XII XII - - - - As an oxidising agent: The nonmetals like carbon, sulphur, phosphorus and iodine are oxidised by nitric acid. C + 4HNO 2H O + 4NO + CO S + 2HNO H SO + 2NO P + 20HNO 4H PO + 4H O + 20NO 3I + 10HNO 6HIO + 10NO + 2H O HNO + F HF + NO F 3H S + 2HNO 3S + 2NO + 4H O As an nitrating agent: In organic compounds replacement of a –H atom with –NO is often referred as nitration. For example. C H + HNO C H NO + H O H SO → Nitration takes place due to the formation of nitronium ion HNO + H SO NO + H O + HSO Action of nitric acid on metals All metals with the exception of gold, platinum, rhodium, iridium and tantalum reacts with nitric acid.
Nitric acid oxidises the metals. Some metals such as aluminium, iron, cobalt, nickel and chromium are rendered passive in concentrated acid due to the formation of a layer of their oxides on the metal surface, which prevents the nitric acid from reacting with pure metal. With weak electropositive metals like tin, arsenic, antimony, tungsten and molybdenum, nitric acid gives metal oxides in which the metal is in the higher oxidation state and the acid is reduced to a lower oxidation state. The most common products evolved when nitric acid reacts with a metal are gases NO , NO and H O.
Occasionally N , NH OH and NH are also formed. HNO NO HNO NO N O + + + + + N NH The reactions of metals with nitric acid are explained in steps as follows: Primary reaction: Metal nitrate is formed with the release of nascent hydrogen M + HNO MNO + (H) Secondary reaction: Nascent hydrogen produces the reduction products of nitric acid. HNO + (H) HNO + H O HNO + (H) Nitrous acid NH OH + 2H O