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Initial Rate Analysis

Determination of the Rate Equation for the Reaction between Hydrogen Peroxide and Iodide Ions

Lab Lore

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Reaction:

$2 I^{-} + H_{2} O_{2} + 2 H^{+} \to I_{2} + 2 H_{2} O$

Objective:

To determine the rate equation and order of the reaction with respect to each reactant.

Experimental Procedure:

Prepare a series of solutions with varying concentrations of iodide ions $[I^{-}]$ and hydrogen peroxide $[H_{2} O_{2}]$ , keeping the concentration of hydrogen ions $[H^{+}]$ constant.
For each solution, measure the initial rate of formation of iodine $[I_{2}]$ by monitoring the absorbance at a specific wavelength using a spectrophotometer.
Record the initial rates and corresponding concentrations of the reactants.

Data:

Experiment	[I^-] (mol/L)	[H₂O₂] (mol/L)	Initial Rate (mol/L·s)
1	0.10	0.10	1.2 × 10^-5
2	0.20	0.10	2.4 × 10^-5
3	0.10	0.20	2.4 × 10^-5

Analysis:

Determine the order with respect to iodide ions $[I^{-}]$ :

Compare Experiments 1 and 2, where the concentration of $[I^{-}]$ is doubled while $[H_{2} O_{2}]$ is kept constant. The initial rate also doubles, indicating that the reaction is first order with respect to iodide ions.

$Order with respect to [I^{-}] = 1$
Determine the order with respect to hydrogen peroxide $[H_{2} O_{2}]$ :

Compare Experiments 1 and 3, where the concentration of $[H_{2} O_{2}]$ is doubled while $[I^{-}]$ is kept constant. The initial rate also doubles, indicating that the reaction is first order with respect to hydrogen peroxide.

$Order with respect to [H_{2} O_{2}] = 1$

Overall Reaction Order:

The overall order of the reaction is the sum of the individual orders:

$Overall order = 1 (order with respect to [I^{-}]) + 1 (order with respect to [H_{2} O_{2}]) = 2$

Rate Equation:

Based on the orders determined, the rate equation for the reaction can be written as:

$Rate = k [I^{-}] [H_{2} O_{2}]$

where $k$ is the rate constant.

Conclusion:

The reaction between hydrogen peroxide and iodide ions is second order, with first-order dependence on both iodide ions and hydrogen peroxide.

This example demonstrates how initial rate analysis can be used to determine the rate equation and order of a reaction. The same approach can be applied to other reactions with different reactants and conditions.