Determining whether Salt Solutions are Acids or Bases

Learning Goals Students will be able: 1) to determine whether salts form acids or bases when dissolved into solutions 2) to determine the pH of salt solutions

Success Criteria Students will display their understanding by: 1) using a problem solving method to determine whether salts are acids or bases in solution. 2) using ICE tables and Ka or Kb values to determine [H + ] or [OH - ] and therefore pH or pOH 3) knowing that the strength of an acid varies inversely with the strength of its conjugate base.

Pre-requisite Knowledge Student must already understand: 1) Kw = K a K b 2) pH = -log[H + ]; [H + ] = 10 -pH ; pOH = -log[OH - ]; [OH - ] = 10 -pOH 3) ICE tables, solving for concentrations given K a (using the very small K method)

We know that compounds such as HCl, H 2 SO 4, HC 2 H 3 O 2 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, CaCO 3, NH 4 Cl?

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 + (H 3 O + ) into solution and as a result their conjugate bases must release very little OH -.

Acids & Conjugate Bases HCl (aq) + H 2 O (aq) ↔ Cl - (aq) + H 3 O + (aq) HC 2 H 3 O 2(aq) + H 2 O (aq) ↔ C 2 H 3 O 2 - (aq) + H 3 O + (aq) HF (aq) + H 2 O (aq) ↔ F - (aq) + H 3 O + (aq) strong acidvery weak conjugate base weak acid ? acid ? conjugate base weak conjugate base Look at page 803 to determine whether an acid is strong or weak

Bases & Conjugate Acids NaOH (aq) ↔ Na + (aq) + OH - (aq) NH 3(aq) + H 2 O (aq) ↔ NH 4 + (aq) + OH - (aq) N 2 H 4(aq) + H 2 O (aq) ↔ N 2 H 5 + (aq) + OH - (aq) very weak conjugate acid ? base weak conjugate acid ? conjugate acid strong base weak base

Let’s Look at a Salt NaF NaF (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) + H 2 O (aq) ↔ F - (aq) + H 3 O + (aq) conjugate acid of a strong base conjugate base of a weak acid very weak base very weak acid

Try this one! KNO 3 KNO 3(s) ↔ K + (aq) + NO 3 - (aq) K + (aq) + NO 3 - (aq) CONCLUSION: think: KOH (aq) ↔ K + (aq) + OH - (aq) and: HNO 3 (aq) + H 2 O (aq) ↔ NO 3 - (aq) + H 3 O + (aq) conjugate acid of a _____ base conjugate base of a _____ acid ________ acid _________ base

Try this one! NH 4 Br NH 4 Br (s) ↔ CONCLUSION: think: ↔ and: ↔ conjugate acid of a _____ base conjugate base of a _____ acid ________ acid _________ base

What if both the conjugate acid and conjugate base form weak solutons NH 4 NO 2 NH 4 NO 2(s) ↔ NH 4 + (aq) + NO 2 - (aq) NH 4 + (aq) + NO 2 - (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 K a and K b values of NH 4 + (aq) + NO 2 - (aq). remember: NH 3(aq) + H 2 O (aq) ↔ NH 4 + (aq) + OH - (aq) HNO 2(aq) + H 2 O (aq) ↔ NO 2 - (aq) + H 3 O + (aq) conjugate acid of a weak base conjugate base of a weak acid weak acid weak base

Comparing K a and K b The K a for NH 4 + (aq) cannot be found in the K a chart on page 803, BUT we have the K b value for NH 3(aq). Remember, K w = K a K b, therefore K a = K w /K b. After substituting the values: K a = 1.0 x /1.8 x = 5.6 x The K b for NO 2 - (aq) cannot be found in the K b chart on page 803, BUT we have the K b value for HNO 3(aq). Remember, K w = K a K b, therefore K b = K w /K a. After substituting the values: K b = 1.0 x /7.2 x = 1.4 x Compare the K a and K b values for the ions. acidic if K a > K b basic if K a < K b neutral if K a = K b Since K a > K b, NH 4 NO 2(s) will form a weak acid in solution

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: AlCl 3(aq) ↔ Al 3+ (aq) + 3 Cl - (aq) Al 3+ (aq) + 6 H 2 O (l) ↔ Al(H 2 O) 6 3+ (aq) Al(H 2 O) 6 3+ (aq) ↔ Al(H 2 O) 5 (OH) 2+ (aq) + H + (aq)

