12.3    Reactions of  Halide Ions

Introduction to halide ions, X⁻

• The halogens are typical non-metals, they:
  1. form singly charge negative ions, X⁻.
  2. form ionic compounds with metals and covalent compounds with non-metals.

Preparation of halogens in the laboratory

• Halogens can be prepared in the laboratory by the oxidation of X⁻ ions using manganese(IV) oxide, MnO2 in the presence of concentrated sulfuric acid

2X⁻ + MnO2 + 4H⁺ → X2 + Mn²⁺ + 2H2O    ; where X = A halogen

• The apparatus needed is shown below:

Note:

1. In each case, hydrogen halide is also formed from the reaction of X⁻ with H2SO4 and must be removed in order to obtain pure halogen.

Test for halide ions(reaction with silver ion, Ag⁺)

• Halide ions are colourless in their aqueous solutions and a test is needed to identify their presence
• Silver ions, Ag⁺ can be used to test halide ions because the silver halide is formed as precipitate

Ag⁺(aq) + X⁻(aq) → AgX(aq or s)    ; where X = A halogen

The silver halides formed can be differentiated by:

1. theircolour
2. their reaction with dilute aqueous ammonia, NH3.

• The test is summarised below: (Fluoride does not form precipitates)
• NH3 is used as a confirmatory test as cream and white precipitate may be hard to distinguish
• Alternatively, concentrated sulfuric acid can be used to test halide ions:

Reaction with concentrated sulfuric acid,  H2SO4

• When halides(NaX) are reacted with concentrated sulfuric acid, the following happens:
• The ease of oxidation of halide ions increases from Cl⁻ to I⁻ because the tendency to be oxidised(the reducing power) increases. The HBr and HI produced are oxidised to Br2 and I2 respectively while the HCl produced is not. (HI is oxidised readily while HBr is not)
• To prepare HI or HBr, phosphoric acid, H3PO4 is used instead because all halides react to give the corresponding hydrides

  2NaX + H3PO4 → 2HX + Na2HPO4           ; where X = A halogen