Nitric acid preparation and properties, Group 15 P Block Elements
Nitric Acid :
→ Nitrogen forms three main oxoacids. These are
- Hyponitrous acid – H2N2O2
- Nitrous acid – HNO2
- Nitric acid – HNO3
→ Among all oxoacids of nitrogen, nitric acid, (HNO3) is the most important acid of nitrogen. Nitric acid, (HNO3) is stable and a strong acid. It acts as a strong oxidising agent. Alchemists named it aqua fortis (meaning strong water), Glauber prepared it first time by the action of sulphuric acid and nitre.
Nitric acid can be prepared by various methods, some of them are as follows:
→ Laboratory Method: In the laboratory, nitric acid is prepared by heating sodium or potassium nitrate (KNO3 or NaNO3) with concentrated sulphuric acid in a glass retort to about 423-475 K.
→ Commercial production of nitric acid by Ostwald’s Process : On commerical scale, nitric acid is prepared by the catalytic oxidation of ammonia. This process is known as Ostwald’s Process. The catalytic oxidation of ammonia is done by atmospheric oxygen at about 500 K and 9 × 105 Pa (9 bar). In presence of Pt or Rh gauge as catalyst.
→ The nitric oxide thus formed is recycled and the aqueous HNO3 is concentrated by distillation to give 68% HNO3 by mass. Further concentration of 98% can be achieved by dehydrating it by concentrated H2SO4.
→ Nitric acid prepared from Oswald’s method is very dilute. It can be concentrated to about 69% by distilling it under reduced pressure. It can be further concentrated by dehydration. It is dehydrated by concentrated H2SO4 upto 98%.
→ The acid obtained by dehydration process contains dissolved NO2. So it is brown in colour. This NO2 gas is thus removed by passing bubbling air through it.
Structure of nitric acid
In gaesous state, HNO3 exist as a planar molecule. Its structure is as follows
Resonating structure of nitric acid is as follows
Nitrate ion formed after the removal of Hion from nitric acid also show the resonating structure as follows
Physical Properties of Nitric Acid
- Anhydrous nitric acid is a colourless liquid which has pungent smell.
- Impure nitric acid is yellow in colour due to the presence of dissolved oxides of nitrogen.
- It boils at 355.6 and freezes at 231.4 K.
- It is soluble in water in all proportions but 68% nitric acid by mass forms an azeotropic or constant boling mixture with water.
- Its specific gravity is 1.504
- It show corrosive action on the skin and causes painful sores.
Chemical Properties of Nitric Acid
The chemical properties of nitric acid are as follows:
Acidic nature : It is a strong monobasic acid which forms only one type of salt. It ionises completely in water.
HNO3(aq) + H2O(l) → H3O+ (aq) + NO3–(aq)
It forms salts with bases like metallic oxides, metallic hydroxides, metallic carbonates and bicarbonates etc.
Decomposition : On heating, nitric acid decomposes into nitrogen dioxide, oxygen and water.
Oxidising Nature : Nitric acid acts as a strong oxidising agent because it readily gives nascent oxygen.
Some important oxidising reactions of nitric acid are as follows:
Oxidation of non-metals : The nascent oxygen coming from nitric acid oxidises various non-metals as follows
Oxidation of metalloids : Semi-metals or metalloids such as arsenic and antimony are also oxidised by conc. HNO3. Involved reactions are as follows
Oxidation of compounds : Both concentrated and dilute nitric acid can oxidise the various compounds.
Oxidation of sulphur dioxide into sulphuric acid
Oxidation of hydrogen sulphide into sulphur
Oxidation of ferrous sulphate Into fernc sulphate in acidic medium
Oxidation of hydrogen iodide into iodine
Similarly HBr also oxidises into Br2
Oxidation of Klinto l2 Both dilute and conc. acid can oxidise.
Oxidation of stannous chloride into stannic chloride in presence of HCI
7SnCl2 + 14HCl + 3HNO3 → 7SnSl4 + NH2OH + NH4NO3 + 5H2O
→ Oxidation of metals: With an exception of gold and platinum (known as noble metals), nitric acid can attack all the metals forming a variety of products. The action of nitric acid on metals can be explained as follows.
→ Action on metals that are more electropositive than hydrogen : Metals like Mg, Al, Mn, Zn, Fe, Pb etc. are lying above hydrogen in electrochemical series. These metals are highly electropositive in nature. When these metals react with nitric acid having different concentration then a variety of compounds are formed. Some reactions are given below
Reaction with zinc (Zn): Zinc forms different products with very dilute, dilute and concentrated acids as follows:
Reaction with Iron : Similar to zinc, iron also reacts with varying concentration of nitric acid and different products are formed.
→ Action on metals that are less electropositive than hydrogen : Metals like Cu, Ag, Hg etc. are lying below in electrochemical series. They form different products with varying concentration of nitric acid. Some of them are as follows:
Reaction with copper
Reaction with silver : Silver (Ag) reacts similarly as copper.
Reaction with Mercury
→ Action on Noble metals Nitric acid as such have no effect on noble metals like gold, platinum, irridium etc. But these metals can dissolve in aqua-regia (3 parts conc. HCl and one part conc. HNO3). Actually aqua-regia release nascent chlorine which reacts with noble metals as follows.
→ Action on metals which become passive : Some metals like Fe, CO, Ni, Cr, Al become passive towards concentrate nitric acid. It is due to the formation of a thin layer of metal oxide over the surface of metal. This layer of oxide protects the surface of metal.
→ It prevents further action of nitric acid on the surface. Thus, we can say that the inertness exhibited by the metals due to the formation of metal oxide layer is known as passivity.
→ Xanthoproteic Test : Nitric acid attacks on protein and forms a yellow coloured nitro compound. This yellow coloured nitro compound is known as xanthoprotein. This, test is knwon as xanthoprotic test and it is used to detect protein.
→ Brown Ring Test : Brown ring test is used to detect NO3– ion in a mixture. In this test first of all an aqueous solution of salt is taken in test-tube. To this aqueous solution now freshly prepared ferrous sulphate solution is added.
→ Now concentrated sulphuric acid is added to this solution drop by drop along the side walls of test tube without disturbing the solution, a dark brown colour ring is obtained at the interface of two layers i.e. oily layer of sulphuric acid and aqueous layer.
The reactions are as follow
Nitric acid Uses
The main uses of nitric acid are as follows
- In manutacturing of various explosive like T.N.T., Picric acid, Dynamite etc.
- In preparation of various nitro compound like ammonium nitrate, basic calcium nitrate etc.
- In manufacturing of artificials silk, dyes, drugs, perfumes etc.
- As a laboratory reagent
- In the puritication of gold and platinum as aqua regia and for pickling (cleaning) of stainless steel
- In manufacturing of some important compounds used in perfumes, dyes and medicines.
- As a solvent for metals, for etching designs on brass and bronze wares.
- In manufacturing of silver nitrate, sodium nitrate and potassium nitrate etc.
- As oxidiser in rocket fuels.