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Date:
21 February, 2013
Title:
Investigating the Effect of Temperature on the Rate of Reaction
Objective:
To investigate the effect of temperature on the rate of reaction
Theory: The reaction between sodium thiosulphate solution and dilute hydrochloric acid i s: Na2S2O3(aq) + 2HCl(aq) gas formed cannot be measured because SO2(g) is very → 2NaCl(aq) + SO 2(g) + S (s) + H2O(l). As the volume of the gas soluble and dissolve quickly in the solution. Therefore, the time taken for the reaction to produce enough yellow precipitate solid to "blot out" the cross and turns the solution opague is measured instead. In order to measure, this experiment is carried in a beaker placed on a white tile with a blue cross on it. Before mixing the sodium thiosulpahte solution and dilute hydrochloric acid in a beaker, they are colourless. After mixing the two solutions in the beaker, at the beginning of the reaction, the cross can easily be seen through the solution in the flask because the reaction mixture is still colourless. As the reaction proceeds, the yellow sulphur precipitates are produced. Therefore, they make the reaction mixture becomes more and more cloudy or opague and lowers the light transmittance of the reaction mixture. Thus, the cross gradually disappear and just to be "blotted out" ultimately. Therefore, by measuring the time for the cross just to be "blotted out", the average rate of this reaction can be determined using the equation 1/time for the cross to be just "blotted out" since the rate of reaction is inversely proportional to the time. Since the objective is to determine the effect of temperature, therefore, it can be achieved by repeating the reaction at different temperature. It means that to heat up the sodium thiosulphate solution at different temperatures and then mix it with the dilute hydrochloric acid, and consequently record the time for the cross to be just "blotted out" and calculate the reciprocal, which is the rate. After that, plot a graph of 1/time against temperature. Therefore, the effect of the temperature on the rate of reaction can be determined by the shape of the graph. In theory, the reaction rate rises exponentially with temperature since for a 10°C temperature rise, the rate of reaction doubles roughly. In other words, the higher the temperature, the faster the rate of reaction. The reason why the higher the temperature, the faster the rate of reaction can be explained by the collision theory. As the temperature increases, the speed of particle increases. Therefore, the kinetic energy of 2 reactant particles increases since kinetic energy = 1/2 x (m) x (v ). Consequently, it increases the fraction of reactant particles having sufficient energy greater than the activation energy to react as well as the collision per unit time increases. Thus, the chance of effective collisions increases and therefore, the rate of reaction increases.
Experimental procedure: 1. A blue cross was marked with a blue "Wytebord" marker on a white tile with the size just smaller than the bottom of a small beaker. 3 3 3 2. 5.0 cm of 0.15 M sodium thiosulphate solution was mixed with 45.0 cm of water in a 50.0 cm measuring cylinder 3. The mixture was added to a small beaker and was placed on the white tile with the blue cross 3 4. 5.0 cm of 2.0 M hydrochloric acid was added into the beaker quickly. The stopwatch timer was started 5. The temperature was recorded and was stirred with a glass rod gently 6. The cross was observed by looking vertically down through the solution. The time required for the cross to be blotted out completely was recorded 3 3 7. Another 5.0 cm of 0.15 M sodium thiosulphate solution was taken and was mixed with 45.0 cm of water 8. The solution was heated with the Bunsen burner until 35°C was reached 3 9. 5.0 cm of 2.0 M hydrochloric acid was added into the beaker quickly and the stopwatch timer was started 10. The time for the blue cross to disappear and not able to be observed was recorded 11. Steps 7-10 were repeated, but with temperature 45°C, 55°C, 60°C, 65°C and 70°C. The result was recorded on the data sheet Observations: Before the reaction:
During the reaction:
After the reaction:
1. The sodium thiosulphate solution is colourless 2. The dilute hydrochloric acid is colourless and has a sour (pungent) smell 1. The reaction mixture turns from colourless to white or pale yellow colour 2. The reaction mixture changes from clear to blur (opaque) 3. Yellow precipitate is formed 4. Irritating smell is detected by the nose 1. The reaction mixture becomes warmed as detected by the hand 2. The blurriness of reaction mixture is the same no matter at what temperature
Data: Temperature of reaction mixture (°C) 25 (room temperature) 35 45 55 60 65 70
Time for cross to be just "blotted out" (s) 296 191 110 73 62 48 40
Calculation: 1. Write the balanced full equation for the reaction involved in this experiment. Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) + H2O(l) 2. Plot a graph of 1/time against temperature. Data processing: -1 Temperature of reaction mixture (°C) Rate (1/time) for cross to be just "blotted out" (s ) -3 25 (room temperature) 3.38 x 10 (3sf) -3 35 5.24 x 10 (3sf) -3 45 9.09 x 10 (3sf) -3 55 13.7 x 10 (3sf) -3 60 16.1 x 10 (3sf) -3 65 20.8 x 10 (3sf) -3 70 25.0 x 10 (3sf) Graph: Refer to the last page of the report 3. With reference of your graph, find the expected time for the cross to be just "blotted out" if the temperature is set at 40°C. -3 -1 The rate for the cross to be just "blotted out" if the temperature is set at 40°C is 7 x 10 s , so the expected time is 143 s (3sf). Diagram: Experimental set-up
Result: The expected time for the cross to be just "blotted out" if the temperature is set at 40°C is 143 s (3sf). Conclusion: The rate of reaction between sodium thiosulphate solution and dilute hydrochloric acid increases with the increase in temperature.
