Nitric acid and nitrous acid are mono basic oxoacids of nitrogen. But they are very different in chemical and physical properties. Nitric acid is widely used in chemical industry for producing other chemicals. Also nitric acid is used as an strong acid and oxidizing acid in laboratories. Though both are acids, their acidic strength are different.
In this tutorial, we are leaning about characteristics, preparation, differences and some reactions of HNO3 and HNO2 acids.
When you are dealing with acids such as nitric acid, you should be very careful to avoid any injuries because nitric acid is an oxidizing acid.
Nitrous acid contains one hydrogen atom, one nitrogen atom and two oxygen atoms. According to the lewis structure of HNO2 acid, nitrogen atom is the center atom and there is one O-H bond, one N-O bond and one N=O bond. Also, there is a lone pair on nitrogen atom.
Nitrous acid is not a stable acid as nitric acid. Therefore, Nitrous acid is prepared when it is required because storage is not effective. Nitrous acid (HNO2) decomposes to Nitric acid (HNO3), Nitric oxide (NO) and water (H2O). When temperature of the aqueous Nitrous acid solution is increased, decomposition rate is also increased as a result.
HNO2 does not dissociate completely in the water. Therefore its' aqueous solution has a H+ concentration is very less when it compare with undissociated HNO2 acid concentration.
Aqueous nitrous acid solution is colourless.
Nitrous acid can be oxidized because nitrogen atom is at +3 oxidation state. Therefore HNO2 acid can reduce acidic potassium permanganate (H+ / KMnO4).
Due to the instability of nitrous acid, it is not stored in the laboratory. At a specific requirement, we prepare the nitrous acid as below.
Solid sodium nitrate (NaNO3) is heated to decompose to solid sodium nitrite (NaNO2) and oxygen (O2) gas. This reaction is a redox reaction (oxidation - reduction reaction)
Produced solid sodium nitrite is dissolved in water to prepare aqueous sodium nitrite solution. There is no reaction in this step, only a dissolving.
Add cold dilute HCl acid to aqueous sodium nitrite solution. It gives dilute nitrous acid and NaCl solution.
As we have studied about Nitrous acid, he we will look a brief summary about Nitric acid.
Add concentrated H2SO4 acid to the solid NaNO3(s) and distill the mixture.
Ostwald process is used in industry to production of nitric acid. Ammonia and oxygen are used as raw materials in the presence of Pt / Rh catalyst. Higher temperatures and pressure are applied in the production to get a successful yield.
When the temperature of the acid solution increases, nitric acid starts to decomposes to nitrogen dioxide, oxygen and water. Nitrogen dioxide (NO2) is a brown gas.
Nitric acid is a strong acid which dissociate completely in the water to H+ ions and NO3-. But, nitrous acid is a weak acid and partially dissociates to H+ ions and NO2-. This happens because conjugate base (NO3-) of nitric acid is more stable. pKa of nitrous acid is 3.3 at 180C. Also, nitrous acid decomposes readily than nitric acid.
Also, oxidation numbers of nitrogen atoms are different. In HNO2, nitrogen atom is at +3 oxidation state . In HNO3, nitrogen atom is at +5.
Nitric acid is a strong acid. But nitrous acid is a weak acid. Therefore acidic strength of nitric acid is much higher than nitrous acid.
When oxidation number increases, acidic characteristics increases. Oxidation number of nitrogen in nitric acid is +5 while it is +3 in nitrous acid. That is one reason why nitric acid is much acidic than nitrous acid.
Oxidation state of HNO3 is +5. So nitrogen atom cannot be oxidized furthermore. Due to +5 oxidation state, nitric acid shows strong acidic properties.
HNO2 is an oxo acid of nitrogen and it is a weak acid. HNO2 is also named as nitrous acid or nitric(III) acid.
Nitrous acid is used very much in organic chemistry such as amine and HNO2 reaction.
Aqueous nitrous acid is decomposed to nitric acid when temperature is increased. So after preparation of nitrous acid, nitric acid and nitrous acid solution mixture can be given.
A strong acidic solution is given.
HNO2 does not contain carbon as a element. So HNO2 is an inorganic compound.
Nitrous acid is a weak monobasic acid.
HNO2 decomposes to HNO3, NO and water easily at room temperature. This decomposition increases with temperature. Therefore, we cannot store nitrous acid for long time in the laboratory. When we require HNO2, at that moment HNO2 is prepared.