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Investigating the Rate of the Reaction between Bromide and Bromate Ions in Acid Solution

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Introduction

Investigating the Rate of the Reaction between Bromide and Bromate Ions in Acid Solution In this investigation, I aim to fully investigate the factors affecting the rate of the reaction between bromide ions and bromate ions in acid solution. The equation of this reaction is given below: 5Br-(aq) + BrO3-(aq) + 6H+(aq) --> 3Br2 (aq) + 3H2O(l) (Equation 1.0.1) I will attempt to find the rate equation for the reaction, in the form: (Equation 1.0.2) where k is the rate constant and x, y, and z are the orders of reaction with respect to Br-, BrO3- and H+ respectively. I will also attempt to find a suitable catalyst for the reaction, as well as the activation enthalpy for the reaction with and without a catalyst. The reaction is a redox reaction: the bromide is oxidised to bromine and the bromate is reduced to bromine. This can be shown by the oxidation states of bromide, bromate and bromine: 5Br-(aq) + BrO3-(aq) + 6H+(aq) --> 3Br2 (aq) + 3H2O(l) Oxidation State: -1 +5 -2 +1 0 +1 -2 I shall be using solutions of Potassium Bromide (KBr) and Potassium Bromate (KBrO3) as sources of bromide and bromate ions for the reaction, and Sulphuric Acid (H2SO4) as a source of H+ ions. A solution methyl orange shall be used as an indicator, which will change colour from pink (in acid solution) to colourless when Bromide ions are produced. In order to be able to measure the amount of time it takes for the indicator to change colour, a small amount of Phenol (C6H5OH) is added to the solution. Phenol reacts instantly with any Bromine produced, returning bromide ions: 3Br2 (aq) + C6H5OH(aq) --> C6H2Br3OH(aq) + 3H+(aq) + 3Br-(aq) (Equation 1.0.3) As soon as all the Phenol has reacted with the Bromine produced, the excess Bromine causes the methyl orange to change colour. The time it takes for this to happen is proportional to the initial rate of the reaction between Bromide and Bromate ions. ...read more.

Middle

Firstly, using the results for varying the concentration of a reactant, I will take an average of the three times that are within the 10% boundary (ignoring outlying results). I will then draw up a table for each of the varied reactants ([reactant] means the concentration of that reactant): [reactant] /mol dm-3 Average time (t) /s /s-1 (Table 2.7.1) The rate of the reaction is proportional to , so a graph of against concentration will produce the same result as a rate/concentration graph. Plotting the graph will give me the order of the reaction with respect to that reactant, which then leads to the rate equation for the reaction (see above). Using a computer spreadsheet, I can calculate a value for the Pearson's Product Moment Correlation Coefficient (PMCC) for against the concentration of each reactant, which is a number between -1 and 1 to indicate the level of linear correlation between two variables. If the value is close to 1, it suggests strong linear correlation, so in this context this would suggest that there is strong evidence that a reaction is first order.xii I can then use the results for varying the temperature to find the activation enthalpy of the reaction. Taking averages of the times and temperatures of the three concordant results I have found, I will then find a value for the rate constant k at each temperature. Since the rate equation will be of the form: (Equation 2.7.1) I can rearrange this to find a value for k at each temperature: (Equation 2.7.2) So to find k I will have to divide the rate of reaction by the product of the concentrations, each to their respective powers x, y and z, that I have chosen to use when varying the temperature. I will then take the natural logarithm of each value of k and write these in a table (The unit of k depends upon the rate equation and hence the order of the reaction with respect to each reactant): Average Time (t) ...read more.

Conclusion

I would also expect the activation enthalpy to be around 50 kJ mol-1, since the majority of reactions that take place at room temperature do. The only part of my investigation that I do not believe produced sufficiently good results was the testing for catalysts, as has been explained above. Whilst I can conclude that iron (III) acts as a catalyst for the reaction, I cannot conclude anything about the affect of the other transition metal ion solutions that I tested, since they did not affect the reaction, whereas I expected more than one of them to have a catalytic effect. 4.6 - Suggestions for Improvements to the Method Whilst my method produced reliable results, there are still ways in which it could be improved to reduce the possibility of error in the investigation. As has been discussed above, the greatest source of error in my investigation was probably the determination of the point at which the colour of the solution changed. To improve this I could have taken a high resolution photograph of my solution at the point of colour change during a preliminary experiment, and used this to determine the end point in all subsequent experiments, ensuring that the end point was exactly the same in all experiments. If I had more resources available to me, there are many ways in which I could have reduced the errors. To reduce the problems of temperature fluctuation, I could have carried out all experiments in a laboratory with a controlled temperature. This would also have allowed me to better control the temperature in the experiments in which I varied the temperature. I could also have used pipettes instead of burettes to measure out the solutions required for each experiment. This would have slightly reduced the error, but would have been unfeasible given the extra time it would have taken. In summary, I believe that my investigation has been a success. I have fulfilled the aims of my investigation and I can be confident that the results are reliable. ...read more.

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