Question

Consider the reaction between Zn and dilute hydrochloric acid forming zinc chloride and liberating hydrogen.

Answer 1

The steps involved in balancing the given reaction by O.N. method are
(i) Write the skeletal equation involving all the known reactants and products.
$Zn+HCl\to ZnC{l}_2+H_2$
(ii) Indicate the O.N. of different atoms involved.
$\stackrel{0}{Zn}+\stackrel{+1}{H}\stackrel{-1}{Cl}\to \stackrel{+2}{Zn}\stackrel{-1}{C{l}_2}+\stackrel{0}{H_2}$
(iii) Identify the elements which undergoes a change in O.N.
$\stackrel{0}{Zn}\to \stackrel{+2}{Zn}$ (O.N. increases)
$\stackrel{+1}{H}\to \stackrel{0}{H_2}$ (O.N. decreases)
(iv) Calculate the increase or decrease of O.N. per atom and multiply this number with the number of atoms which are undergoing the change.
(i) $[\stackrel{0}{Zn}\to \stackrel{+2}{Zn}$ $2$ per atom $]\times 1$
(ii) $[\stackrel{+1}{H}\to \stackrel{0}{H_2}$ $1$ per atom $]\times 2$
(v) Balance other atoms by simple method (if required)
$Zn+2HCl\to ZnC{l}_2+H_2$
This is the balanced equation.