Discussion: Precautions Firstly, handle all chemicals, which are sodium thiosulphate solution and dilute hydrochloric acid, with care because they are corrosive. Secondly, beware of the hot temperature because the reaction mixture needed to be heated by the Bunsen burner. Thirdly, since only the effect of temperature on the rate of reaction is studied, other factors affecting the rate of reaction, such as concentration, quantities of reactants used, surface area etc., should be same for all the sets. Fourthly, the cross mark is to be viewed vertically from the top at a certain and constant height and not from the side. Fifthly, always use the same cross on the white tile because a different cross could be more or less visible and this may affect the time for the cross to be just "blotted out", and in turn may affect the result. Sixthly, wash and clean all the glass apparatus, which are the beaker for the reaction to take place and the measuring cylinders for the measurement of the volume of water, sodium thiosulphate solution and dilute hydrochloric acid, before and after every experiments Lastly, start the stopwatch timer immediately after the addition of the dilute hydrochloric acid so as to reduce the error of reaction time. Sources of error There were a number of problems in my experiment that could explain my anomalous result. The most dramatic problem was keeping the temperature the same throughout the experiments. Errors in temperature control could easily account for the anomalous result, as the result would be recorded as either too fast (as
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with my result, placing it above the best line – if the time was too low for the reaction, time would be greater, so the measure of the rate would be too high) or too slow. Another source of error was Sulphur build-up on the sides of the conical flask. This reduced visibility and could have caused the cross to disappear earlier with each experiment, as it would be less visible to begin with. This would cause the results to be too high. A final problem came with the cross. As the experiments progressed, the cross became damp and its visibility reduced. This could cause times to be recorded too fast – the cross would be fainter, so would appear to disappear sooner – whilst redrawing the cross could cause times to be too slow – the cross would be bolder, so would be visible for longer. Suggestions / Improvements Firstly, a temperature sensor could be used to record the temperature at regular intervals throughout the experiment. The temperature could be measured throughout the experiment, rather than simply taking the temperature at the start and end of the experiment – a method fraught with error. A computer could be used to record these readings, eliminating human error entirely. An overall average temperature for the experiment could then be calculated. A colorimeter (light/visibility meter) could also be used. Placed around the conical flask and connected to a digital timer, this would stop the timer at the point where visibility reached a certain level – a light source and sensor would face each other across the flask, as the Sulphur reached a certain level, the light sensor would no longer be able to detect the light source, stopping the timer. The timer would stop at the same point each experiment, making the tests fair. This equipment would eliminate human error and remove the need for a cross, further improving accuracy. I could have obtained better results by laminating the cross to prevent fading, or better still using a colorimeter. I could also have used to temperature sensor to take my temperature readings. I could have measured my volumes more accurately, using a burette, for example. Repeating my experiments more times would also have improved the accuracy, as more precise mean results could have been calculated.
