Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 – 007 Date of experiment: 22/04/09
PRACTICAL 15 – Kinetics: Investigating the relationship between concentration of reactants and rate of reaction using iodine clock reaction.
Aim To study the effect of manipulating the concentration of hydrogen peroxide on rate of oxidation of iodide, I- ions by hydrogen peroxide, H2O2 using the iodine clock reaction. Introduction The iodine clock reaction was discovered by Hans Heinrich Landolt1 and is mainly used to demonstrate kinetics in Chemistry. The basis of its mechanism is that two clear solutions of known concentrations are mixed together and after a delay, the solution will suddenly turn yellow (or dark blue with addition of starch). In this experiment, the solutions used are hydrogen peroxide, H2O2 and potassium iodide, KI with thiosulphate ions, S2O32- as a delaying agent. Starch is added to the system for a clear indication of the end-point of reaction by a change in colour to blue-black. The time taken for the colour to change will be measured and the rate of reaction can be determined by taking the reciprocals of the measured times. Several factors influence the rate of reaction of a substance. According to the collision theory these factors include concentration of reactants. As such, the effect of concentration of hydrogen peroxide on the oxidation of iodide ions will be tested in this experiment. The reaction which will take place is as below: H2O2(aq) + 2H+(aq) + 2I-(aq)
2H2O(l) + I2(aq)
However, the reaction would take place too quickly if a delaying mechanism is not introduced into the system. As such, sodium thiosulphate solution is added to introduce thiosulphate ions in the system. These ions act as a reducing agent which will reduce the iodine, I2 formed in the first reaction to back to iodide ions, I- according to the equation below:
2S2O32-(aq) + I2(aq)
S4O62-(aq) + 2I-(aq)
Once all the thiosulphate ions have been exhausted, there will be free iodine in the system which will form the blue-black starch complex.
1
“Iodine Clock Reaction” Wikipedia: The Free Encyclopedia, Wikipedia 2009 th <http://en.wikipedia.org/wiki/Iodine_clock_reaction#cite_note-0> 19 April 2009
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 – 007 Date of experiment: 22/04/09
Hypothesis According to the Collision Theory2, a reaction will occur only when reactant particles fulfil these requirements:
The reactant particles collide with one another The collision must be energetic enough to overcome the activation energy of the reaction The collision must occur in the correct geometrical alignment
In addition, the Collision Theory states that several factors will affect the rate of reaction. The factor being studied in this experiment is the concentration of reactant particles, i.e. hydrogen peroxide. According to this theory, the higher the concentration of hydrogen peroxide, the higher the rate of reaction. This is because as the concentration of hydrogen peroxide increases, the number of hydrogen peroxide particles per unit volume will increase. Thus, the number of effective collisions will thus increase. This however only holds true when the volume of iodide ions and all other variables are kept constant. As the volume of the entire system is kept constant, increasing the concentration of hydrogen peroxide would decrease the relative concentration of iodide ions. At high concentrations of hydrogen peroxide, the chances of an effective collision to happen will be higher. The addition of a delaying mechanism should not interfere with the rate of reaction as the concentration and volume added throughout the experiment is kept constant.
As such, the hypothesis for this experiment is the higher the concentration of hydrogen peroxide, the higher the rate of oxidation of iodide ions.
2
John Green & Sadru Damji, Chemistry for the International Baccalaureate, p. 234
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 – 007 Date of experiment: 22/04/09
Variables Variable Measured Dependent Variable
Independent Variable
Method of measuring variable
Time taken for blue-black starch complex to form
The time taken will be measured using a digital stopwatch (±0.1s). To reduce random errors, three readings will be taken and an average will be calculated.
Rate of reaction
The rate of reaction will be calculated by taking the reciprocal of the average time taken for the complex to form.
Concentration of hydrogen peroxide solution
Using a stock solution of 6% hydrogen peroxide, a serial dilution will be performed to achieve solutions of 3%, 1.5%, 0.75% and 0.375% concentrations using a micropipette (±0.003cm3) Method of controlling variables
Controlled Variables
Concentration and volume of sodium thiosulphate solution added
The solution of known concentration will be prepared.
