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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

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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