28th of September 2018
How do Changes in Temperature Affect the Rate of Reactions?
To study the effect that temperature has on the rate of a chemical reaction, a reaction between sodium thiosulphate and dilute hydrochloric acid can be used. In this particular reaction, it produces sulfur as a precipitate. Using this precipitate and by changing the temperature of one of the chemicals in the reaction, while keeping all other variables constant, can allow for the rate of the reaction to be estimated. The reaction that occurs is shown below:
Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + SO2(aq) + H2O(l) + S(s)
Independent Variable: The independent variable for this experiment is the different temperatures of the Na2S2O3.
Dependent Variable: The dependent variable will be the time it takes for the reaction to occur. This will be measured by using a stopwatch and a cross drawn onto a piece of white paper. When the cross disappears from view, the time will stop and that will represent how long it took for the reaction to occur.
Volume of Na2S2O3 (50.0 cm3) and HCl (5.0 cm3) – By using the same volume of each solution for the experiment it allows for a fair test because if there was more solution in one experiment compared to another it would mean that there were more reactants and thus would produce more precipitate and the cross would disappear sooner, and would be unrepresentative of the actual rate.
The cross – The exact same piece of paper with the cross drawn on it will be used in all experiments. This is because if the cross were to vary between experiments then some experiments may take longer or be shorter than expected as the cross was too thin or too thick.
The concentration of the solutions – The same concentration will be used for each solution throughout. This is because concentration also affects rate and so it must remain constant. (2.0 mol of HCl will be used and 0.05 mol of Na2S2O3)
Size of the beaker – The size of the beaker will remain the same because that will determine how much solution there is between the human eye and the cross. The larger the beaker the more spread out the solution is, and vice versa.
Distance from the eye to the cross – The distance between the cross and the person’s eye must remain constant. If they move closer or further away that can affect how they view the cross and will result in inaccurate data. To limit this the same person will be used to watch the cross for all experiments.
Small measuring cylinder
Medium-sized measuring cylinder
White paper and pen
350.0 cm3 of 0.025 mol dm-3 sodium thiosulphate solution
35.0 cm3 of 2.00 mol dm-3 hydrochloric acid
Heating equipment (triangle stand, gauze, Bunsen burner, heat protectant gloves, matches)
There are multiple chemical hazards when performing this experiment. Both of the reactants, the hydrochloric acid and sodium thiosulphate, are irritants at their specific concentrations. The sulfur dioxide gas that is produced is toxic, and when breathed in can trigger asthmatic attacks.
As this experiment does require the use of a Bunsen burner and a flame, this is a potential hazard as it may burn you if not following the safety procedures.
How to Reduce the Risks
When dealing with the hydrochloric acid and sodium thiosulphate, as they are only irritants gloves do not have to be worn, however if they make contact with skin, make sure to wash the area immediately with water. Also, as the sulfur dioxide can trigger asthmatic attacks, those with asthma should not be the ones performing the data collection. However they can also wear a mask if they wish to do so. Finally, when lighting the Bunsen burner and heating the sodium thiosulphate, precaution should be taken so as to not get burned, such as through using heat protectant gloves, and making sure the Bunsen burner is set to the correct dials before lighting it. Lab attire (goggles, lab coat, closed-toe shoes) should also be worn at all times as a precaution.
50.0 cm3 of thiosulphate solution was poured into a beaker using the medium-sized measuring cylinder. The temperature was then taken using a thermometer.
331470024066500A small cross was drawn onto a piece of white paper and the beaker was then placed on top of the cross.
5.0 cm3 of hydrochloric acid was measured into the small-sized measuring cylinder.
The HCl was then poured into the beaker, and the stopwatch was started.
3543300835025Figure 1: Apparatus setup (“Method of Measuring the Rate of Reaction”)
0Figure 1: Apparatus setup (“Method of Measuring the Rate of Reaction”)
The time it took for the cross to disappear when looking from above was timed.
The beaker was then washed and dried thoroughly after the reaction completed.
Steps 1-6 were repeated five more times while changing the temperature of the Na2S2O3 solution by heating it carefully with a Bunsen burner.
Steps 1-6 were then repeated one final time, however the beaker containing the Na2S2O3 solution was placed into ice to cool it after Step 1, before continuing on with Steps 2-6.
Data Collection and Processing
Raw Data and Data Processing
Temperature (°C) ±0.5Time (s) ±0.001Rate (s-1) (3SF)
15.0 305.75 3.27
26.0 135.31 7.39
40.0 71.56 14.0
48.0 44.19 22.6
55.0 36.98 27.0
63.0 24.83 40.3
73.0 14.97 66.8
To calculate the rate of each individual temperature, the following equation was used: rate=1000time (s) where 1000 represents a constant unit for each reaction.
When conducting this experiment, various points of qualitative observations were made. Firstly, when the HCl was added to the Na2S2O3, it would turn the solution cloudy. This was because a precipitate, sulfur, was being formed, and so it made the solution turn milky and opaque. Also when the precipitate was being formed, it seemed to occur all of a sudden. The solution would begin to look a little cloudy but it would remain at that point for a while. However after that, in a short span of time the entire solution would turn opaque. Another observation that was made was that there was no effervescence when the Na2S2O3 was being heated by the Bunsen burner. The highest temperature reached was 73°C so by having no effervescence it means that Na2S2O3 most likely has a higher boiling point compared to water, as water begins to have slight effervescence at around that temperature.
