Question

The following reduction potentials are measuredat:$\text{pH}0$

${\text{BrO}}_3^{-}+6{\text{H}}_3{\text{O}}^{+}+5e^{-}\to \frac{1}{2}{\text{Br}}_2\left(\mathcal{l}\right)+9{\text{H}}_2\text{O\hspace{1em}}{E}^{\text{o}}=1.52\text{V}$

${\text{Br}}_2\left(\mathcal{l}\right)+2e^{-}\to 2{\text{Br}}^{-}{E}^{°}=1.065\text{V}$

(a) Will bromine disproportionate spontaneously in acidic solution?

(b) Which is the stronger reducing agent at$\text{pH}0:{\text{Br}}_2\left(\mathcal{l}\right)$or${\text{Br}}_2$?

Short answer

a) The bromine would not disproportionate spontaneously in an acidic solution.

b) The species that has more negative standard potential is ${\text{Br}}^{-}$.

Step-by-step solution

The reducing agent. 

The strongest reducing agent will be the one that has the more negative standard reduction potential.

Check if bromine is disproportionate spontaneously in an acidic solution. 

a)

Write a reduction potential diagram in which the species are arranged in sequence with the most oxidized form on the left and the most reduced form on the right:

${\text{BrO}}_{-}{3}^{-}\stackrel{1.52\text{V}}{\to }{\text{Br}}_{-}2\stackrel{1.065\text{V}}{\to }{\text{Br}}^{-}$.

The standard reduction potential for each of the three half-reactions is written above the arrow. A species will undergo a disproportionation reaction when the reduction potential for the reaction that connects the species in$\left({\text{Br}}_2\right)$ to the one on the right$\left({\text{Br}}^{-}\right)$ is more positive than the reduction potential for the reaction that connects the species in the center $\left({\text{Br}}_2\right)$to the one on the left $\left({\text{BrO}}_3^{-}\right)$. In this case, the bromine would not disproportionate spontaneously in an acidic solution because the reduction potential for the half-reaction connecting the species in the center to the one on the right is less positive than the reduction potential for the half-reaction connecting the species in the center to the one on the left.

The stronger reducing agent. 

b)

The species that have more negative standard potential is the stronger reducing agent. In this case, it is ${\text{Br}}^{-}$.