Question 14 Marks

Starting from zinc how would you obtain hydrogen using $– (a)$ Steam $(b)$ A dilute acid $(c)$ An alkali $[$Give balanced equations for each $\&$ name the product formed in each case other than hydrogen$]$. Name a metal which will not react with the reactants above to give hydrogen.

Answer

$(a)$ Steam : Metals above $[ H ]$ in activity series of metals, i.e. $\ce{Zn, Fe}$ and $Pb$ react with steam and form corresponding oxides.
$\ce{3Fe + 4 H_2 O \rightleftharpoons Fe_3O_4 + 4 H_2}$
$\text { Steam [Magnetic oxide of iron] }$
or 
$\text { [Trionr tetroxide] }$
$Zn +\underset{\text { Steam }}{H_2O} \longrightarrow \underset{\text { Zinc oxide }}{\ce{ZnO} + H_2}$
Metals below $[ H ]$ do not react even with steam. Metal are $\ce{Cu , Hg , Ag , Pt}$ and $Au$.
$(b)$ With dil. acids : Metals like $\ce{Zn, Fe, Pb}$ form corresponding salt.
$Zn +$ dil. $\ce{2 HCl} \longrightarrow \underset{\text { Zinc chloride }}{ \ce{ZnCl_2} + H_2}$
$\ce{Fe + H_2 SO_4} \longrightarrow \underset{\text { Ferrous sulphate }}{ \ce{FeSO_4+ H_2}}$
$\ce{Pb + 2 HCl} \text { dil. } \longrightarrow \underset{\text { Lead chloride }}{\ce{ PbCl_2} + H_2}$
$Cu$ and metals below $Cu$ do not react with dil. acids.
$(c)$ An alkali: $\ce{Zn, Pb}$ react with concentrated alkali on boiling to give the corresponding metallic salt and liberate hydrogen.
$ \ce{Zn + 2 NaOH \longrightarrow Na_2 ZnO_2 + H_2}$
$\text { Zinc [Sodium zincate] }$
$\ce{Pb + 2 NaOH} \longrightarrow \underset{\text { [Sodium plumbite] }}{\ce{Na_2 PbO_2 + H_2}}$

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