Concentration and volume of potassium iodide used
The volume used will be accurately measured using a micropipette (1.000±0.003)cm3 The solution of known concentration will be prepared.
Volume of starch solution used Temperature of reactants
The volume used will be accurately measured using a micropipette (1.000±0.003)cm3 5 drops of starch will be added for each sample. Temperature of the reactants was kept at a constant by conducting the experiment at room temperature.
Table 1: List of variables
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 – 007 Date of experiment: 22/04/09
Apparatus and Materials o o o o o o o o o o
Electronic balance (±0.001g) Micropipette (±0.003cm3) Pipette (10 ± 0.02cm3) 6 beakers (50cm3) 2 Volumetric flasks (100cm3) 3 conical flasks Droppers Spatulas Filter funnel Wash bottle
o o o o o
6% Hydrogen peroxide solution Potassium iodide powder Sodium thiosulphate powder Distilled water Starch solution
Method Design A preliminary experiment was conducted to obtain the optimal volumes of reactants to be used in order to yield results. A trial and error approach was utilised. The volume of sodium thiosulphate solution had to be altered several times for the experiment to finally work. Other possible weaknesses and shortcomings were also identified.
Procedure A) Preparation of 0.01M potassium iodide solution: 1. 1.66g of potassium iodide powder is weighed in a small beaker. 2. Some distilled water is added to the powder until it dissolves and the solution is transferred into a 100cm3 volumetric flask via a filter funnel. 3. The beaker and filter funnel are washed down with distilled water to make sure all the potassium iodide is transferred into the volumetric flask. 4. Distilled water is added into the volumetric flask up to the calibration mark. 5. The volumetric flask is capped and inverted several times until a homogenous solution is formed. B) Preparation of 0.03M sodium thiosulphate solution: 1. 0.745g of sodium thiosulphate powder is weighed in a small beaker. 2. Some distilled water is added to the powder until it dissolves and the solution is transferred into a 100cm3 volumetric flask via a filter funnel. 3. The beaker and filter funnel are washed down with distilled water to make sure all the sodium thiosulphate is transferred into the volumetric flask. 4. Distilled water is added into the volumetric flask up to the calibration mark. 5. The volumetric flask is capped and inverted several times until a homogenous solution is formed. Page | 4
Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 â&#x20AC;&#x201C; 007 Date of experiment: 22/04/09 C) Preparation of hydrogen peroxide solution of different concentrations: 10cm3 distilled water
10cm3
6% H2O2
10cm3
3% H2O2
10cm3
1.5% H2O2
10cm3
0.75% H2O2
0.375% H2O2
Diagram 1: Serial dilution of 6% hydrogen peroxide solution. 1. 20cm3 of 6% hydrogen peroxide was transferred into a beaker using a pipette. 2. 10cm3 of the 6% hydrogen peroxide in the beaker is transferred into another beaker using a pipette as seen in Diagram 1. 10cm3 of distilled water is added into the beaker to form a hydrogen peroxide solution of 3%. 3. Step 2 is repeated consecutively until a hydrogen peroxide concentration of 0.375% is achieved. D) Iodine Clock Reaction: 1. 3 test tubes are labelled A, B and C respectively. 2. Using a micropipette, 2.00cm3 of 0.01M potassium iodide solution is transferred into test tube A. 3. Using a micropipette, 2.00cm3 of 6% hydrogen peroxide solution is transferred into test tube B. 4. 5 drops of 1M hydrochloric acid is added into test tube B to acidify the hydrogen peroxide. 5. Using a micropipette, 0.25cm3 of 0.03M sodium thiosulphate solution is transferred into test tube C. 6. Solutions from test tubes A and C are added together in a conical flask. 7. 