Ammonium (NH4+)
Table of Contents
The ammonium ion (NH4+) is a small, positively charged cation. It forms when ammonia (NH3) combines with a hydrogen ion (H+). Ammonium can be considered the “charged form” of ammonia. Because of its positive charge, ammonium readily combines with negatively charged nonmetal ions to form stable salts, most of which dissolve easily in water. [1–4]
Ammonium salts are widely used in daily life. They are important in fertilizers, where they supply nitrogen needed for plant growth. Some salts are also used in smelling salts, which release ammonia and give off a sharp, pungent odor.
Structure and Bonding
The ammonium ion has the formula NH4+. It carries a +1 charge, similar to alkali metal ions, and its molar mass is 18.04 g/mol. It forms when the nitrogen atom in ammonia donates its lone pair of electrons to bond with a hydrogen ion. This bond is called a coordinate covalent bond (or dative bond). [1]
After this bond forms, all four N–H bonds in NH4+ become equivalent. The nitrogen atom lies at the center, while four hydrogen atoms are arranged symmetrically around it. This arrangement gives the ion a tetrahedral shape with bond angles nearly 109.5°. The Lewis structure is shown below.
Common Ammonium Compounds [1]
Several ammonium compounds have significant industrial, agricultural, and medicinal applications:
| Compound | Formula | Uses and Applications |
|---|---|---|
| Ammonium chloride | NH4Cl | Dry cells, metalworking, cough medicines |
| Ammonium sulfate | (NH4)2SO4 | Fertilizer, water treatment, food additive |
| Ammonium nitrate | NH4NO3 | Fertilizer, explosives, cold packs |
| Ammonium carbonate | (NH4)2CO3 | Leavening agent, smelling salt, cough syrup |
| Ammonium bicarbonate | NH4HCO3 | Leavening agent, fertilizer, cough syrup |
| Ammonium dichromate | (NH4)2Cr2O7 | Pyrotechnics, laboratory “volcano reaction”, dyeing |
Preparation [2]
1. Industrial Preparation
Ammonium salts are commonly prepared by reacting ammonia gas with acids such as hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3):
NH3 (g) + HCl (aq) → NH4Cl (aq)
2 NH3 (g) + H2SO4 (aq) → (NH4)2SO4 (aq)
NH3 (g) + HNO3 (aq) → NH4NO3 (aq)
2. Laboratory Preparation
In the laboratory, ammonium salts can be prepared by mixing aqueous ammonia with dilute acids. For example:
NH3 (aq) + HCl (aq) → NH4Cl (aq)
Reactions of Ammonium Ion
The ammonium ion is chemically active. It acts as the conjugate acid of ammonia and releases a proton (H+) under suitable conditions. Its main reactions include: [2]
1. Acid–Base Reactions
In aqueous solution, the ammonium ion is in equilibrium with ammonia and a proton:
NH4+ (aq) ⇌ NH3 (aq) + H+ (aq)
This equation indicates that ammonium is a weak acid (pKa = 9.25). The position of the equilibrium depends on the pH of the solution: in acidic solutions, NH4+ is favored, while in basic solutions, NH3 dominates.
2. Reaction with Bases
When an ammonium salt is treated with a strong base, such as sodium hydroxide (NaOH), ammonia gas is released:
NH4Cl (aq) + NaOH (aq) → NH3 (g) + H2O (l) + NaCl (aq)
This reaction is used as a test for ammonium ions. The liberated ammonia can be detected by its sharp smell and its ability to turn damp red litmus paper blue.
3. Thermal Decomposition of Ammonium Salts
When heated, ammonium salts decompose in different ways depending on the anion:
i. Ammonium chloride sublimes, temporarily breaking into NH3 and HCl gases:
NH4Cl (s) ⇌ NH3 (g) + HCl (g)
ii. Ammonium nitrate decomposes on strong heating to produce nitrous oxide and water:
NH4NO3 (s) → N2O (g) + 2 H2O (g)
iii. Ammonium dichromate undergoes a redox decomposition to give chromium(III) oxide, water vapor, and nitrogen gas:
(NH4)2Cr2O7 → Cr2O3 (s) + 4 H2O (g) + N2 (g)
Substituted Ammonium Cations
Apart from the simple ammonium ion NH4+, many related ions exist in which alkyl or aryl groups (R) replace the hydrogen atoms. They are called substituted ammonium cations, and classified as: [4]
1. Primary Ammonium Cations (RNH3+): One R group replaces a hydrogen atom
Example: Methylammonium ion (CH3NH3+)
2. Secondary Ammonium Cations (R2NH2+): Two R groups replace hydrogen atoms
Example: Dimethylammonium ion ((CH3)2NH2+)
3. Tertiary Ammonium Cations (R3NH+): Three R groups replace hydrogen atoms
Example: Trimethylammonium ion ((CH3)3NH+)
4. Quaternary Ammonium Cations (R4N+): All four hydrogens are replaced by R
Example: Tetramethylammonium ion ((CH3)4N+)
Quaternary ammonium ions are permanently charged and cannot lose a proton. They are widely used as disinfectants, antiseptics, and surfactants. A common example is benzalkonium chloride, found in household cleaners and medical wipes.
The ammonium ion is a fundamental chemical species that plays a central role in both nature and industry. Its ability to form stable, water-soluble salts makes it invaluable in agriculture, medicine, and food processing.