In this graph, a general pattern can be seen. As the temperature of the Na2S2O3 increases, the rate of the reaction also increases. The curve that is created on this graph shows that the rate is quite slow at the lowest temperature, 15°C with a rate of 3.27 s-1 whereas the highest temperature, 73°C, had a rate of 66.8 s-1. This shows a very large increase in rate, and is able to show the direct correlation between temperature and rate.
Propagation of Uncertainties
Percentage uncertainty = Total uncertaintyExperimental value×100Temperature of Na2S2O3: 15.0°C ±0.5Percentage uncertainty for the thermometer : 0.515.0×100=3.33% This is just an example of one of the percentage uncertainties for the thermometer, however this same process can be applied to the remaining six temperatures.
Longest time for reaction to occur: 135.31s ±0.001Percentage uncertainty for the stopwatch: 0.001135.31×100= 0.000327%
Volume of HCl: 5.0 cm3
Percentage uncertainty for the small graduated cylinder: 0.25.0×100=4.0%Volume of Na2S2O3: 50.0 cm3
Percentage uncertainty for the medium graduated cylinder: 0.550.0×100=1.00%Total Percentage uncertainty: 1.00+4.00+0.000327+3.33=8.33% (3SF)
Using this percentage uncertainty, it can be applied to the data points to see if they are statistically significant or if it is due to the error in apparatus.
Data point 1: rate = 3.27 s-1, and when applying the percentage uncertainty it becomes a range of 3.01 to 3.53 – this range was found by using these steps:
3.27100×8=0.262. Then 3.27-0.262=3.01 and 3.27+0.262=3.53
As can be seen by the orange error bar in the image above, the second data point shown on the far right, has a rate of 7.39 s-1, and is out of range of the percentage uncertainty and thus makes it statistically significant. This can then be applied to the remaining data points, and it shows how each point is in fact statistically significant.
The aim of this experiment was to see how changes in temperature affect the rate of a reaction. After completing the reaction it can be seen that there is a direct correlation between an increasing temperature and an increasing rate. This experiment had a total percentage uncertainty of 8.33%, and when applying this to the graph, it can be seen how each data point is outside the range of error and reveals how each point is statistically significant, and thus there is a direct correlation between temperature and its affect on the rate of a reaction. The reasoning for this is because when the solution is heated, the increase in temperature provides the particles with more energy. This added energy allows for more of the particles to collide. Thus by having more collisions, and more particles reaching or surpassing their activation energy because of the added energy, the amount of collisions with correct orientation and sufficient energy increases. This allows for the overall rate of reaction to occur at a much faster rate. (Clark)
Systematic Error Influence on Results Possible Modification to Reduce Impact
Human Eye When completing the experiment, the human eye was used to determine when the cross had disappeared from view. However, there is some bias in this technique and could result in inaccurate data. As it is based off of judgment, it is very difficult to gauge when the cross had completely disappeared. This would result in slight error because there is inconsistency in telling when the cross had fully disappeared for each experiment.
Also the distance between the eye and the cross could have varied between experiments and would also influence how the person saw the cross and would result in inaccurate data. One possible modification that could be made to reduce the impact of the error of the human eye would be to use a spectrophotometer. This instrument measures the intensity of light relative to wavelength. (Martin) By using this instrument it allows for you to see once you’ve reached a certain level of cloudiness, numerically. This would allow for the data collection to be accurate and constant. The second modification for the distance, if a spectrophotometer could not be obtained and the human eye is still used, could be to place a 30cm ruler vertically next to the beaker and then placing your nose onto the other end of the ruler. By repeating this each time before collecting data, then the distance will remain the same for all experiments and increase the accuracy.
Temperature Rising/Cooling During the experiment, the temperature of the Na2S2O3 continued to rise after taking it off of the Bunsen burner. However the single temperature value that was recorded only represented the temperature straight after removing it from the heat, and is not representative of the continual increase that occurs. This would result in inaccurate results, as the reactions would be first occurring at a higher temperature than what was recorded. Also as the reaction is taking place, the temperature of the solution will then start to cool, slowing down the rate of the reaction. This would also result in inaccurate results. A modification that could be made to reduce this impact would be to place the beaker straight into a vacuum after it has been heated. By doing this it will allow for the temperature of the Na2S2O3 to remain constant throughout the entire experiment and will allow for more accurate results.
Mixing of the Solutions When adding the HCl into the Na2S2O3, the solution was not stirred. This could have lead to inaccurate results as it would have taken longer for the HCl and Na2S2O3 to react, as the solution was not evenly distributed throughout the beaker. To reduce this error, a stirring rod could have been used to mix the HCl and Na2S2O3 together. Another modification could have been to use a conical flask rather than a beaker. This would have allowed for the solution to be swirled and would have resulted in a more even mix of Na2S2O3 and HCl.
Clark, Jim. “The Effect of Temperature on Rate of Reaction.” Chemistry LibreTexts, Libretexts, 31 Jan. 2014, chem.libretexts.org/Textbook_Maps/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/Rate_Laws/Reaction_Mechanisms/Reaction_Mechanisms/The_Effect_of_Temperature_on_Rate_of_Reaction.
“Kinetics Study on the Reaction between Sodium Thiosulphate and Hydrochloric Acid.” OLABS, amrita.olabs.edu.in/?sub=73&brch=8&sim=142&cnt=1.
Martin, Paul. “What Is a Spectrophotometer?” What Is a Microspectrophotometer?, www.microspectra.com/support/learn/what-is-a-spectrophotometer.
“Method of Measuring the Rate of Reaction.” Exampro. http://ahammondbiology.weebly.com/uploads/3/7/6/6/37663423/c8_rates_and_equilibrium_exam_pack_and_mark_scheme.pdf