In the reaction $3{\text{Br}}_2+6{\text{CO}}_3^{2-}+3{\text{H}}_2\text{O}\to 5{\text{Br}}^{-}+{\text{BrO}}_3^{-}+6{\text{HCO}}_3^{-}$, what is the change in oxidation state of bromine?

Solution

1. In ${\text{Br}}_2$, the oxidation state of bromine is $0$.
2. In ${\text{Br}}^{-}$, the oxidation state of bromine is $-1$.
3. In ${\text{BrO}}_3^{-}$, let the oxidation state be $x$, so $x+3(-2)=-1$, which gives $x=+5$.
4. The bromine atom is both reduced to ${\text{Br}}^{-}$ and oxidized to ${\text{BrO}}_3^{-}$.
5. Hence the answer is (A).