Nitric acid properties Part 2

Nitric acid

source : greener-industry.org.uk

1)  Oxidation of metal :

a) Reaction of Mg or Mn with very dilute HNO₃ (6%):   Hydrogen is formed.

Mg + 2HNO₃  ———-> Mg(NO₃)₂ +H₂ 

Mn + 2HNO₃ ———-> Mn(NO₃)₂ +H₂

b) Reaction of copper with  HNO₃:

i) 3 Cu + 8HNO₃ (dilute 20%)   ———> 3Cu(NO₃)₂ + 2NO+ 4H₂O

ii) Cu + 4HNO₃ (Concentrated)   ——–> Cu(NO₃)₂+ 2NO₂ + 2H₂O

c) Oxidation of silver with  HNO₃ :

i) 3 Ag + 4HNO₃ (dilute)  ——–> 3AgNO₃(Lunar caustic) +NO + 2H₂O

ii) Ag + 2HNO₃ (concentrated) ——> AgNO₃ +NO₂ +H₂O

d) Oxidation of lead:

i)  3Pb +  8HNO₃ (dilute) ——>  3Pb(NO₃)₂ +  2NO +  4H₂O

ii) Pb +  4HNO₃ (dilute) ——>  Pb(NO₃)₂ +  2NO₂ +  2H₂O

e) Oxidation of Mercury :

i)  6 Hg + 8HNO₃ (dilute) ——> 3Hg₂(NO₃)₂ (mercurous nitrate) + 2NO + 4H₂O

ii) Hg + 4HNO₃ (concentrated) ——>Hg(NO₃)2 (mercuric nitrate) +  2NO₂ +  2H₂O

f) Oxidation of Iron :

Iron does not react with highly conc. nitric acid (80%).It becomes passive due to formation of  a protective layer of ferric oxide. 

i)  4Fe + 1o HNO₃ (dilute) ——> 4Fe(NO₃)₂ (ferrous nitrate) +  N₂O +  5H₂O

ii) Fe + 6HNO₃ [ conc. (70%)] ——> Fe(NO₃)₃ (ferric nitrate) + 3NO₂ + 3H₂O

g) Oxidation of Zinc :

i) 4 Zn + 10 HNO₃ (dilute) ——> 4 Zn(NO₃)₂ (zinc nitrate)+  N₂O +  5H₂O

ii) Zn +  4HNO₃ (concentrated) ——> Zn(NO₃)₂ (zinc nitrate) +  2NO₂ + 2H₂O

h) Oxidation of Tin:

i)  4Sn + 10 HNO₃ (dilute) ——> 4Sn(NO₃)₂ (stannous nitrate)+  NH₄NO₃ +  3H₂O

ii) Sn + 4 HNO₃ (concentrated) ——> H₂SnO₃ (meta stannic acid) + 4NO₂ + H₂O

2) Reaction of Al with Nitric acid:

Aluminium does not react with dilute or concentrated HNO₃. It  is due to the formation of protective layer of Al₂O₃ at the surface of aluminium.

3) Reaction with protein :

Concentrated HNO₃ burns the skin and produces yellow blisters to  make the skin yellow it is due to the formation of xanthoprotein.

source : colinb-sciencebuzz.blogspot.com

Question: What is aqua regia ?

Solution :

The mixture of three part concentrated HCl and one  part concentrated HNO₃ is called aqua regia. It is strong oxidizing agent . Most of the metals react with aqua regia . Nascent chlorine is liberated due to which aqua regia is very reactive  . Noble metals like Au , Pt dissolves in aqua regia.

3HCl + HNO₃  ——–> NOCl(nitrosyl chloride) + 2Cl (nascent chloride)+ 2H₂O

Reaction with Gold :

3HCl + HNO₃ —–> NOCl + 2Cl + 2H₂O ] x 3

Au + 3Cl ——> AuCl₃ ] x 2

9 HCl + 3HNO₃ ——-> 3 NOCl + 6Cl + 6H₂O

2Au + 6Cl   ——-> 2AuCl₃

Equation as a whole,

9 HCl + 3HNO₃ + 2Au  ——-> 3 NOCl + 6H₂O + 2AuCl₃