Are the following salts acidic, basic, or neutral? Explain your answer. Write chemical equations and show appropriate calculations. KBr NH4CN
The Correct Answer and Explanation is:
To determine whether a salt is acidic, basic, or neutral in aqueous solution, we analyze the properties of its cation and anion, especially their behavior as conjugate acids or bases of strong or weak acids/bases.
1. KBr (Potassium Bromide)
Step 1: Identify the ions
- K⁺ (potassium ion)
- Br⁻ (bromide ion)
Step 2: Analyze the ions
- K⁺ comes from KOH, a strong base → does not hydrolyze (no effect on pH).
- Br⁻ comes from HBr, a strong acid → does not hydrolyze (no effect on pH).
Conclusion: Neither ion reacts with water. The solution remains neutral.
Chemical equation in water:KBr (s)→K⁺ (aq)+Br⁻ (aq)\text{KBr (s)} \rightarrow \text{K⁺ (aq)} + \text{Br⁻ (aq)}KBr (s)→K⁺ (aq)+Br⁻ (aq)
Since neither ion affects pH:
KBr is a neutral salt.
2. NH₄CN (Ammonium Cyanide)
Step 1: Identify the ions
- NH₄⁺ (ammonium ion)
- CN⁻ (cyanide ion)
Step 2: Analyze the ions
- NH₄⁺ is the conjugate acid of NH₃, a weak base → acts as a weak acid, donates H⁺.
- CN⁻ is the conjugate base of HCN, a weak acid → acts as a weak base, accepts H⁺.
So, we must compare the strengths of NH₄⁺ (acid) and CN⁻ (base).
Given:
- pKa of NH₄⁺ ≈ 9.25
- pKa of HCN = 9.21, so Kb of CN⁻ can be calculated using:
pKb=14−pKa=14−9.21=4.79\text{pKb} = 14 – \text{pKa} = 14 – 9.21 = 4.79pKb=14−pKa=14−9.21=4.79
NH₄⁺ has a weaker acidity than CN⁻’s basicity (since CN⁻ has a lower pKb → stronger base than NH₄⁺ is an acid).
Conclusion: The base (CN⁻) is stronger than the acid (NH₄⁺), so the solution will be basic.
Chemical equations:
- NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ (acidic)
- CN⁻ + H₂O ⇌ HCN + OH⁻ (basic)
Net effect: More OH⁻ produced → NH₄CN is a basic salt.
Final Answers:
- KBr: Neutral
- NH₄CN: Basic
These conclusions are based on the conjugate acid-base behavior of the salt’s ions in water.
