* Research question:
Does the change in concentration of sodium thiosulfate and the fixed concentration of hydrochloric acid result a change in time taken for the yellow sulfur precipitate to form, thus lead to a change in time taken for the cross to disappear and the rate of reaction?
* Variables:
* Independent variable: The concentration of sodium thiosulfate / M.
* Dependent variable: The time taken for the cross to disappear / second.
* Controlled variables:
* The concentration of hydrochloric acid / M.
* The temperature in each conical (Erlenmeyer) flask prior to every reaction / oC.
* The absence of unnecessary substances or ions.
* The angle to view the cross.
* Prediction:
* For many reactions involving liquids or gases, increasing the concentration of the reactants increases the rate of reaction. In order for any reaction to happen, particles must first collide. This is true whether both particles are in solution, or whether one is in solution and the other a solid. If the concentration is higher, the chances of collision are greater [1].
* In the reaction between sodium thiosulfate solution and dilute hydrochloric acid, yellow sulfur (S(s)) is formed in the flask:
2HCl (aq) + Na2S2O3 (aq) –> 2NaCl (aq) + SO2 (g) + S (s) + H2O (l) [2]
* In this experiment, I decide to alter the concentration of sodium thiosulfate by constantly increasing the concentration of sodium thiosulfate, the concentration of hydrochloric acid however is remained the same. My prediction is, the higher the concentration of sodium thiosulfate is, and the less time taken for the cross to stop being seen is.
This should be because higher concentration of sodium thiosulfate will result the yellow sulfur precipitate to be formed more quickly.
* Method:
* Apparatus:
* 500 ml Hydrochloric acid 2.000 M.
* 500 ml Sodium thiosulfate 0.1000 M.
* 500 ml Distilled water.
* 250 ml conical flask.
* Casio stop watch (Uncertainty: � 0.01 seconds).
* Square of blank paper.
* 3 x 50 ml burettes (Uncertainty: � 0.5 ml).
* 3 x funnels.
* Thermometer (Uncertainty:� 0.05 �C).
* 3 x retort stands.
* Bench mat.
* Risk assessment:
* The procedure uses corrosive hydrochloric acid and the reaction produces poisonous sulfur dioxide. Safety is therefore needed to be highly maintained.
* Goggles and lab coat are worn throughout the experiment.
* Procedures:
1. Close the tap and run some distilled water into the top of the burette, then swish the burette up and down to let the water clean all the inside of the burette. Open the tap, let the water drain out. Repeat this step with the other 2 burettes.
2. Attach 3 burette to 3 retort stands and take care that the burettes are upright and stable.
3. Close the tap of the first burette and pour sodium thiosulfate into the burette through the funnel. Open the tap and release 10ml of sodium thiosulfate 0.1 M into the conical flask.
4. Close the tap of the second burette and pour distilled water into the burette through the funnel. Open the tap and release 40ml of distilled water into the same conical flask.
5. Draw a dark cross on the paper, put the paper on the bench mat, and then put the conical flask on top of the paper. Then adjust the height of the burette so that the tip of the burette is just above the lip of the conical flask
6. Close the tap of the third burette and pour hydrochloric acid into the burette through the funnel. Open the tap and release 10ml of hydrochloric acid 2.00 M into the conical flask containing dilute sodium thiosulfate.
7. When the cross fully disappears, immediately stop the watch and record the time taken for the cross to disappear.
8. Wash out the flask thoroughly.
9. Repeat the experiment using other volumes of sodium thiosulfate and water in the table next page.
Volume of sodium thiosulfate 0.10 M
Table 1.1 shows the intended composition of dilute sodium thiosulfate.
* Range and repetitions of experiment:
* There are 9 different ranges (The lowest value: 0.02 M & the highest value: 0.1 M, Please refer to Data Collection and Processing -> Processed data).
* The whole experiment is repeated 3 times and 27 results are recorded.
* Control of variables:
* The concentration of hydrochloric acid remains the same for each test. This variable is controlled because 10.0ml of hydrochloric acid 2.00 M is measured by the burette. In addition, the funnels should be removed from the burette after pouring solutions to make sure no air bubble inside the burette. If present, they may lead to inaccuracies while reading the volumes.
