Use oxidation numbers to find the oxidant and the reductant!
$Cr_2O_7^{2-}$ $(o.n.(Cr)=6)$ is the oxidant which is reduced to $Cr^{3+}$ $(o.n.(Cr)=3)$. $Fe^{2+}$ $(o.n.(Fe)=2)$ is the reductant which is oxidised to $Fe^{3+}$ $(o.n.(Fe)=3)$.
How many electrons are exchanged by the oxidant and the reductant?
$Cr_2O_7^{2-}$ catches 6 $e^-$. $Fe^{2+}$ looses 1 $e^-$.
Write the partial oxidation and reduction reactions in an acid solution.
$Cr_2O_7^{2-}$ $+$ $6e^-$ $+$ $14H^+$ $\longrightarrow$ $2Cr^{3+} $ $+$ $7H_2O$ $Fe^{2+}$ $-$ $e^-$ $\longrightarrow$ $Fe^{3+}$
Balance by the number of exchanged electrons and write the redox reaction!
$Cr_2O_7^{2-} $ $+$ $6e^-$ $+$ $14H^+$ $\longrightarrow$ $2Cr^{3+} $ $+$ $7H_2O$
$6Fe^{2+}$ $-$ $6e^-$ $\longrightarrow$ $6Fe^{3+}$
$Cr_2O_7^{2-} $ $+$ $6Fe^{2+}$ $+$ $14H^+$ $\longrightarrow$ $2Cr^{3+} $ $+$ $6Fe^{3+}$ $+$ $7H_2O$
Complete for the ions which are not involved et annotate!
$2K^+$ $+$ $Cr_2O_7^{2-} $ $+$ $6Fe^{2+}$ $+$ $6SO_4^{2-}$ $+$ $14H^+$ $+$ $7SO_4^{2-}$ $\longrightarrow$ $2K^+$ $+$ $SO_4^{2-}$ $+$ $ 2Cr^{3+}$ $+$ $3SO_4^{2-} $ $+$ $6Fe^{3+}$ $+$ $9SO_4^{2-}$ $+$ $7H_2O$ Potassium dichromate, iron(II) sulfate and sulfuric acid produce potassium sulfate, chromium(III) sulfate, iron(III) sulfate and water nbsp;