The average rate of consumption of Br− is 1.06×10−4 M/s over the first two minutes

The average rate of consumption of Br− is 1.06×10−4 M/s over the first two minutes. What is the average rate of formation of Br2 during the same time interval?

Answer – In the reaction

5Br⁻(aq) +BrO₃^–(aq) +6H⁺(aq) →3Br₂(aq)+3H₂O(l)

5 bromide ions react also produce 3 bromine molecules. The rate of reaction equals the rate of disappearance of a substance divided by their stoichiometric coefficient:

r=−∆[Br−]5∆tr=−5∆t∆[Br−]​.

The similar relation is lawful for the rate of formation of the bromine:

r=∆[Br2]3∆tr=3∆t∆[Br2​]​ .

Using these two equations, we can easily compute the rate of formation of Br₂:

∆[Br2] ∆t=−3∆[Br−]5∆t=1.06⋅10−4⋅3/5=6.36⋅10−5∆t∆[Br2​]​=−5∆t3∆[Br−]​=1.06⋅10−4⋅3/5=6.36⋅10−5 M/s.

Answer: the average rate of formation of Br₂ during the same time interval is 6.36·10^-5 M/s.