* The temperature in each conical flask prior to every reaction is the same. This variable is controlled because a thermometer is used to measure the temperature to make sure that there are no differences in temperature. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10 �C rise in temperature [3].
* The burettes are rinsed carefully with distilled water prior to the experiment to ensure that the later collection of sodium thiosulfate, hydrochloric acid from these burettes may not contain any unnecessary substances or ions. They may react with the solutions to form products if present.
* I need to try my hardest to be at the same angle (180o) each time to watch the cross disappearing. Making different angles could result uncertainties for when to stop the watch.
DATA COLLECTION AND PROCESSING
* Raw data table:
Na2S2O3
(� 0.0500 ml)
H2O
Table 2.1 shows the collected raw data table.
* Processed data:
* Calculating the concentration of sodium thiosulfate in each sample:
* Formula:
Concentration = 0.1 x (Volume of sodium thiosulfate 0.100 M divided by Volume of the solution).
Multiplying 0.1 is because the given sodium thiosulfate is 0.1000 M.
Volume of sodium thiosulfate 0.10 M
10.00
40.00
50.00
0.0200
15.00
35.00
50.00
0.0300
20.00
30.00
50.00
0.0400
25.00
25.00
50.00
0.0500
30.00
20.00
50.00
0.0600
35.00
15.00
50.00
0.0700
40.00
10.00
50.00
0.0800
45.00
5.000
50.00
0.0900
50.00
0.000
50.00
0.1000
Table 2.2 shows the processed different concentrations of sodium thiosulfate used.
* Calculating the mean time taken for the cross to disappear:
* Formula: Mean time taken = (1st + 2nd + 3rd experiment data) divided by 3
Concentration of
sodium thiosulfate/ M
Mean time taken for the cross
to disappear (� 0.0100 seconds)
0.0200
259.0
0.0300
140.0
0.0400
103.0
0.0500
76.70
0.0600
65.30
0.0700
56.30
0.0800
48.00
0.0900
43.00
0.1000
36.00
Table 2.2 shows the processed mean time taken for the cross
to disappear of each tested concentration of sodium thiosulfate.
* Calculating the rate of reaction:
* Rate of reaction = 1 divided by the mean time taken for the cross to disappear.
Concentration of sodium thiosulfate / M
The rate of reaction / seconds-1
0.0200
0.0039
0.0300
0.0071
0.0400
0.0097
0.0500
0.0130
0.0600
0.0153
0.0700
0.0178
0.0800
0.0208
0.0900
0.0233
0.1000
0.0278
Table 2.2 shows the processed rate of reaction
of each tested concentration of sodium thiosulfate.
* Presentation of processed data:
Graph 2.1 shows the relationship between the concentration of sodium thiosulfate and the mean time taken for the cross to disappear.
Graph 2.2 shows the relationship between the concentration of sodium thiosulfate and the mean time taken for the cross to disappear
(excluding the concentration of 0.02 M).
Graph 2.3 shows the rate of reaction.
* Treatment of uncertainties:
* I need to try my hardest to be at the same angle (180o) each time to watch the cross disappearing. Making different angles could result uncertainties for when to stop the watch.
* Qualitative observations:
* One qualitative observation is the colour of precipitate. I observe that the colour of sulfur after the cross disappears does vary, in respect of the difference in the concentration of sodium thiosulfate. The observation is, the higher the concentration of sodium thiosulfate is, the darker the yellow precipitate gets. This observation can also prove that the predicted equation (listed in Design -> Aim) is correct as the yellow sulfur precipitate is produced.
* Another qualitative observation is the irritating odour familiar as the smell of a just-struck match demonstrated by CFC StarTec LLC (2007) [4]. The odour certainly comes from the production of sulfur dioxide. This observation agrees with the predicted equation (Please refer to Design -> Prediction).
CONCLUSION AND EVALUATION
* Graph analysis:
* According to the presented graph of the mean time taken for the cross to disappear, overall, there is a relatively negative exponential correlation between the concentration of sodium thiosulfate and the mean time taken for the cross to disappear. Furthermore, there is a fairly stronger negative correlation appears between Na2S2O3 0.04 M and 0.1 M because the variables appear to be closer to the line of best fit when I exclude the variable 0.02 M on the graph (Please refer to Data Collection and Processing -> Presentation of processed data -> Graph 2.2).
