base

4.10 E.g. 3) What is the Bronsted-Lowry acid base equilibria that results from the following reactants? NaHSO4 + NaH2PO4 NaHSO4  Na+ + HSO4- NaH2PO4  Na+ + H2PO4- HSO4- + H2PO4- ↔ SO42- + H3PO4 Both reactants dissociate and Na+ is a spectator These are our reactants You should notice that this is really the same question as example 2.

Learning Check When given 2 weak acids can you determine which one will act as the acid and which one will act as the base? Can you identify whether the equilibrium favours the reactants or the products? Can you do the above with amphiprotic substances? Salts?

4.10 Read & Highlight Self Notes (p. 6 – 12) Problems Hebden # 38-46