Acids and Bases: Salts

1. Acids and Bases: Salts Determining whether Salt Solutions are Acids or Bases

2. Learning Goals • Students will be able: • to determine whether salts form acids or bases when dissolved into solutions • to determine the pH of salt solutions

3. Success Criteria • Students will display their understanding by: • using a problem solving method to determine whether salts are acids or bases in solution. • using ICE tables and Kaor Kb values to determine [H+] or [OH-] and therefore pH or pOH • knowing that the strength of an acid varies inversely with the strength of its conjugate base.

4. Pre-requisite Knowledge • Student must already understand: • Kw = KaKb • pH = -log[H+]; [H+] = 10-pH; pOH= -log[OH-]; [OH-] = 10-pOH • ICE tables, solving for concentrations given Ka (using the very small K method)

5. We know that compounds such as HCl, H2SO4, HC2H3O2 are acids from the H in their formulas, similarly NaOH, Ca(OH)2 and KOH are bases from the hydroxide in their formulas. • How do we know the acidity or alkalinity of salts (ionic compounds) with formulas that have neither H or OH such as NaCl, CaCO3, NH4Cl?

6. Laying the foundation • We can use the knowledge of acids and their conjugate bases (and bases and their conjugate acids) • The diagram shows that as the strength of an acid increases, the strength of its conjugate base decreases. • This makes sense because strong acids release large amounts of H+ (H3O+) into solution and as a result their conjugate bases must release very little OH-.

7. Acids & Conjugate Bases Look at page 803 to determine whether an acid is strong or weak • HCl(aq) + H2O(aq) ↔ Cl-(aq) + H3O+(aq) • HC2H3O2(aq) + H2O(aq) ↔ C2H3O2 -(aq) + H3O+(aq) • HF(aq) + H2O(aq) ↔ F-(aq) + H3O+(aq) strong acid very weak conjugate base weak conjugate base weak acid ? conjugate base ? acid

8. Bases & Conjugate Acids • NaOH(aq) ↔ Na+(aq) + OH-(aq) • NH3(aq) + H2O(aq) ↔ NH4 +(aq) + OH-(aq) • N2H4(aq) + H2O(aq) ↔ N2H5+(aq) + OH-(aq) very weak conjugate acid strong base weak base weak conjugate acid ? conjugate acid ? base

9. Let’s Look at a Salt • NaFNaF(s) ↔ Na+(aq) + F-(aq) • Na+(aq) + F-(aq) • since a weak acid is stronger than a very weak base, NaF forms an acidic solution. remember: NaOH(aq) ↔ Na+(aq) + OH-(aq) and: HF(aq) + H2O(aq) ↔ F-(aq) + H3O+(aq) conjugate acid of a strong base conjugate base of a weak acid very weak base very weak acid

10. Try this one! • KNO3KNO3(s) ↔ K+(aq) + NO3-(aq) • K+(aq) + NO3-(aq) • CONCLUSION: think: KOH(aq) ↔ K+(aq) + OH-(aq) and: HNO3 (aq)+ H2O(aq) ↔ NO3-(aq)+ H3O+(aq) conjugate acid of a _____ base conjugate base of a _____ acid ________ acid _________ base

11. Try this one! • NH4Br NH4Br(s) ↔ • CONCLUSION: think: ↔ and: ↔ conjugate acid of a _____ base conjugate base of a _____ acid ________ acid _________ base

12. What if both the conjugate acid and conjugate base form weak solutons • NH4NO2NH4NO2(s) ↔ NH4+(aq) + NO2-(aq) • NH4+(aq) + NO2-(aq) • PROBLEM: Is this neutral? NO! We have to determine which is stronger- the weak acid or the weak base? To do this we must look at the Ka and Kb values of NH4+(aq) + NO2-(aq). remember: NH3(aq)+ H2O(aq)↔ NH4+(aq) + OH-(aq) HNO2(aq) + H2O(aq) ↔ NO2-(aq) + H3O+(aq) conjugate acid of a weak base conjugate base of a weak acid weak acid weak base

13. Comparing Kaand Kb The Ka for NH4+(aq)cannot be found in the Ka chart on page 803, BUT we have the Kb value for NH3(aq). Remember, Kw = KaKb, therefore Ka = Kw/Kb. After substituting the values: • Ka = 1.0 x 10-14/1.8 x 10-5 = 5.6 x 10-10 The Kbfor NO2-(aq)cannot be found in the Kbchart on page 803, BUT we have the Kb value for HNO3(aq). Remember, Kw = KaKb, therefore Kb= Kw/Ka. After substituting the values: • Kb= 1.0 x 10-14/7.2 x 10-4= 1.4 x 10-11 • Compare the Ka and Kb values for the ions. • acidic if Ka > Kb • basic if Ka < Kb • neutral if Ka = Kb • Since Ka > Kb, NH4NO2(s)will form a weak acid in solution

14. Small, highly charged metal cations • Metallic ions with small atomic radii and high ionic charges often react with water to form acidic solutions. • For example, an aluminum salt follows this 3-step process to produce hydrogen ions in solution: • AlCl3(aq) ↔ Al3+(aq) + 3 Cl-(aq) • Al3+(aq) + 6 H2O(l)↔ Al(H2O)63+(aq) • Al(H2O)63+(aq) ↔ Al(H2O)5(OH)2+(aq) + H+(aq)

15. The effect of metallic ions on the acidity of water • Note that the number of waters in the hydrate complex is double the charge. • Chromium (III) nitrate follows this 3-step process to produce hydrogen ions in solution: • Cr(NO3)3(aq) ↔ Cr3+(aq) + 3 NO3-(aq) • Cr3+(aq) + 6 H2O(l)↔ Cr(H2O)63+(aq) • Cr(H2O)63+(aq) ↔ Cr(H2O)5(OH)2+(aq) + H+(aq)

16. Metal Oxides • Metal oxides such as CuO, CaO, PbO, and Fe2O3 will react with H2O to form basic solutions. • CuO + H2O → Cu2+ + 2 OH- • Copper (II) oxide reacts to form a basic solution. • This process often occurs in mining areas where metal ores and metal-laden mine tailings react with groundwater to form toxic alkaline run-off.

17. Non-metal oxides • Non-metal oxides such as SO2, CO2, N2O will react with H2O to form acids. • We often see this occur in our lower atmosphere. The burning of fossils fuels releases a lot of SO2 into the atmosphere where it combines with water in the air (clouds) to form acids. These acids rain down upon us as acid rain

18. Salts: Determining Acid-Base

19. Salts: Determining Acid-Base

20. Practice Questions Problems: Identifying Salt Solutions as Acid, Neutral or Basic: • KCl • K2CO3 • NH4NO3 • NH4NO2 • Na2SO3 • N2H5Cl • AlCl3 • N2O • NaClO4 • AlF3 • NaC2H3O2 • PbO