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Introduction: The iodine clock experiment shows chemical kinetics in action. The experiment involves the reaction of hydrogen peroxide, ascorbic acid, potassium iodide and starch solution which when added together turn a blue-black colour, due to the formation of the triiodide-starch complex.
Method: Measuring and adding all of the reactants named while increasing the volume of ascorbic acid 10ųl each time in order to see the effect on the time taken for the solution to turn blue-black.
Results: The time taken for the colour change reaction to occur is seen to increase as the volume of ascorbic acid within the solution increases.
Discussion and Conclusion: The experiment was successful, and the predicted trend occurred in the results despite the fact there were a few factors which could have skewed the data.
The iodine clock experiment is a reaction which shows chemical kinetics in action (Lin, 2009). The experiment involves the reaction of hydrogen peroxide, ascorbic acid, potassium iodide and starch solution.
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Once all of the reactants are added, after a short time delay, the solution instantly turns a blue-black colour, due to the formation of the triiodide-starch complex. (Science Buddies Staff, 2018)
The reactions occur in two steps, the first is where triiodide is produced: 3I- + 2H+ + H2O2 → I3- + 2H2O. The second step is where the starch is added to form the blue-black solution: I3- + starch blue-black complex (Clark, 2019). When adding ascorbic acid, the triiodide ion is converted back into the iodide ion which is unable to form a complex with the starch solution as this is a dynamic equilibrium (Oliveira and Faria, 2005): Ascorbic acid + I3- Dehydroascorbic acid + 3I- + 2H+.
Therefore, the solution will not turn blue-black until the ascorbic acid is exhausted and sufficient triiodide is left for the starch to react with it. (Ferrier, 2018)
Thus, the aims of this experiment were to examine the effect of changing the volume of ascorbic acid on the reaction time of the iodine clock experiment then predict the volume of ascorbic acid required for a specific reaction time. The hypothesis for this work was that the greater the volume, and therefore the concentration, of ascorbic acid the longer the reaction time will be for the solution to turn blue-black.
The materials required were, 10 ml 3% hydrogen peroxide solution, 10 ml 1% potassium iodide solution, 5 ml 50 mm ascorbic acid, 5 ml starch water (starch solution), deionised water, 2 test tubes, a stopwatch or stop clock, a p1000 micropipette and several blue tips and a p200 micropipette and several yellow tips.
At first 7 test tubes were labelled with the numbers 1 to 7; then this step was repeated 2 more times so that there were 3 test tubes with the same numbers on them.
In the first test tube labelled 1, in immediate succession, 250ųl of hydrogen peroxide, 250ųl of starch solution, 250ųl of potassium iodide and 50ųl of ascorbic acid were all added. As soon as the ascorbic acid was added the stopwatch was started. The stopwatch was then stopped once the solution turned a blue-black colour. The time taken for the colour change to occur was then recorded and this step was repeated 2 more times for the remaining test tubes labelled 1.
After this, the test tubes labelled 2 were taken and the above steps were repeated however this time adding 60ųl of ascorbic acid instead of 50ųl; while the rest of the reactants were kept the same. The time taken for the solution to turn a blue-black colour for all 3 of the test tubes were recorded.
This was then repeated for all the test tubes while each time adding 10ųl extra of ascorbic acid when a higher numbered test tube is used.
Next, the mean reaction time for each of the increases of ascorbic acid were calculated and plotted on a graph with ascorbic acid on the x-axis and the reaction time on the y-axis. Then a line of best fit was drawn through the majority of the plotted points. After this from 52 seconds on the y-axis draw a line was drawn across until it touched the line of best fit drawn, then a line was drawn down in order to predict the volume of ascorbic acid required for the rate of reaction to be 52 seconds.
The results of the experiment show that there is a gradual increase on the reaction time of the iodine clock experiment as the volume of ascorbic acid is increased. The graph shows a clear linear increase on the data when a line of best fit is drawn; when the volume of ascorbic acid was 50ųl the reaction time was 30 seconds whereas when the volume was 80ųl the reaction time was 50 seconds and when the volume was 100ųl the reaction time was 71 seconds.
When using the graph to calculate the volume required to control the reaction to 52 seconds the graph shows 76ųl and when trying this volume out a reaction time of 50.7 to 3 significant figures was calculated.
The aim of the experiment was to examine the effect of changing the volume of ascorbic acid on the reaction time of the iodine clock experiment and from the experiment being carried out it is clear that there is an increase in the reaction time as the volume of the ascorbic acid is increased. This also supports the hypothesis that the greater the volume, and therefore the concentration, of ascorbic acid the longer the reaction time will be for the solution to turn blue-black.
Although the results of the experiment were as expected there are many factors that make the experiment planned and carried out unreliable. One example is the intervals between each reactant being added to the test tube, due to it being put in one after the other it is hard to specifically say the time difference between each reactant being added and this could have varied the results. As well as this although the same person stopped the stopwatch once the solution turned blue-black, due to the reaction being instantaneous it is hard to specifically say exactly when it turns blue-black as there is a slight graduation within the blue-black solution where it becomes slightly darker.
If repeating the experiment, it would be a good idea to have a continuous stopwatch running in the background so that reactants can be added every 2 or 3 seconds and give it a framework rather than just putting the reactants in as soon as possible. As well as this, another improvement could be allowing the same person who adds the last reactant, ascorbic acid, to also be the person starting the stopwatch. Human error plays a big part of this experiment therefore to reduce that the same person should carry out the same tasks throughout the experiment, such as, the same person should pipette throughout the experiment or the same person should stop the stopwatch each time.
When designing my experiment, the main skill I learnt was organisation, this is due to the fact the designing process was carried out between 3 people therefore delegating tasks and meeting deadlines were important as they impacted the whole group.
Additionally, another skill I learnt was the ability to manipulate a plan/method to fit the materials I was given. I was told that I was going to have no more reactants than the amounts specified in the materials section therefore the volumes of the reactants I used each time had to be calculated so that I did not run out. I also made sure to set aside a small amount in order to be able to do any extra repeats due to unreliable data.
Another skill learnt was the ability to design a data collection table, as this meant I had to decide what was important information to collect and decide on the dependant and independent variables of the experiment which in turn influences the whole experiment. Designing the table was difficult as it meant I needed to decide how many repeats or how many pieces of data I was going to collect when carrying out our experiment. The table I decided on can be seen below.
When carrying out my experiment a skill I practised was pipetting, I made sure to use the correct method each time and change tips when I changed reactants so that no contamination occurred.
Furthermore, another skill I learnt was working as a group to systematically get things done. Within the group we gave everyone a specific role in order to ensure the experiment was systematic and reduced of human error. This also meant that when one person was doing one thing another person was able to prepare the next step in order to ensure the steps were followed smoothly.
The main skilled I learned while carrying out the experiment was the ability to adapt my experiment and redefine it until a near to ideal result was given then record this. For example, we had no basis given in order to plan an appropriate start volume for ascorbic acid therefore I had to do a few trials of the iodine clock experiment to form a basis. Another thing that enhanced my adaptation skills was the fact that in the beginning we were given a weak starch solution, therefore, we did not receive the desired effect from our experiment. However, we were able to get a hold on stronger starch solution which gave the desired effect by changing the solution blue-black.
Investigation of Ascorbic Acid Concentration Effect on Iodine Clock Reaction Kinetics. (2024, Feb 22). Retrieved from https://studymoose.com/document/investigation-of-ascorbic-acid-concentration-effect-on-iodine-clock-reaction-kinetics
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