MediumChemistryHigh School (11–12)Chemical KineticsNumerical

Bromate ions react with bromide ions in acid: BrO₃⁻ + 5Br⁻ + 6H⁺ → 3Br₂ + 3H₂O…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Bromate ions react with bromide ions in acid: $\text{BrO}_3^- + 5\text{Br}^- + 6\text{H}^+ \rightarrow 3\text{Br}_2 + 3\text{H}_2\text{O}$ . The initial rates were measured at a fixed temperature. The numbers are chosen for practice.

Run 1: $[\text{BrO}_3^-] = 0.10\ \text{mol/L}$ , $[\text{Br}^-] = 0.10\ \text{mol/L}$ , $[\text{H}^+] = 0.10\ \text{mol/L}$ . Rate $2.1 \times 10^{-4}\ \text{mol L}^{-1}\ \text{s}^{-1}$ .
Run 2: $[\text{BrO}_3^-] = 0.20\ \text{mol/L}$ , the others as in run 1. Rate $4.2 \times 10^{-4}\ \text{mol L}^{-1}\ \text{s}^{-1}$ .
Run 3: $[\text{Br}^-] = 0.30\ \text{mol/L}$ , the others as in run 1. Rate $6.3 \times 10^{-4}\ \text{mol L}^{-1}\ \text{s}^{-1}$ .
Run 4: $[\text{H}^+] = 0.20\ \text{mol/L}$ , the others as in run 1. Rate $8.4 \times 10^{-4}\ \text{mol L}^{-1}\ \text{s}^{-1}$ .

Find the order in each reactant, the overall order and the rate constant.

Need a reminder? The key ideaShow

The rate law is $\text{rate} = k[\text{A}]^x[\text{B}]^y$ , and the orders come from experiment. When only $[\text{A}]$ changes by a factor $f$ , the rate changes by $f^x$ . For overall order $n$ , the unit of $k$ is $\text{L}^{n-1}\ \text{mol}^{1-n}\ \text{s}^{-1}$ .

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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