the complete equation for the reaction is
Cr₂O₇²⁻ + 6Cl⁻ + 14H⁺ ---> 2Cr³⁺ + 3Cl₂ + 7H₂O
in the oxidation half reaction the oxidation number of the species increases.
in the reduction half reaction the oxidation number of the species decreases.
the oxidation number of Cr reduces from +6 in Cr₂O₇²⁻ to +3 in Cr³⁺
the oxidation number of Cl increases from -1 in Cl⁻ to Cl₂ therefore oxidation takes place
oxidation half reaction is as follows
2Cl⁻ --> Cl₂ + 2e
