Zn --> Zn²⁺ + 2e
this is an oxidation reaction, since the oxidation number of Zn increases from 0 to +2
2H⁺ + 2e --> H₂
this is a reduction reaction, since the oxidation number of H decreases from +2 to 0.
we can directly add the 2 equations as number of electrons are balanced,
Zn + 2H⁺ ---> Zn²⁺ + H₂
we add 2 Cl⁻ ions to both sides of the equation, then the balanced redox reaction is,
Zn(s) + 2HCl(aq) ---> ZnCl₂(s) + H₂(g)
