Variables
- Independent Variable : Concentration of iodide solution (or concentration of KI solution used)
- Dependent Variable : Time taken for formation of blue-black solution, rate of reaction
iii) Fixed Variable : Volume of sodium peroxodisulphate Na2S2O8 solution used, volume of
sodium thiosulphate Na2S2O3 solution used, volume of starch
solution used, volume of iodide I- solution used
Pre-Experiment Preparation / Method Designing
Using trial and error approach, a pre-experiment exercise was performed to determine the suitable amount of solute and concentration of chemical solutions used. Most notably, the amount of sodium thiosulphate used was adjusted several times to produce a reasonable delay.
A trial run was performed to identify possible weaknesses and shortcomings, and thus in the process, design actual experimental procedures.
Control of Variables
Concentration of iodide solution
The concentration of iodide solution is equivalent to the concentration of potassium iodide KI solution since potassium ions are spectator ions. The concentration of iodide solution would be manipulated, acting as independent variable.
Using a standard KI 1.0 mol dm-3 solution as stock solution, a dilution will be performed to produce solutions of varying concentrations (0.8, 0.4, 0.2, and 0.1 mol dm-3).
Time taken for blue-black solution to form
The time taken for the formation of blue-black solution will be measured using a digital stop watch to reduce uncertainty. The time taken is therefore the dependent variable.
To reduce random errors, three repetitions will be performed for each set of concentration.
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Rate of reaction
The rate of reaction is the reciprocal of time taken for formation of blue-black solution. Since there are three repetitions for each set of concentration, the rate of reaction is the reciprocal of the average time taken. Primarily, this is the dependent variable being studied.
Volume of sodium peroxosulphate Na2S2O8 solution used
The volume of sodium peroxodisulphate will be fixed at 5 cm3 throughout the experiment.
Volume of sodium thiosulphate Na2S2O3 solution used
The volume of sodium thiosulphate used will be fixed at 2 cm3 throughout the experiment.
Volume of starch solution used
The volume of starch solution used will be fixed at 5 drops throughout the experiment.
Volume of iodide I- solution used
The volume of iodide solution will be fixed at 10 cm3 throughout the experiment. The volume is
related to concentration through the equation Ž ₃ ₃ŧ where M is concentration, V is volume
???
and n is amount of solute.
Since the manipulated variable is the concentration of iodide solution, it is imperative that the volume of iodide solution be made constant.
Apparatus / Materials
Pipette (25.00 ± 0.03) cm3, pipette (10.00 ± 0.02) cm3, pipette (5.00 ± 0.01 ) cm3, burette (50.00 ± 0.05) cm3, micropipette (1.000 ± 0.003) cm3, digital stop watch (± 0.01s) , electronic balance (± 0.001g), sodium peroxodisulphate Na2S2O8 0.04 mol dm-3 solution, standard potassium iodide KI 1.0 mol dm-3 solution, sodium thiosulphate Na2S2O3 0.05 mol dm-3solution, starch solution, test tubes, test tube rack, beakers, volumetric flasks, conical flask, distilled water, retort stand with clamp, pipette filler, spatula, dropper
Experimental Procedures
i) Preparation of chemical solutions
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To prepare 1000 cm3 of standard potassium iodide KI 1.0 mol dm-3 solution, 166.000g of KI is needed. However, to prevent wastage, only 83.000g (to 3 d.p) was instead used to produce 500 cm3 of standard potassium iodide KI 1.0 mol dm-3 solution. After weighing, the weighted mass of KI solute was
diluted with distilled water to produce 500 cm3 of standard KI 1.0 mol dm-3 solution.
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A dilution was then performed using the standard KI solution as stock solution. For concentration 0.8 mol dm-3, 80 cm3 of stock solution was transferred into a 100 cm3 container using a burette. Using a second burette, 20 cm3 of distilled water was added. For concentration 0.4, 0.2 and 0.1 mol dm-3, the stock to distilled water ratios are 4:6, 2:8, and 1:9 respectively.
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Figure 1.1 Stock to distilled water ratios for different concentrations
3. To prepare 250 cm3 of sodium peroxodisulphate Na2S2O8 0.04 mol dm-3 solution, 2.381g of Na2S2O8 was weighed on the balance. After weighing, all the solute was diluted and transferred into a volumetric flask. Distilled water was added drop-wise till the calibration mark.
4. To prepare 250 cm3 of sodium thiosulphate Na2S2O30.05 mol dm-3solution, 1.977g of Na2S2O3was weighed on the balance. After weighing, all the solute was diluted and transferred into a volumetric flask. Distilled water was added drop-wise till the calibration mark.
ii) The Iodine Clock Reaction
- Three test tubes were labeled A, B, and C respectively.
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Using a pipette (10.00 ± 0.02) cm3, 10 cm3 of standard potassium iodide KI 1.0 mol dm-3 solution was transferred into test tube A.
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Using a pipette (5.00 ± 0.01) cm3, 5cm3 of sodium peroxodisulphate Na2S2O8 0.04 mol dm-3 was transferred into test tube B.
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Using a micropipette (1.000 ± 0.003) cm3, two batches of 1cm3 of sodium thiosulphate Na2S2O3 0.05 mol dm-3 were transferred into test tube C. Sodium thiosulphate acts as a delaying mechanism.
- Solutions from test tube A and C were added together in a conical flask. Then, 5 drops of starch solution were added to allow the formation of blue-black complex in the presence of iodine.
