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Generally, when metals are treated with mineral acids, hydrogen gas is liberated but when metals (except Mn and Mg), treated with HNO3, hydrogen is not liberated, why?

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Ans.
Nitric acid (HNO3) is a strong oxidising agent. As soon as hydrogen gas is formed in the reaction between a metal and dilute nitric acid, the nitric acid oxidises this hydrogen to water and itself gets reduced to NO2 or NO or N2O.
Mg+2HNO_{3}\rightarrow Mg\left ( NO_{3} \right )_{2}+H_{2}
4Zn+10HNO_{3}\rightarrow 4Zn\left ( NO_{3} \right )_{2}+N_{2}O+5H_{2}O
4Sn+10HNO_{3}\rightarrow 4Sn\left ( NO_{3} \right )_{2}+NH_{4}NO_{3}+3H_{2}O
3Pb+8HNO_{3}\rightarrow 3Pb\left ( NO_{3} \right )_{2}+2NO+4H_{2}O
So, in the reaction of metals (except Mn and Mg), with dilute nitric acid, no hydrogen gas is evolved.

 

 

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