5 drops of starch solution is added into the conical flask and the conical flask is lightly swirled. 8. The contents of test tube B are inserted into the conical flask and the stopwatch is immediately started. The conical flask is swirled to ensure a homogenous mixture. 9. When the blue-black starch complex is formed, the stopwatch is stopped. 10. The time taken for the formation of the blue-black starch complex is recorded in a table. 11. Steps 1 to 10 are repeated for two more replicates. 12. Steps 1 to 11 are repeated using hydrogen peroxide concentrations of 3%, 1.5%, 0.75% and 0.375% Page | 5
Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 â&#x20AC;&#x201C; 007 Date of experiment: 22/04/09
E) Calculations: 1. The average time, tAVE for the blue-black starch complex to form is calculated using the formula: đ?&#x2018;Ą1 + đ?&#x2018;Ą2 + đ?&#x2018;Ą3 đ?&#x2018;Ąđ??´đ?&#x2018;&#x2030;đ??¸ = 3 2. The rate of reaction for each concentration is calculated using the formula: Rate of reaction =
1 đ?&#x2018;Ą đ??´đ?&#x2018;&#x2030;đ??¸
Data Collection and Processing Quantitative Data: Concentration of hydrogen peroxide solution / %
Time taken for blue-black starch complex to form, t/s (Âą0.01s) t1 t2 t3 tAVE 6.0 57.75 81.09 85.00 74.6 3.0 143.37 197.50 205.50 182.2 1.5 220.71 271.34 207.09 233.1 0.75 302.34 322.22 251.28 292.0 0.38 Table 2: Time taken for the blue-black starch complex to form and the average times for all concentrations tested.
1 Concentration of hydrogen tAVE /s Rate of reaction, / s-1 đ?&#x2018;Ą đ??´đ?&#x2018;&#x2030;đ??¸ peroxide solution / % 6.0 74.60 0.0134 3.0 182.2 0.0055 1.5 233.1 0.0043 0.75 292.0 0.0034 0.38 Table 3: Rates of reaction for different concentrations of hydrogen peroxide solution.
Qualitative Data: ď&#x201A;ˇ
When hydrogen peroxide (from test tube A) was added to sodium thiosulphate solution (from test tube C), the solution remained colourless and no reaction was evidently occurring. When starch was added, the solution still remained colourless. It is ready to be reacted with the potassium iodide (from test tube B).
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 â&#x20AC;&#x201C; 007 Date of experiment: 22/04/09 ď&#x201A;ˇ ď&#x201A;ˇ
After addition of potassium iodide, the solution did not change colour immediately and took some time for the blue-black starch complex to form. For hydrogen peroxide concentration of 0.38%, the solution did not change colour even after a very long time and so the experiment was stopped.
Uncertainties Uncertainty due to dilution of hydrogen peroxide solution (shown on the x-bar of graph): Uncertainty due to 10cm3 pipette = Âą0.02cm3 Concentration of hydrogen peroxide solution / %
Uncertainties Volume of Volume of Total percentage Absolute hydrogen peroxide distilled water error for uncertainty for solution added/ added / cm3 concentration of concentration of 3 cm hydrogen hydrogen peroxide/ % peroxide/ % 6.0 3.0 10.00 Âą0.02cm3 10.00 Âą0.02cm3 Âą0.4 0.012 = 10.00 Âą 0.2% = 10.00 Âą 0.2% 3 1.5 10.00 Âą0.02cm 10.00 Âą0.02cm3 Âą0.4 0.006 = 10.00 Âą 0.2% = 10.00 Âą 0.2% 0.75 10.00 Âą0.02cm3 10.00 Âą0.02cm3 Âą0.4 0.003 = 10.00 Âą 0.2% = 10.00 Âą 0.2% 0.38 10.00 Âą0.02cm3 10.00 Âą0.02cm3 Âą0.4 0.002 = 10.00 Âą 0.2% = 10.00 Âą 0.2% Table 4: Uncertainty table for concentration of hydrogen peroxide solution
Uncertainty due to time taken for blue-black starch complex to appear (shown on the y-bar of graph): Uncertainty due to digital stopwatch = Âą0.01s Concentration of hydrogen peroxide solution / % 6.0 3.0 1.5 0.75
tAVE /s 74.60 Âą 0.01 = 74.60 Âą 0.013% 182.2 Âą 0.01 = 182.2 Âą 0.0055% 233.1 Âą 0.01 = 233.1 Âą0.0043% 292.0 Âą 0.01 = 292.0 Âą0.0034%
Uncertainties 1 / s-1(%) đ?&#x2018;Ą đ??´đ?&#x2018;&#x2030;đ??¸
1 đ?&#x2018;Ą đ??´đ?&#x2018;&#x2030;đ??¸
/ s-1( x 10-4)
