Initial Rate Analysis

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

Reaction:

Objective:

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

Experimental Procedure:

  1. Prepare a series of solutions with varying concentrations of iodide ions and hydrogen peroxide , keeping the concentration of hydrogen ions constant.
  2. For each solution, measure the initial rate of formation of iodine by monitoring the absorbance at a specific wavelength using a spectrophotometer.
  3. 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:

  1. Determine the order with respect to iodide ions :

    Compare Experiments 1 and 2, where the concentration of is doubled while is kept constant. The initial rate also doubles, indicating that the reaction is first order with respect to iodide ions.

  2. Determine the order with respect to hydrogen peroxide :

    Compare Experiments 1 and 3, where the concentration of is doubled while is kept constant. The initial rate also doubles, indicating that the reaction is first order with respect to hydrogen peroxide.

Overall Reaction Order:

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

Rate Equation:

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

where 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.