* The rate of reaction is investigated to have a fairly strong positive correlation with the concentration of sodium thiosulfate (Please refer to Data Collection and Processing -> Presentation of processed data -> Image 2.2).
* Conclusion:
* The results demonstrate that, by increasing the concentration of sodium thiosulfate, the time taken for the cross to disappear is reduced and the rate of reaction is therefore increased. From the experiment, I can conclude that increasing the concentration of sodium thiosulfate means that the sulfur precipitate is produced more quickly, and there is less time before the cross can no longer be seen.
* The results suggest that the collision theory is responsible for the increase in the rate of reaction. Jim Clark (2007) [5] stated that: As the quantity of sodium sulfate particles in a given volume increases, the number of collisions between the reactant particles (sodium thiosulfate and hydrochloric acid) will increase, therefore the rate increases.
* Evaluation of procedures:
* Strengths:
* Safety in the laboratory is highly maintained (by wearing goggles, lab coat and being careful with glass apparatus to avoid any chemicals that may splash).
* Standard ranges and repetitions are met, a very strong positive correlation between the concentration of sodium thiosulfate and the rate of reaction is observed.
* Qualitative observations, along with quantitative investigation successfully prove that the expectations based on scientific knowledge are totally correct.
* Weaknesses:
* Several inevitable uncertainties occur throughout the whole experiment which may account for inaccuracies in the collected data.
* The chosen value Na2S2O3 0.02 M and 0.03 M eventually result a much longer the mean time taken for the cross to disappear (259 � 0.5 seconds and 142 � 0.5 seconds, respectively) than other values.
* There is irritating odour familiar as the smell of a just-struck match. Although it provides the qualitative observation that supports the hypothesis, the odour takes very long time to be completely removed by the air ventilator.
* Improving the investigation:
* The procedures can be partially replaced by computer data logging suggested by Laurence Rogers (1995) [6] to prevent uncertainties from human errors when stopping the watch. The experiment can be programmed to collect the data (Time taken for the cross to disappear) automatically.
* The consequence of very low concentration of sodium thiosulfate (for instance, Na2S2O3 0.02 M and 0.03 M) is, these concentration value although still support the hypothesis (lower concentration of sodium thiosulfate -> more time taken for the cross to disappear), do not fit the expected very strong negative correlation between the concentration of sodium thiosulfate and the mean time taken for the cross to disappear. Attention therefore needs to be paid in the ranges from Na2S2O3 0.04 M to 0.1 M. The ranges are therefore can be altered to 7, from 0.04 M , 0.05 M , to 0.09 M, 0.1 M.
* The enclosed bung can be used to cover the lip of the conical flask to prevent the release of sulfur dioxide gas.
Bibliography
[1] Clark, J. (2002). Collisions involving two particles. In The effect of concentration on reaction rates. Retrieved March 9, 2009, from Chem Guide Web site: http://www.chemguide.co.uk/physical/basicrates/concentration.html
[2] France, C. (2008). The reaction between sodium thiosulfate solution and dilute hydrochloric acid. In Rates of Reaction. Retrieved March 9, 2009, from GCSE Science Web site: http://www.gcsescience.com/rc7-increase-concentration.htm
[3] Clark, J. (2002). The facts. In The effect of temperature on rates of reaction. Retrieved March 9, 2009, from Chem Guide Web site: http://www.chemguide.co.uk/physical/basicrates/temperature.html
[4] CFC StarTec LLC. (2007, May 27). General Characteristics. In Sulfur Dioxide. Retrieved March 9, 2009, from http://www.c-f-c.com/specgas_products/so2.htm
[5] Clark, J. (2002). Collisions involving two particles. In The effect of concentration on reaction rates. Retrieved March 9, 2009, from Chem Guide Web site: http://www.chemguide.co.uk/physical/basicrates/concentration.html
[6] Rogers, L. (1995, May). Sensors and The Data-Logger. In Hardware and software. Retrieved March 9, 2009, from School of Education, University of Leicester Web site: http://www.le.ac.uk/se/lto/logging/test1.html
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