Characteristic Reactions of Halide Ions

This lesson covers: 

1. The properties of halide ions
2. Using silver nitrate to test for halide ions

Halide ions form molecules by losing electrons

Halide ions are the negatively charged ionic forms of the halogens fluorine, chlorine, bromine and iodine. They have a 1- charge and are named by adding ‘-ide’ to the halogen name (e.g. chloride, bromide, iodide).

Halide ions react by losing an electron to form neutral halogen molecules. For example:

X^- ➔ 1⁄2X₂ + e^-

Where X represents a halogen atom.

This electron loss results in the halide being oxidised, as its oxidation number increases from -1 to 0.

As the halide ion loses an electron, it causes another substance to be reduced - so halide ions act as reducing agents.

Testing for halide ions

Aqueous silver nitrate (AgNO₃) solution is used to test for the presence of halide ions (Cl^-, Br^-, I^-) in a sample.

The method is:

1. Add dilute nitric acid to the sample to remove any ions like carbonates that may interfere by also forming precipitates with silver nitrate, confounding the results.
2. Add silver nitrate solution; silver ions react with halide ions to form silver halide precipitates:

Ag⁺_(aq) + X^-_(aq) ➔ AgX_(s) 

Where X = Cl, Br or I

1. Observe precipitate colour to identify which halide ion is present:

Some silver halides have similar colours so to confirm the identity of the halide present, add excess ammonia solution. Silver halides have different solubilities in dilute and concentrated ammonia solution: