Identification of non-metallic ions and gases

YOU SHOULD BE ABLE TO:

■ Identify the following gases – H2, O2, NH3, SO2, HCl, Cl2, NO2, H2O

■ Identify the following anions – CO32-, SO42-, SO32-, NO3-, Br-, I-, Cl

QUICK POINTS

Non-metallic ions, or anions, are generally identified based on the evolution and identification of gases produced when the compounds are heated or reacted with acids.

The colour and solubility of precipitates formed on reaction with barium ions and ammonia (for the halide ions) are also used.

IDENTIFICATION OF NON-METALLIC IONS

The anions of interest include CO32-, SO42-, SO32-, NO3-, Br-, Cl- and I-.

One method of identifying these ions is to heat the compound and identify the gas produced. When heated, CO32- ion produces the gas CO , NO ion gives off O and/or 2 3- 2 NO2 gases, and SO 2-ion gives off SO gas. 3 2

These gases can be identified by their characteristic tests.

QUICK CHECK

What are the characteristic tests for hydrogen and chlorine gases?

Some anions react with dilute and concentrated acids (if they are added to the solid compound). These reactions are summarised in the table below.

The halide ions, in solution, can be identified differently based on their reaction with silver nitrate solution (AgNO ) and the solubility of the precipitate formed in 3 ammonia.

Cl- forms a white precipitate with AgNO3 solution, which is soluble in aq NH3.

Br- forms a cream precipitate with AgNO3, which is partially soluble in aq NH3.

I- forms a light yellow precipitate, which is insoluble in aq NH3.

Some anions can also be identified based on the formation of precipitates with a solution of Ba2+ ions.

CO32- forms a white precipitate, which dissolves on addition of acid to form CO2.

SO32- forms a white precipitate, which dissolves on addition of acid to give SO2.

SO42- forms a white precipitate, which is insoluble in dilute acid.

FURTHER PRACTICE