Nitric acid properties, Passivity phenomenon & Economic importance of the 5A group elements
Properties of nitric acid HNO3
Nitric acid is colorless liquid , It turns to red color when adding a few drops of litmus solution , Strong oxidizing agent – especially the concentrated acid – and this is illustrated from the action of heat on it , Its action on metals and passivity phenomenon .
Preparation of nitric acid in lab
Put potassium nitrate and concentrated sulphuric acid in the glass retort and the receiver in a trough containing cold water , Heat the contents of the retort gently , but not to exceed the temperature 100° C , Collect the acid formed in the receiver .
2KNO3 ( s ) + H2SO4 ( l ) → K2SO4 ( aq ) + 2HNO3 ( l )
The action of heat on nitric acid
It decomposes by heat giving brown fumes of nitrogen dioxide ( NO2 ) , It is a strong oxidizing agent , Because oxygen gas evolves as a result of its thermal decomposition .
4HNO3 ( l ) → 2H2O ( v ) + 4NO2 ( aq )+ O2 ( g ) ↑
Action of the acid on metals
The chemical reaction : Nitric acid reacts with metals above hydrogen in the electrochemical series , giving metal nitrate and the atomic hydrogen which reduces the acid forming nitric oxide gas NO and water .
The diluted nitric acid reacts with iron , forming iron (III) nitrate and liberating nitric oxide gas ( colorless ) which turns into reddish brown color at the top of the tube .
4HNO3 ( aq ) + Fe ( s ) → Fe(NO3)3 ( aq ) + 2H2O ( l ) + NO ( g ) ↑
Nitric acid reacts with metals below hydrogen in the electrochemical series , Because it is a strong oxidizing agent which oxidizes the metal forming a basic oxide , which reacts with acid forming the acid salt and water , The type of the produced gas differs according to the concentration of acid .
The diluted nitric acid reacts with copper and nitric oxide gas evolves .
8HNO3 ( aq ) + 3Cu ( s ) → 3Cu(NO3)2 ( aq ) + 4H2O ( l ) + 2NO ( g )
When the conc. nitric acid is added to copper , brown vapors of nitrogen dioxide gas are formed and copper ions Cu2+ from green color in the solution .
4HNO3 ( aq ) + Cu ( s ) → Cu(NO3)2 ( aq ) + 2H2O ( l ) + 2NO2 ( g )
The conc. nitric acid does not affect some metals such as ( iron , chromium and aluminum ) , due to the oxidizing property of the acid , a non-porous layer of the metal oxide is formed which protects the metal from further reaction , what is known by passivity .
Detection of nitrate ion NO3 − ( the brown ring experiment )
Add nitrate salt solution to a freshly prepared conc. solution of iron (II) sulphate in a test tube , Add carefully on the internal wall of the tube a few drops of conc. sulphuric acid .
Observation : A brown ring appears at interface and disappears by shaking or heating .
4H2SO4 (l) + 2NaNO3 (aq) + 6FeSO4 (aq) → 2NO (g) + Na2SO4 (aq)+ 4H2O (l) + 3 Fe2(SO4)3 (aq)
NO ( g ) + FeSO4 ( aq ) → FeSO4.No ( s ) Brown ring compound
Differentiation between nitrite and nitrate salts
To differentiate between nitrite and nitrate salts , add to each solution of them a solution of potassium permanganate acidified with concentrated sulphuric acid , if :
- The violet color of the permanganate disappears , the salt is nitrite .
- The violet color of the permanganate does not disappear , the salt is nitrate .
3H2SO4 (l) + 2KNO2 (aq) + 2KMnO4 (aq) → 5KNO3 (aq) + K2SO4 (aq)+ 3 H2O (l) + 2MnSO4 (aq)
The economic importance of the 5A group elements
Nitrogen gas is used in manufacturing ammonia gas , It is used in manufacturing nitrogenous fertilizers , It is used in filling of car tires , Because nitrogen reduced the probability of their explosion , as nitrogen is not affected easily by changing of the temperature , in addition to its leakage rate is less than that of air .
Nitrogen gas is used in filling chips packaging bags , To keep potato’s slices crunchy , because it is relatively inactive , Liquified nitrogen is used in preserving and transferring the living cells , It is used in treating some banging tumors .
Phosphorus is used in manufacture of matches , fireworks , phosphorus fertilizers , Many alloys such as phosphorus-bronze alloy ( Cu + Sn + P ) , which is used in the manufacture of pushing fans in ships .
Arsenic is used as a preservative substance for wood , Due to its poisonous effect on insects , bacteria & fungi , It is used in preparing of arsenic trioxide compound , which is used in treatment of blood cancer ( Leukemia ) , Transferring arsenic and its compounds is very dangerous , Because it is very toxic element .
Antimony is used in the manufacture of lead-antimonal alloy which is used in the acidified car batteries , because it is harder than lead , It is used in the manufacture of semiconductors technology which are used in manufacturing instruments of detecting the infrared rays ( IR ) .
Bismuth is used in manufacturing bismuth , lead and cadmium alloys , which are used in manufacturing electric fuses due to its low melting point .