Dichromate ion in acidic medium oxidizes stannous ion as:

$x{\mathrm{Sn}}^{2+}+y{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+z{\mathrm{H}}^{+}\to a{\mathrm{Sn}}^{4+}+b{\mathrm{Cr}}^{3+}+{\mathrm{cH}}_2\mathrm{O}$ then which is not true