Aim: To calculate the activation energy (EA) for the reaction between Br- and BrO3- in acid solution

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Title: Calculation of Activation Energy of a Reaction

Aim: To calculate the activation energy (EA) for the reaction between Br- and BrO3- in acid solution

Introduction: Using the kinetic theory and probability theory, Maxwell and Boltzmann showed that the fraction of molecules with energy greater than EA was given by:

Where: R is the gas constant = 8.314472  −1 −1 (in this experiment 2 decimal places will be used)

        T is the absolute temperature

        e is the exponential function

        EA is the activation energy of a reaction

This suggests that at a given temperature

The rate of reaction  

But as k, the rate constant for a reaction, is measured of the rare of reaction, we can write:

k  

Savante Arrhenius developed this equation as:

K=  , also known as the Arrhenius equation

This equation allows us to determine the activation energy of a reaction from the temperature dependence of its rate constant. Usually this is based on the above equation, rearranged by taking natural logarithms:

lnK=  + lnA

In this experiment the following reaction will take place:

5Br-(aq) + BrO3- (aq) + 6H+  3Br2(aq) + 3H2O(l)

The reaction will take place in the presence of a fixed number of phenol and a small amount of methyl red indictor. The bromine produced will react with the phenol, until all the phenol has been consumed. At this point the bromine will bleach the indicator – so  sudden change in the colour of the indicator is used as a measurable “end point” for this reaction. By carrying out this reaction at different temperatures the time taken to react this end point in each case can be recorded.

If we let the concentration of Br2 (from start to end point) = c

(and note this is a constant in the reaction at different temperatures)

And the time taken for this change = t

Then:                k  α reaction rate = change in concentration/ time = c/t

Substituting into the equation:                 ln K=  + ln A

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Gives us the following equation:                 ln(c/t) =  + ln A

Or                                                 ln t =  +ln e – ln A

As both ln c and in ln A are contants, this is the equation of a straight line. Experiment data can thus be used to generate this straight line and use it to calculate activation energy.

Hypothesis: As the temperature increases (with the concentration kept at a constant) the rate of the reaction increases, as the temperature decreases (with the concentration kept at a constant) the rate of the reaction decreases.

Procedure: This experiment was done in pairs. First 10 cm3 of ...

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