Chemistry Experiments - Metals In Nirtric Acid

Nitric acid is one of the strongest acids in chemistry. Nitric acid is still used today in the mining of Gold as it dissolves common metals very effectively. It even attacks Silver but not Gold – which is why gold is so easily divided from other metals.
Gold does not react with Nitric acid. The Magnesium and the Copper shavings react very quickly, releasing a reddish brown gas. The Zinc granules react after a little while in the same way. What is left in the flasks is a solution of the metal salts created by the reaction – the Copper Nitrate solution is green, the Zinc and Magnesium Nitrate solutions are yellow.
Nitric acid attacks almost all common and even some of the precious metals, such as Silver and Copper. The only exceptions are the ‘noble’ metals, Gold, Platinum and Iridium. A few other metals, namely Iron, Chrome and Aluminium, are also immune because of their passivation. They form a protective oxidized layer which cannot be dissolved by the acid. Zinc also forms such a layer but it does depend on the distribution of the metal – the finer the dissemination in the acid, the stronger and quicker the reaction.

Oxidation: 3 Mg ->3 Mg2+ +6e-

Reduktion: 2 NO3- + 8H3O+ + 6e- -> 2 NO + 12 H2O

3 Mg + 2 NO3- + 8 H3O+ -> 3 Mg2+ 2NO + 12 H2O

The released Nitrogen Monoxide is colourless but reacts immediately with the Oxygen to form brown Nitrogen Oxide.

2 NO (uncoloured) + O2 -> 2 NO2 (brown)