Acid-Base Properties of Ionic Compounds and Identifying Ions

Acid-Base Properties of Ionic Compounds

Formation of Salts

When an acid neutralizes a base, an ionic compound called a salt is formed. The resulting solution can be neutral, acidic, or basic, depending on the acid-base properties of the cations and anions produced.

• General Reaction:

$\text{HCl} + \text{NaOH} \rightarrow \text{H}_2\text{O} + \text{NaCl}$ $\text{Na}^+ + \text{Cl}^-$

• Key Point: The nature of the salt solution (acidic, basic, or neutral) depends on the acid-base properties of the ions.

Rules for Identifying Your Ions

Cations (Positive Ions)

Cations can be classified as acidic, basic, or neutral based on their charge and type.

1. Transition Metals

• If the transition metal has a charge of +2 or higher, it is acidic.

• If the charge is less than +2, it is neutral.

• Examples:

  • $\text{Zn}^{2+}$, $\text{ZnCl}_2$ — acidic

  • $\text{AgBr}$ — Ag+ is neutral

2. Main-Group Metals

• If the main-group metal has a charge of +3 or higher, it is acidic.

• If the charge is less than +3, it is neutral.

• Examples:

  • $\text{Al}^{3+}$, $\text{Al}_2(\text{SO}_4)_3$ — acidic

  • $\text{Ba}^{2+}$, $\text{BaSO}_4$ — neutral

3. Positive Amines

• Ammonium ($\text{NH}_4^+$) and other amines are acidic.

• Examples:

  • $\text{NH}_4^+$, $\text{CH}_3\text{NH}_3^+$ — acidic

  • $\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$ — neutral

Anions (Negative Ions)

Anions are classified based on their ability to accept protons (H+).

1. Negative Ions

• If you have a negative ion, then add an H+ to it. If you create a weak acid, your negative ion is basic.

• If you create a weak base, your negative ion is acidic.

• Examples:

  • $\text{Br}^-$ — Br- from HBr (strong acid) is neutral

  • $\text{CN}^-$ — CN- from HCN (weak acid) is basic

  • $\text{NO}_2^-$ — NO2- from HNO2 (weak acid) is basic

  • $\text{ClO}_4^-$ — ClO4- from HClO4 (strong acid) is neutral

Examples and Applications

Determine if each of the following compounds will create an acidic, basic, or neutral solution:

• a) NaCl Na+ (main-group metal, neutral) + Cl- (from strong acid, neutral) = neutral solution

• b) PbCl2 Pb2+ (transition metal, acidic) + Cl- (neutral) = acidic solution

Practice Problems

• Li2CrO4 Li+ (neutral) + CrO42- (from weak acid, basic) = basic solution

• C6H5NH3Br C6H5NH3+ (acidic) + Br- (neutral) = acidic solution

• Co(H2SO4)2 Co2+ (acidic) + SO42- (neutral) = acidic solution

• Sr(H2SO4)2 Sr2+ (neutral) + SO42- (neutral) = neutral solution

Summary Table: Acid-Base Properties of Ions

Ion Type	Charge	Acidic/Basic/Neutral	Example
Transition Metal Cation	+2 or higher	Acidic	Zn2+, Fe3+
Main-Group Metal Cation	+3 or higher	Acidic	Al3+
Main-Group Metal Cation	+1 or +2	Neutral	Na+, Ca2+
Ammonium/Amines	Any	Acidic	NH4+, CH3NH3+
Anion from Strong Acid	Any	Neutral	Cl-, Br-
Anion from Weak Acid	Any	Basic	CN-, NO2-

Additional info: The above rules are essential for predicting the pH of salt solutions in aqueous chemistry, a key concept in GOB Chemistry. Practice problems reinforce the identification of acidic, basic, or neutral salts based on their constituent ions.