Ammonium Salts: General Properties, Chemistry and effect of Heat on Ammonium Salts

Ammonium salts share some very important characteristics or properties. They are all white crystalline solids that are soluble in water. They also electrolytes that ionize completely in solution.

Ammonium salts are compounds that contain the ammonium ion, NH₄⁺, as their cation. They are prepared from gaseous or aqueous ammonia by reacting them with in acids. For example, ammonium chloride, NH₄Cl, is formed by dissolving gaseous ammonia in hydrochloric acid.

NH₃(g) + HCl(aq) → NH₄Cl(aq)

Reaction of aqueous ammonia with hydrochloric acid can be expressed in an equation of reaction as evident in,

NH₄⁺(aq) + OH^-(aq) + HCl(aq) → NH₄Cl(aq) + H₂O(l)

Whereas, water is formed as a product of reacting aqueous ammonia with hydrochloric acid, dissolving ammonia gas in hydrochloric acid only produce one product that is the ammonium chloride.

Ammonium trioxonitrate(V) is also produced in a similar way as ammonium chloride. One method involves dissolving ammonia gas in trioxonitrate(V) acidHNO₃, to produce only one product: ammonium trioxonitrate(V) salt, NH₄NO₃.

NH₃(g) + HNO₃(aq) → NH₄NO₃(aq)

Water is formed with the ammonium trioxonitrate(V) salt when aqueous ammonia is reacted with the trioxonitrate(V) acid.

NH₄⁺(aq) + OH^-(aq) + HNO₃(aq) → NH₄NO₃(aq) + H₂O(l)

An important point to note is that, ammonia is separated out of solution after preparation by crystallization as they easily decompose by heat so that heating techniques such as evaporation are discouraged.

Ammonium salts decompose to different types of products. A unique aspect is the sublimation of some common ammonium salts like ammonium chloride. Here, we will be dealing with the effect of heat on five examples of ammonium salt that include ammonium dioxonitrate(III), ammonium chloride, ammonium tetraoxosulphate(VI), ammonium trioxonitrate(V) and ammonium trioxocarbonate(IV).

Ammonium dioxonitrate(III)

Nitrogen gas and steam are produced when ammonium dioxonitrate(V) is heated.

NH₄NO₂(s) → N₂(g) + 2H₂O(g)

Ammonium chloride

The effect of heat on ammonium chloride is an example of sublimation and condensation. When heated, ammonium chloride dissociates to produce ammonia and hydrogen chloride gas. The ammonium chloride is reformed when the products are cooled.

heat + NH₄Cl(s) ↔ NH₃(g) + HCl(g) + cold

Ammonium tetraoxosulphate(VI)

Volatile ammonia gas and liquid tetraoxosulphate(VI) acid are formed when ammonium tetraoxosulphate(VI) salt is decomposed by heat.

(NH₄)₂SO₄(s) → 2NH₃(g) + H₂SO₄(l)

Ammonium trioxonitrate(V)

In the case of ammonium trioxonitrate(V), nitrogen (I) oxide and water is formed instead of nitrogen gas and water as in the case of the decomposition of ammonium dioxonitrate(III).

NH₄NO₃(s) → N₂O(g) + 2H₂O(g)

Ammonium trioxocarbonate(IV)

Ammonium trioxocarbonate(IV) is easy to handle with heat as it is unstable and decomposes very readily to ammonia, carbon(IV) oxide and water.

(NH₄)₂CO₃(s) → 2NH₃(g) + CO₂(g) + H₂O(g)

The difference between ammonium salt and normal salt can be established by reacting the salt with a base or alkali. The liberation of ammonia by ammonium salt (and not normal metallic salt) is confirmatory for an ammonium salt when the salt is heated with a base or alkali.

NH₄⁺(s) + OH^-(aq) → NH₃(g) + H₂O(l)

This reaction is also used in the laboratory preparation of ammonia such as where calcium hydroxide is heated with ammonium dioxonitrate(III) salt.

Test for ammonium salt involves the detection of ammonia gas in an unknown sample. Here, the sample is heated with sodium hydroxide to initially detect the characteristic choking smell of ammonia. Then, the gas is tested with moist red litmus paper. The sample is confirmed to contain ammonium salt if the gas is able to turn the moist red litmus paper blue.

NH₄⁺(s) + NaOH^-(aq) → NH₃(g) + H₂O(l) + Na⁺(aq)