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(NO 3 ) 3(aq) ↔ Cr 3+ (aq) + 3 NO 3 - (aq) Cr 3+ (aq) + 6 H 2 O (l) ↔ Cr(H 2 O) 6 3+ (aq) Cr(H 2 O) 6 3+ (aq) ↔ Cr(H 2 O) 5 (OH) 2+ (aq) + H + (aq)

Metal Oxides Metal oxides such as CuO, CaO, PbO, and Fe 2 O 3 will react with H 2 O to form basic solutions. CuO + H 2 O → Cu 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.

Non-metal oxides Non-metal oxides such as SO 2, CO 2, N 2 O will react with H 2 O to form acids. We often see this occur in our lower atmosphere. The burning of fossils fuels releases a lot of SO 2 into the atmosphere where it combines with water in the air (clouds) to form acids. These acids rain down upon us as acid rain

Salts: Determining Acid-Base Type of SaltpH of Solutionexampleexplanation 1 Cations associated with strong bases Anions associated with strong acids Neutral NaCl KNO 3 K 2 SO 4 NaOH → Na + + OH - Na + is the very weak conjugate acid of a strong base HCl → H + + Cl - Cl - is the very weak conjugate base of a strong acid 2 Cations associated with weak bases Anions associated with strong acids The conjugate acid of a weak base is itself a weak acid. Acid NH 4 Cl NH 4 NO 3 NH 3 + H 2 O → NH OH - NH 4 + is the weak conjugate acid of a weak base HCl → H + + Cl - Cl - is the very weak conjugate base of a strong acid 3 Cations associated with strong bases Anions associated with weak acids The conjugate base of a weak acid is itself a weak base. Base NaC 2 H 3 O 2 KNO 2 NaOH → Na + + OH - Na + is the very weak conjugate acid of a strong base HC 2 H 3 O 2 → H + + C 2 H 3 O 2 - C 2 H 3 O 2 - is the weak conjugate base of a weak acid 4 Anions associated with weak acids and cations associated with weak bases are acidic. The acid–base nature of the salt can be determined by comparing the relative values of K a and K b for the ions. In this situation, normally the cation is acidic and the anion is basic. Acid or BaseNH 4 C 2 H 3 O 2 NH 4 F NH 4 C 2 H 3 O 2 → NH C 2 H 3 O 2 - NH 3 + H 2 O → NH OH - NH 4 + is the weak conjugate acid of a weak base HC 2 H 3 O 2 → H + + C 2 H 3 O 2 - C 2 H 3 O 2 - is the weak conjugate base of a weak acid Compare the K a and K b values for the ions. acidic if K a > K b basic if K a < K b neutral if K a = K b In this case, K a for NH 4 + is 5.8 x and the K b for C 2 H 3 O 2 - is 5.6 x , so K a > K b the solution is acidic

Salts: Determining Acid-Base 5 Cations are small, highly charged (+2 or +3) metal cations Anions associated with strong acids highly charged (+2 or +3) metal cations are hydrated in water and act as acidic cations. Acid AlCl 3 CuSO 4 AlCl 3 → Al Cl - Al H 2 O → Al(H 2 O) 6 3+ Al(H 2 O) 6 3+ → Al(H 2 O) 5 (OH) 2+ + H + and a weak acid forms HCl → H + + Cl - Cl - is the very weak conjugate base of a strong acid 6Metal OxidesBasic CuO Fe 2 O 3 CuO + H 2 O → Cu OH - Copper (II) oxide reacts to form a basic solution. 7Non-metal OxidesAcidicSO 2 CO 2 H 2 O + SO 2 → H 2 SO 3 Sulfur dioxide gas reacts with water to form sulfurous acid

Practice Questions 1) KCl 2) K 2 CO 3 3) NH 4 NO 3 4) NH 4 NO 2 5) Na 2 SO 3 6) N 2 H 5 Cl 7) AlCl 3 8) N 2 O 9) NaClO 4 10) AlF 3 11) NaC 2 H 3 O 2 12) PbO Problems: Identifying Salt Solutions as Acid, Neutral or Basic: