A-Level Investigation - Rates of Reaction – The Iodine Clock

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A-Level Investigation

Rates of Reaction – The Iodine Clock

Aim: carry out investigations to determine the rate equation for the oxidation of iodide ions by hydrogen peroxide in acid solution.  Investigate the effect of various transition metal catalysts on the reaction rate.

Introduction to research

The research of this project involved investigation of a range of scientific literature available from Oxford University’s online science library, various chemistry texts and other Internet sources.

The Iodine clock reaction

This reaction has been used repeatedly down the years in the study of reaction rates.  Most famously by Harcourt and Esson in the late 18th – early 19th century.  The reaction between hydrogen peroxide and iodide ions is still known as the Harcourt-Esson reaction.

The Iodine Clock reaction can be represented by the half equations:

                  H2O2 (aq) + I- (aq)  →  IO- (aq) + H2O (l)                         Slow  [1]

               IO- (aq) + H3O+ (aq)    HIO (aq) + H2O (l)                         Fast [2]

HIO (aq) + H3O+ (aq) + I- (aq)  →  I2 (aq) + 2H2O (l)                           Fast [3]

Overall:

        2H+ + H2O2 + 2I-   →  2H2O + I2

Iodide ions are oxidised to diatomic iodine molecules.  These are soluble in water and will turn it a pale brown color.  In this experiment the presence of starch will emphasize the color change at the end-point of the reaction by forming a deep blue complex with the iodine.

Use of Sodium Thiosulphate  [ Na2S2O3 ]

Sodium Thiosulphate(VI) is a reducing agent.  In the iodine clock reaction, it plays the following part:

Half equations;  

   2S2O32- (aq)  →  S4O62- (aq) + 2e-

   I2 (aq) + 2e-  →  2I- (aq)

   

Overall:  

2S2O32- (aq) + I2 (aq)  →  S4O62- (aq) + 2I- (aq)

The Thiosulphate has the effect of reducing iodine to iodide ions.  Since the Iodine Clock Reaction is basically the conversion of iodide ions to iodine, the Thiosulphate actually does the reverse.  The function of the Thiosulphate is to slow the reaction down, as without the Na2S2O3 the reaction would prove to be too fast to provide suitable results.

Introduction to Reaction Rates

Increasing the concentration of a reactant normally increases the rate of reaction, but this is not always the case.  The different reactants can affect the particular reaction in different ways e.g.: doubling concentration can double or even quadruple a reaction rate, but it could also have no effect whatsoever.

In a reaction with more than one step the rate will be dictated by the rate of the slowest stage in the reaction mechanism – this is called the rate-determining or rate-limiting step.

In a reaction; if the doubling of the concentration of a reactant has the effect of doubling the reaction rate – it is said to be first order with respect to the concentration of that chemical.  If a doubling of concentration causes the rate to quadruple - then it is referred to as second order.  Similarly, if the increase in concentration has no effect, then it is known as zeroth order with respect to that reactant.

The order with respect to a particular reagent is known as a singular order, however, the overall order of a reaction is found by adding together all the singular order involved.

E.g.

  In the reaction   A + B    C

Doubling the concentration of reactant A doubles the rate ∴First order w.r.t. [A]

Doubling the concentration of reactant B quadruples the rate ∴second order w.r.t. [B]

The rate equation for this reaction can therefore be written as:

Rate of Reaction  [A]1[B]2  =  k  [A] [B] 2

 

The overall order would be equal to three.

To determine the order of a reaction by graphical means:

By performing an experiment repeatedly, varying the concentrations of a particular variable whilst keeping the other volumes and conditions constant, it is possible to determine the order of reaction with respect to that chemical.  When enough values of reaction time are obtained a graph may be plotted: χ-axis concentration of reactant, y-axis 1/reaction time (rate of reaction).  From the shape of this graph it is possible to determine the order of the reaction with respect to that reactant.  For example:

Figure 1.1

Zeroth Order

First Order

Second Order

Prediction of Rate Equation

For the Iodine Clock Reaction I predict that the reaction will be;

Zero order with respect to [H+]

First order with respect to [H2O2]

First order with respect to [I-]

Therefore I predict that the overall rate equation will be:

Rate =   k [ H+ ]0 [ H2O2 ]1 [ I- ]1

Which gives an overall reaction order of two.

Variables

In this investigation it is necessary to investigate the effect of varying the concentration of three chemicals in the Iodine Clock Reaction.  I am therefore required to perform three different experiments, changing the concentration of a single variable in each one.  This will mean that the variables, independent and controlled, will change in each ‘set’.  All experiments should be performed at standard temperature and pressure (S.T.P.) – 298K and 1 ATM pressure.

For example;

When varying the volume of Hydrogen Peroxide in the mixture it will be the independent variable, the controlled variables will be the volumes of the Thiosulphate, Potassium Iodide and Sulphuric Acid.  The overall volume will be kept constant by the addition of a measured volume of distilled water that will make the solution up to 200ml (including 1ml of 1% starch solution).  The dependant variable will obviously be the time taken for the reaction to reach end point (signified by the solution color change clear to deep blue).

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Expt. (1)

Independent variable:

Volume Potassium Iodide

Controlled variables:

Volume Thiosulphate

Volume Hydrogen Peroxide

Volume Sulphuric Acid

Temperature

Total solution volume

Volume starch

Pressure

Dependent variable:

Time

Expt. (2)

Independent variable:

Volume Hydrogen Peroxide

Controlled variables:

Volume Thiosulphate

Volume Potassium Iodide

Volume Sulphuric Acid

Temperature

Total solution volume

Volume starch

Pressure

Dependent variable:

Time

Expt. (3)

Independent variable:

Volume Sulphuric Acid

Controlled variables:

Volume Thiosulphate

Volume Potassium Iodide

Volume Hydrogen Peroxide

Temperature

Total solution volume

Volume starch

Pressure

Dependent variable:

Time

...

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