When metals react with Dil.HNO3 why isn't H2 evolved?

Question

Please explain with equations

Answer 1

First of all it depends on the metal.

For example Copper has E0=+0.34. Thus it cannot displace H+ from acids (only metals with negative E0 can).

Nitric acid is not only an acid but also a strong oxidizing agent (by reducing its N atom). The exact reaction depends on the conditions. E.g.

NO3- + 4 H+ + 3 e- -> NO + 2 H2O with E0= + 0,96 V

This means that nitric acid can oxidize all compounds with standard electrode potentials lower than +0,96 V, through the N.

For copper you have

3Cu + 2NO3- +8H+ -> 3Cu+2 + 2NO + 4H2O or
Cu + 4 HNO3 -> Cu(NO3)2 + 2 NO2 (g) + 2 H2O

Metals with negative E0 are tough cases. E.g. Zinc has E0=-0.76

thus Zn can either reduce H+

Zn + 2HNO3 -> Zn(NO3)2 + H2(g)

or the N from the NO3- group

3Zn + 2NO3- +8H+ -> 3Zn+2 + 2NO + 4H2O or
Zn + 4 HNO3 -> Zn(NO3)2 + 2 NO2 (g) + 2 H2O depending on concentration
Which reaction takes place depends on the conditions.
In fact Zn and dilute HNO3 can give H2 and
Zn + concentrated HNO3 can give NO2 according to:

http://www.cci.ethz.ch/experiments/HNO3_Met/en/stat.html

Answer 2

once a metal is added to an acid...the substance that is most susceptible to evolve is oxygen...eg. zinc + nitric acid = zinc hydroxide and oxygen and NO gas...

eg. with equation.
Zn(s) + 2HNO3(aq) ===> Zn(OH)2 (aq) + 2NO (g) + O2(g).

Answer 3

because O2 gas is more easily evolved and drives the reaction to completion