Aim: Using an iodine clock reaction to find the order of hydrogen peroxide and Ethanoic acid present in that experiment.

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Aim: Using an iodine clock reaction to find the order of hydrogen peroxide and Ethanoic acid present in that experiment.

Relevant Chemicals Equations

Below are the two steps of the reaction. The reactants in red are the reactants whose volumes were changed, and are the reactants that made up solution B. The other reactants in both equations are the substances which make up solution A.

H2O2(aq) + 3 I(aq) + 2 H+ = I3 + 2 H2O Slow

I3(aq) + 2 S2O32−(aq) = 3 I(aq) + S4O62− Fast

In the equations above, the hydrogen peroxide and H+ ion (from CH3COOH) causes the iodine ions to combine into an I3 molecule. The iodine does not immediately react with the starch since it is paired with potassium, (which in this case is a spectator). The slow reaction produces I3-, which then reacts with sodium thiosulphate to produce individual I- ions, which then bind with the starch, causing the dark blue – black coloring of the solution.



  1. Collect all necessary apparatus and chemicals
  2. Measure out 20ml of solution A using a measuring cylinder and pour it into a beaker marked “Solution A”. Solution A consists of starch solution, sodium thiosulphate and potassium iodide.
  3. Measure out the appropriate amount of H2O2, acid and water depending on that experiment. The reactant (either H2O2 or CH3COOH) whose volume is being varied is one component, the other reactant (either H2O2 or CH3COOH) is another, and water should be added in order to make the total solution have a volume of 20 ml.
  4. Add H2O2, the acid and the water together into one beaker, labeled “Solution B”.
  5. Pour one solution into a larger, marked beaker marked to indicate it is the beaker where the reaction takes place.
  6. Pour the second solution into the larger beaker and start the timer as soon as the second solution hits the first solution.
  7. Stop the time when the entire solution has turned black.
  8. Repeat with different concentration of the liquids making up solution B.
  9. Solutions of different concentrations can be created by increasing the amount of reactant that is being changed by two. An example of this is while changing the volume of H2O2; the other reactants’ volumes should be held constant, and H2O2’s volume should increase by 2 for each experiment.
  10. Carry out the experiment with volumes of 2, 4, 6, 8 and 10 for both H2O2 and CH2COOH while keeping solution A at 20ml and either H2O2 or CH3COOH at 10, depending which reactant is being varied.
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Data Collection and Processing

Table Showing The Time Taken for Each Different Type of Solution to React

As the results above show, as the volume (which is used here instead of concentration, but they are equivalent) of the H2O2 is doubled, the rate falls, but not at a constant or perfectly exponential rate. This is probably ...

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