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Investigating how concentration affects rate of reaction

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Introduction

Investigating How Concentration and Temperature Affect the Rate of Reaction Aim I aim to investigate how the concentration of potassium bromide affect the rate of reaction when added to a solution made up of potassium bromate(V), sulphuric acid and phenol. The general equation for the reaction between bromide and bromate ions in acidic aqueous solution is: (1) BrO3-(aq) + 5Br -(aq) + 6H+(aq) 3Br2(aq) + 3H2O(l) I am going to alter the concentrations of potassium bromide, potassium bromate and sulphuric acid to find the orders of reaction with respect to each reactant. This will allow me to prove that the rate equation for this reaction is: (1, pg 230) Rate = k[BrO3-][Br -][H+]2 I will be working out a value for the rate constant, k, and will also be investigating the affect temperature has on rate of reaction, using my results and the Arrhenius equation to work out the activation enthalpy. Theory: The colour change in this reaction is from an orange, brought about by the methyl orange indicator, to a colourless solution. As bromine is produced in the reaction it, at first, bonds to the phenol. However, when all binding sites on phenol have been used up, the presence of excess bromine will turn the solution colourless. This shows that I am measuring the time taken for all binding sites on phenol to be used up, which explains the change from orange to colourless in the solution. This can be shown in the following equation: 2C6H5OH(aq) + 3Br2(aq) 2C6H2Br3OH(aq) + 6H+(aq) + 6Br -(aq) The term 'rate of reaction' refers to how fast reactants are converted into products in a given reaction. A way to measure rate of reaction directly has not yet been discovered, and therefore we must measure the change in amount of product/reactant in a certain time. This can be shown by the following equation: (1, pg 228) Rate = Change in Property Time Taken However, orders of reaction must also be taken into account when writing a rate equation. ...read more.

Middle

To achieve cooler temperatures put ice cubes into the beaker and monitor the temperature using a thermometer. Place the boiling tubes containing the correct solutions in the water bath so that they are also heated or cooled to this temperature. 5. Mix the two solutions and start the stopwatch. Record the time taken for the solution to go colourless. Ensure that the temperature of the water is kept constant. This can be done by having a thermometer in the beaker. Although the higher temperatures should be easily maintained by the heating apparatus, lower temperatures must be kept constant by adding more ice cubes if required. 6. Repeat 5 times with each temperature to ensure accurate and fair results. Results With Respect to Potassium Bromate(V) The table below shows my results when varying the concentration of potassium bromate(V): Concentration of Potassium Bromate(V) (mol/dm-3) Time Take For the Mixture to Turn Colourless (seconds) Repeat 1 Repeat 2 Repeat 3 Repeat 4 Repeat 5 Average Reaction Rate (seconds-1) 0.01 31.9 32.2 31.7 31.8 32.0 31.92 0.0313 0.008 45.6 45.1 45.3 45.4 45.2 45.32 0.0221 0.006 62.8 63.4 62.7 62.9 63.2 63.00 0.0159 0.005 88.8 88.3 89.0 88.5 88.7 88.66 0.0113 0.004 104.5 105.2 105.6 104.9 105.2 105.08 0.0095 0.003 121.3 122.1 121.9 121.7 122.0 121.80 0.0082 Graph 1 on the next page shows the concentration of potassium bromate(V) plotted against the average time taken for the solution to turn colourless. From this graph I can see that there is a negative correlation because as concentration is increased, the time taken for the solution to turn colourless decreases. However, this graph does not provide enough information to work out the rate equation data for potassium bromate(V). For this, I had to draw up a graph of concentration against reaction rate. Reaction rate is worked out using the following equation: Reaction Rate = 1 Time This is presented in Graph 2. Graph 2 has a line of best fit which is a straight diagonal line, rather than a curve. ...read more.

Conclusion

This could be due to not washing out the apparatus thoroughly enough. Another possible reason is simply that I did not notice the colour change at the right time. This is human error but I don't think that this was very likely, as the colour change is very easy to see. I think that the most likely reason was making up the solution incorrectly, as this is most likely to affect the reaction time as greatly as the anomaly suggests. For this reason I disposed of the sulphuric acid solution and made up a fresh batch. This appears to have worked, as in my results table for sulphuric acid the values were much closer together and continued the trend, after the first repeat had been ignored. This human error can easily be avoided by ensuring that each solution is measured out very carefully. Another potential problem that I could have encountered would have been if I had added too much phenol, or had added phenol of a stronger concentration. This would mean that there were more phenol molecules. If there are more phenol molecules then the bromine made in the reaction may not exceed the amount of phenol. This would in turn mean that all of the phenol binding sites are not filled, and bromine does not get a chance to react with the methyl orange indicator. This would mean that the solution would not turn colourless. Overall, I think that my investigation was a success. I managed to prove that the rate equation for the bromine clock reaction was as follows: Rate = k[BrO3-][Br -][H+]2 This is exactly as I had predicted in my aim. I investigated the affect that temperature has on the rate of reaction, and found that the rate constant, 'k', changes with temperature. I then used this knowledge, along with the Arrhenius equation to find the activation enthalpy of this reaction. I found that the activation enthalpy in my investigation was 60.594kJmoles-1. This seems to be a reasonable value, and I can therefore conclude that I achieved all of my aims. ...read more.

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