Method
In each experiment, the independent variable is the concentration of the substance which is being changed. The dependant variable is the rate of reaction, shown by the time taken for the solution to become colourless. Controlled variables are the total volume of the solution, temperature and the concentration of the other two substances.
Experiment 1: Changing the concentration of Iodine
- Measure out 35ml of propanone in the 50ml measuring cylinder and 10ml of sulfuric acid in the 25ml measuring cylinder. Measure out 5ml of the 0.0142M solution of Iodine in the 10ml measuring cylinder.
- Place the clean conical flask on the white tile and keep the stopwatch at the ready. Firstly, add the 35ml propanone, followed by the 10ml sulfuric acid and finally the 5ml iodine.
- When the last drop of iodine is in the flask, start the stopwatch.
- Swirl once and leave the solution to react, observing from above.
- Once the solution is colorless, stop the stopwatch and record the time.
- Repeat the procedure with clean apparatus using 0.0038M, 0.0076M and 0.0114M concentrations of iodine. Two trials should be taken for each concentration of iodine used.
Experiment 2: Changing the concentration of H2SO4
- Measure out 10ml of propanone in the 25ml measuring cylinder and 5ml of the 0.0038M iodine in the 10ml measuring cylinder. First take 35ml of sulfuric acid using the 50ml measuring cylinder. Note the total volume of the solution will always add up to 50ml.
- Pour the propanone and sulfuric acid in to the conical flask, placed on a white tile. Next add the iodine, starting the stopwatch as soon as the last drop is added.
- Swirl once and leave the solution to react, observing from above.
- Once the solution is colorless, stop the stopwatch and record the time.
- 4 other different concentrations of sulfuric acid should be taken, with 2 trials for each concentration taken.
- The other concentrations will be made by adding distilled water, with the total volume of sulfuric acid remaining the same. 30ml sulfuric acid and 5ml water, then 25ml acid and 10ml water, the 20ml acid and 15ml water, then 15ml acid then 20ml water.
Experiment 3: Changing the concentration of propanone
- Measure out 10ml of sulfuric acid in the 25ml measuring cylinder and 5ml of the 0.0038M iodine in the 10ml measuring cylinder. First take 35ml of propanone using the 50ml measuring cylinder. Note the total volume of the solution will always add up to 50ml.
- Pour the propanone and sulfuric acid in to the conical flask, placed on a white tile. Next add the iodine, starting the stopwatch as soon as the last drop is added.
- Swirl once and leave the solution to react, observing from above.
- Once the solution is colorless, stop the stopwatch and record the time.
- 4 other different concentrations of propanone should be taken, with 2 trials for each concentration taken.
- The other concentrations will be made by adding distilled water, with the total volume of propanone remaining the same. 30ml propanone and 5ml water, then 25ml propanone and 10ml water, the 20ml propanone and 15ml water, then 15ml propanone then 20ml water.
Data Collection
Experiment 1: Changing the concentration of Iodine
The four concentrations of iodine were already given to me, along with the 1M solutions of propanone and H2SO4. I worked out number of moles using the formula: Moles = concentration x volume
Observations: The time taken for the reaction was similar for all concentrations of iodine, and therefore the average rate.
Experiment 2: Changing the concentration of H2SO4
Experiment 3: Changing the concentration of Propanone
Data Presentation
To determine the order of the reaction, graphs of rate with respect to concentration were plotted for each experiment
Experiment 1
Experiment 2
Experiment 3
Errors
Measuring cylinder (50ml): ±0.5
Measuring cylinder (25ml): ±0.25
Measuring cylinder (5ml): ±0.05
Stopwatch: ±0.01
To find the total percentage error in each reading of the trial, we must add the total percentage error of the stopwatch to the total percentage error of the measuring cylinder. To find the percentage error of each measurement, divide the error in the measuring cylinder by the reading and multiply by a hundred.
First Experiment
Second Experiment
Third Experiment
Conclusion
The order of reaction for each reactant can be concluded from the shape of the graphs, and therefore the overall order of the reaction.
For zero order, the graph should have no slope
For first order, the graph should have a straight line
For second order, the graph should give a curve
As iodine had no slope, and both propanone and sulfuric acid gave straight-line graphs, I can conclude that the order for each of the reactants is as follows:
Iodine: zero order
Propanone: first order
Sulfuric acid: first order
Overal order = 1 + 1 + 0 = 2
The rate expression can therefore be defined as
Rate = k [CH3COOH] [H2SO4]
Evaluation
Since the rate expression found from my calculations qualified with the theoretical rate expression, my experiment was successful in its aims. There was, however, room for improvement to reduce the margin for error:
Systematic Errors
- These produce results that fluctuate by a fixed amount from the true results. This can be viewed in the graphs for changing the concentration of propanone as well as sulfuric acid, as the line of best fit did not pass through the origin. This could have been avoided by checking that all measuring instruments were correctly calibrated.
Random Errors
- These are dependant on the precision of the measuring instruments, so using stopwatches and measuring cylinders to a smaller degree would reduce this, also not eliminating it completely.
- Error in time could also have been minimized by taking by more trials to obtain a more accurate value overall.
- Measuring from the bottom of the meniscus each time could have also reduced parallax error due to the measuring cylinder.