- The mixture in the flask was swirled to ensure a homogenous mixture. As the solution from test tube B was added into the flask, the digital stop watch was started instantaneously.
- The flask was swirled gently as the formation of blue-black complex was observed. Once the blue-black solution was formed, the stop watch was stopped.
- The time taken was recorded in a table. The experiment was then repeated to obtain two more readings.
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Step 1 till 8 were repeated using different concentrations of iodide solution in the order of 0.8, 0.4, 0.2, and 0.1 mol dm-3.
CALCULATION
10. The average time TAVE was calculated using the formula:
₃§AVE₃ ₃₃ şÎŏşŝŏş₃ .
₃
11. The rate of reaction for each concentration was calculated using the formula:
₃ ±₃₃₃₃₃₃₃ ŷ±5₃Ž ₃₃ ₃₃$₃₃
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12. A graph of rate of reaction against concentration was plotted after an error analysis was conducted. A best fit line was drawn to connect the plots.
Data Collection
OBSERVATION
All solutions in test tube A (KI 1.0 mol dm-3), B (Na2S2O8 0.04 mol dm-3), and C (Na2S2O3 0.05 mol dm-3) was colourless. As solutions of both test tube A and C were added into the conical flask, the mixture remained colourless. After 5 drops of starch solution were added, the mixture resembled rice water.
After solution in test tube B was added into the flask, there was no initial reaction observed. For some time, no changes were detected. Suddenly, the mixture in the flask turned into a blue-black solution.
The blue-black intensified as time passed by.
Table 1.1 Concentration of iodide solution and time for blue-black solution to form
Data Processing and Analysis
Table 1.2 Rates of reaction for different concentrations
Inference on observations
When potassium iodide (test tube A) was added together with sodium thiosulphate (test tube C) in a flask, no reaction was occurring. Rather, all ions were present in aqueous form, preparing to react with peroxodisulphate ion to be added later. Thus, the mixture remained colourless.
The mixture resembled rice water after the addition of starch solution because the starch solution was homogenous with the mixture. Starch solution by itself resembles rice water. Physical diffusion occurred but no chemical reaction was occurring.
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After peroxodisulphate ion (test tube C) was added into the solution, no initial reaction was observed due to the delaying mechanism in action. As iodide ion is oxidized by peroxodisulphate ion to form iodine, the formed iodine is reduced by thiosulphate ion back into iodide.
As a result, no iodine is free to react with starch to form blue-black starch complex which forms the distinct blue-black solution.
However, once the thiosulphate is exhausted, iodine is free to react with starch. Thus, there seemed to be a sudden formation of blue-black solution. The blue-black intensified as time passed by due to the increasing concentration of blue-black complex formed. As time passed by, more free iodine reacts with starch, thus the blue-black seemed to be more intense.
Error Analysis
Uncertainty on x -bar
Uncertainty due to dilution of KI
Table 1.3 Uncertainties due to dilution of KI solution
*The uncertainties due to preparation of solutions are minimal and assumed to be negligible.
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Table 1.4 X-bar uncertainties
Uncertainty in y - bar
Table 1.5 Y-bar uncertainties
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The graph of rate of reaction against concentration is plotted to determine the relation between the two variables. A best fit line is then constructed to relate the x-bar and the y-bar. The proposed model and R-squared value of the model, which is a measure of correlation between the two variables, are also attached. Both x and y-error bars are also set according to the uncertainties we have processed, though on the graph they look slightly small.
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From the graph, it seems there exists a linear relationship between the concentration factor and the rate of reaction. The R-squared value of the model is at R2 = 0.993, a relatively high value that signals that only a small residual variability remains unexplained. In order words, the linear relationship model between the two variables is statistically very strong.
Conclusion and evaluation
Based on the experimental findings, it can thus be concluded that it is very likely that the concentration factor is affecting the rate of reaction of the iodide oxidation linearly. Based on the graph, as the concentration of the iodide solution increases, the rate of reaction of iodide oxidation increases too. This linear relationship only applies to the named reaction in this experiment; other named reaction may or may not behave the same.
These findings support our hypothesis that the concentration factor does affect the number of effective collisions and by extension, rate of reaction.
As the concentration of the iodide solution increases, the number of particles per volume increases which increases the probability of collision. Given the higher probability in collision, the number of effective collisions – those with sufficient activation energ y and minimal steric stress – increases. In effect, the rate of reaction also increases proportionally.
Possible sources of error
Throughout the experiment, very precise instruments were used. With greater apparatus accuracy, and higher precision, the total uncertainty for the experiment is very low. Nevertheless, there may be some error introduced during:
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the prepared iodide solutions was kept overnight in 100cm3 air-tight containers since the experiment cannot be completed in a single day. On the next day where the experiment commenced, some solutions in their containers was slightly yellowish suggesting the presence of iodine. That indicated the iodide solution may have been oxidized by oxygen since there was a small volume of air stored in each container.
- all the solutions involved were colourless; the solutions may have been contaminated during the transfer and addition of solutions. If contaminants were introduced into the solutions, it is very difficult for the experimenter to take note since all solutions are colourless.
Suggestions to improve future experiments
- the experiment should be conducted in batches. In view that the experiment may not be completed in one single go, dilution of KI solutions should be done prior to the iodine clock reaction. Overnight storage of iodide solution should be avoided.
Reference
Green, John & Damji, Sadru. 2001, Chemistry for International Baccalaureate, 2nd Edition, IBID Press, Victoria, Australia.
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