I could improve my method by carrying out my experiments in the water baths. By keeping my reactants out of the baths for as little time as possible, a more constant temperature could be maintained Use the same person to start and stop the stopwatch to record the time for the cross to be just "blotted out" and observe the fading of the cross to reduce the reaction time error and prevent the situation of different perception of "blot out" by different people respectively. Other relevant information Since this reaction Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO 2(g) + S (s) + H 2O(l) is exothermic in nature (∆H < 0), it indicates that the products, NaCl(aq), SO2(g), S(s) and H2O(l), are energetically more stable than the reactants, Na2S2O3(aq) and HCl(aq). Also, it indicates that the energy absorbed for the bond breaking of the reactants is smaller than that of the energy released for the bond forming of the products Thus, it is a bond breaking reaction overall. Also, this reaction is often misunderstood that it is a redox reaction. Yet, the correct oxidation number 2of the thiosulphate ion, S2O3 , is 0 and +4 observed from the structure, but not the calculated one, which is +2. Therefore, the oxidation number of Na, S, O, H and Cl remains +1, 0 and +4, -2, +1 and -1 respectively throughout the reaction. Thus, it is not a redox reaction because there are neither oxidation nor reduction reactions occur. Moreover, this reaction is also known as a "clock reaction", because when the sulphur solid precipitate reaches to a certain concentration, the reaction mixture turns from colourless to pale yellow, which is a observable and obvious change and indication. In addition, this is also an example of irreversible reactions. It is because the activation energy for the backward reaction, which is 2NaCl (aq) + SO 2(g) + S (s) + H 2O(l) → Na 2S2O3(aq) + 2HCl(aq), is too high. Also, there is a loss of sulphur dioxide gas since it is very soluble in solution and dissolve in it. Therefore, the reaction goes in one direction only and can go to completion. Answers to questions a. What is/are the source(s) of the error in this experiment? Suggest some possible solutions if there are any. There were a number of problems in my experiment that could explain my anomalous result. The most dramatic problem was keeping the temperature the same throughout the experiments. Errors in temperature control could easily account for the anomalous result, as the result would be recorded as either too fast (as with my result, placing it above the best line – if the
1
time was too low for the reaction, time would be greater, so the measure of the rate would be too high) or too slow. Another source of error was Sulphur build-up on the sides of the conical flask. This reduced visibility and could have caused the cross to disappear earlier with each experiment, as it would be less visible to begin with. This would cause the results to be too high. A final problem came with the cross. As the experiments progressed, the cross became damp and its visibility reduced. This could cause times to be recorded too fast – the cross would be fainter, so would appear to disappear sooner – whilst redrawing the cross could cause times to be too slow – the cross would be bolder, so would be visible for longer. Firstly, a temperature sensor could be used to record the temperature at regular intervals throughout the experiment. The temperature could be measured throughout the experiment, rather than simply taking the temperature at the start and end of the experiment – a method fraught with error. A computer could be used to record these readings, eliminating human error entirely. An overall average temperature for the experiment could then be calculated. A colorimeter (light/visibility meter) could also be used. Placed around the conical flask and connected to a digital timer, this would stop the timer at the point where visibility reached a certain level – a light source and sensor would face each other across the flask, as the Sulphur reached a certain level, the light sensor would no longer be able to detect the li ght source, stopping
the timer. The timer would stop at the same point each experiment, making the tests fair. This equipment would eliminate human error and remove the need for a cross, further improving accuracy. To laminate the cross to prevent fading, or better still using a colorimeter. I could also have used to temperature sensor to take my temperature readings. I could have measured my volumes more accurately, using a burette, for example. Repeating my experiments more times would also have improved the accuracy, as more precise mean results could have been calculated. I could improve my method by carrying out my experiments in the water baths. By keeping my reactants out of the baths for as little time as possible, a more constant temperature could be maintained Use the same person to start and stop the stopwatch to record the time for the cross to be just "blotted out" and observe the fading of the cross to reduce the reaction time error and prevent the situation of different perception of "blot out" by different people respectively. b. Is the reaction rate directly proportional to temperature? Explain your answer briefly. No. If the reaction rate is directly proportional to temperature, the shape of the graph should be a straight line, which is not the case in this experiment. From the graph, it seems that reaction rate rises exponentially with temperature. c. Why do we used the same concentrations and quantities of reactants at the start in all cases? It is because to keep other conditions constant except temp, so only the effect of temperature, but not other factors, could be studied and achieve the objective. d. State one other example in our daily life to show the understanding of the effect of temperature on reaction rate is very important. Fresh food can be kept fresh for a longer time in a refrigerator. As refrigerator provides a low temperature environment (~4°C) and low temperature would slow down the rate of bacterial activities, which deteriorate the food, thus fresh food can consequently be kept fresh for a longer time in a refrigerator. References: 1. Aristo 2. xxxxx 3. iii
