Question

Determine the chemical equation for the reaction between Fe³⁺ and Cu²⁺ with NH₃

Answer 1

Ammonia is a weak base and is most commonly used in the aq. form. Thus NH₃ is actually NH₄OH as shown in the following equation

NH₃ (aq) + H₂O (l) NH₄OH (aq)

This reaction occurs first to form OH^- ions and then the reactions with Fe (III) and Cu (II) occurs.

Reaction with Fe3+

3NH₃ + 3H₂O + Fe³⁺ Fe(OH)₃ + 3NH₄⁺

Fe(OH)₃ is a red brown insoluable precipitate.

Reaction with Cu2+

There are two reactions which can occur.

1. In dilute NH3

Cu²⁺ + 2NH₃ + 2H₂O Cu(OH)₂ + 2NH₄⁺

Cu(OH)₂ is a gelatinous pale blue soild.

2. Excess NH3

Upon further addition of NH3, the pale blue gelatinous precipitae dissolves to form a blue solution of tetraaminecopper(II) complex.

Cu²⁺ + 4NH₃ [Cu(NH₃)₄]²⁺