Consider the oxidation of iodide ion by hydrogen peroxide in an acidic solution.
${\mathrm{H}}_2{\mathrm{O}}_2\left(aq\right)+3{\mathrm{I}}^{-}\left(aq\right)+2{\mathrm{H}}^{+}\left(aq\right)\to {\mathrm{I}}_3^{-}\left(aq\right)+2{\mathrm{H}}_2\mathrm{O}\left(l\right)$
The rate of formation of the brown-coloured tri-iodide ion $\mathrm{\Delta }\left(I_3^{-}\right)/\mathrm{\Delta }t$  can be
determined by measuring the rate of appearance of the colour. Following are initial
rate data at 25°C.

Experiment	Initial $\left[{\mathrm{H}}_2{\mathrm{O}}_2\right]$	Initial $\left[I^{-}\right]$	Initial rate of formation of $I_3^{-}(M/S)$
1	0.100	0.100	$1.15\times {10}^{-4}$
2	0.100	0.200	$2.30\times {10}^{-4}$
3	0.200	0.100	$2.30\times {10}^{-4}$
4	0.200	0.200	$4.60\times {10}^{-4}$

Solution:

The rate of formation of the brown-coloured tri-iodide ion ΔI3−/Δt\Delta \text{I}_3^-/\Delta tΔI3−/Δt can be determined by measuring the rate of appearance of the colour. Given the initial rate data at 25°C, we can analyze how the concentration of H2O2\text{H}_2\text{O}_2H2O2 and I−\text{I}^-I− affect the rate.

From the data:

1. Experiment 1: [H2O2]=0.100[ \text{H}_2\text{O}_2 ] = 0.100[H2O2]=0.100 M, [I−]=0.100[ \text{I}^- ] = 0.100[I−]=0.100 M, Initial rate = 1.15×10−41.15 \times 10^{-4}1.15×10−4 M/s
2. Experiment 2: [H2O2]=0.100[ \text{H}_2\text{O}_2 ] = 0.100[H2O2]=0.100 M, [I−]=0.200[ \text{I}^- ] = 0.200[I−]=0.200 M, Initial rate = 2.30×10−42.30 \times 10^{-4}2.30×10−4 M/s
3. Experiment 3: [H2O2]=0.200[ \text{H}_2\text{O}_2 ] = 0.200[H2O2]=0.200 M, [I−]=0.100[ \text{I}^- ] = 0.100[I−]=0.100 M, Initial rate = 2.30×10−42.30 \times 10^{-4}2.30×10−4 M/s
4. Experiment 4: [H2O2]=0.200[ \text{H}_2\text{O}_2 ] = 0.200[H2O2]=0.200 M, [I−]=0.200[ \text{I}^- ] = 0.200[I−]=0.200 M, Initial rate = 4.60×10−44.60 \times 10^{-4}4.60×10−4 M/s

From these experiments, it's observed that doubling the concentration of either H2O2\text{H}_2\text{O}_2H2O2 or I−\text{I}^-I− doubles the initial rate of formation of I3−\text{I}_3^-I3−. Therefore, the reaction is first order with respect to both H2O2\text{H}_2\text{O}_2H2O2 and I−\text{I}^-I−.