0.013
0.017
0.0055
0.003
0.0043
0.002
0.0034
0.001
0.38 Table 4: Uncertainty table for time taken for blue-black starch complex to form.
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 â&#x20AC;&#x201C; 007 Date of experiment: 22/04/09 Data Presentation: The graph of average time of reaction was plotted against the concentration of hydrogen peroxide solution to ascertain the relationship between the two. A line of best fit is drawn with the equation and R-squared value of the line shown. Error bars on both the x-values and the y-values are also included.
Graph of average time of reaction, t against concentration of hydrogen peroxide solution/% 350 300
Average time, t/s
250 200 150
y = -101.4ln(x) - 195.3 R² = 0.9686
100 50 0
0.00%
1.00%
2.00%
3.00%
4.00%
5.00%
6.00%
7.00%
8.00%
Concentration of hydrogen peroxide solution/%
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 â&#x20AC;&#x201C; 007 Date of experiment: 22/04/09
A graph of rate of reaction against concentration is drawn to show the relationship between these two variables. A line of best fit is drawn with the equation and R-squared value of the line shown. Error bars on both the x-values and the y-values are also included.
Graph of rate of reaction/s-1 against concentration of hydrogen peroxide solution/% 0.014 0.012
y = 0.1843x + 0.0014 R² = 0.9527
Rate of reaction,/s-1
0.01 0.008 0.006 0.004
0.002 0 0.00%
1.00%
2.00%
3.00%
4.00%
5.00%
6.00%
7.00%
8.00%
Concentration of hydrogen peroxide solution/%
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Candidate Name: Muhammad Azam bin Ismail Candidate Number: 02206 â&#x20AC;&#x201C; 007 Date of experiment: 22/04/09
Conclusion Based on the data acquired from this experiment, we can see a trend in the rate of reaction when concentration of its reactants are manipulated. The relationship between concentration of hydrogen peroxide and rate of reaction fits a linear regression line as seen in the graphs above. The higher the concentration of hydrogen peroxide solution, the higher the rate of reaction. As such the hypothesis is accepted.
Evaluation Based on the calculations of uncertainties, the percentage error in this experiment is not very big and the results are quite reliable. It is relatively safe to say that there is a linear correlation between concentration of hydrogen peroxide and rate of reaction.
However, there are several limitations to this experiment. Firstly is in the time taken for the blueblack starch complex to form. This colouration should appear suddenly and the digital stopwatch should be stopped when this happens. However in this experiment, the colour did not appear suddenly but gradually formed in the conical flask. As a result, there was some confusion as to when to stop the digital stopwatch. To remedy this, a white tile with a mark may be placed under the conical flask and the time taken for the mark to disappear can be taken instead.
Also, as seen in both graphs there is a large uncertainty when it comes to concentration of hydrogen peroxide prepared. This is due to the limitation of the instrument used. Conversely, a micropipette with a smaller uncertainty can be used to prepare the solutions of desired concentrations in minute amounts. This will also prevent wastage but the experiment must be done on a smaller scale. Also, at 0.38% concentration of hydrogen peroxide, the time could not be taken. The limitation could be due to the concentration of potassium iodide being too low. Thus, an increase in the concentration of potassium iodide would be more effective as more data points can be plotted and a better line of best fit can be drawn.
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