Variables
The variables in this experiment, and they ways in which they may be controlled, are as follows:
- Temperature. This will affect rate of reaction as stated above, and will be altered in this investigation.
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Concentration of reactants. This will affect rate of reaction as stated above, and will be altered in this investigation. Since there are two reactants, only the concentration of one will be altered and the other kept constant. The acid will be constant at 2.00moldm-3.
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Volume of reactants. This will be kept constant as far as possible. 5cm3 of acid and 50cm3 of thiosulphate will be used throughout.
- Any stirring or swirling of the mixture necessary to make the cloudy suspension develop evenly. A preliminary test will be carried out to determine whether this is required. Otherwise it would be very difficult to control this variable.
- The actual conical flask used. As the experiment is based on the visibility of a cross through the glass and the solution, then the same thickness and quality of glass must be used. Also some flasks are scored and discoloured.
- The pen used to mark the cross. Some inks will be darker than others and so this can also affect the visibility of the mark.
- The light level in the lab. This will also affect the visibility of the cross and so must be kept constant. It is also possible that light could increase the rate of the reaction. Some reactions are catalysed by light, and indeed some substances will break down in the presence of strong light. Examples of these are food additives and chemicals which are supplied in brown glass bottles.
Method
The investigation will use the reaction between sodium thiosulphate and hydrochloric acid. The equation for this is shown in the Introduction. As can be seen from this equation, one of the products is sulphur. Sulphur is a yellow powder-like solid which is insoluble in water, so as it builds up over the course of the reaction it will appear as a cloudy suspension in the liquid which starts colourless. This fact will be used in order to measure the rate of reaction. Figure 1 shows the apparatus.
Method for the concentration experiment
Sodium thiosulphate will be used in five different concentrations. 50cm3 of one of these solutions of thiosulphate will be added to a conical flask and then 5cm3 of 2.00moldm-3 hydrochloric acid will be quickly poured in, mixed and at the same time a stopwatch will be started. The solutions will be added in this order as the largest volume first followed by the smallest volume. This is because it will take a shorter time to pour 5cm3 into the flask than it would to pour 50cm3 in. This is important because the reaction will start as soon as the first particles from one solution mix with the other. Also, while something is being poured in, the concentration is changing. It is best to mix them as rapidly as possible, and this is done by pouring in the smallest volume. The flask will be placed on a white sheet of paper with a large red cross drawn in ball point pen. The cross will be observed down though the open top of the conical flask and through the reaction mixture. When the cross could no longer be seen though the sulphur suspension then the stopwatch was stopped. The time will be recorded and the rate of the reaction will be calculated as in the equation above.
The experiment will be repeated with the other four concentrations of thiosulphate. In total, the experiment will be carried out twice with each concentration and average results calculated. The results will be recorded in a table and the rate of reaction will be plotted against concentration of thiosulphate.
Method for the temperature experiment
Temperature was also used as a variable. In this case, the experiment was carried out as above but at five different temperatures starting from 22°C and rising in intervals of 10°C to 62°C. In order to be safer, only the sodium thiosulphate was heated. It was assumed that as the acid was of smaller volume that the effect of not heating it would be minimal. Sodium thiosulphate for this experiment was used at a constant concentration of 0.03moldm-3, and the acid was also constant at 2.00moldm-3. Heating was carried out by placing the sodium thiosulphate into the conical flask and placing over a blue Bunsen flame. The temperature was monitored with a thermometer, and when at the correct temperature it was removed from the heat. It was then placed on the cross and the acid added immediately. The procedure was then as described in the ‘Concentration’ section.
Safety
When using acids in the laboratory, goggles should be worn at all times, anyone with long hair should tie it back and tuck it in. Any acid spilled on skin should be washed in plenty of cold water. In the temperature section, acid will not be heated. Care will also be taken when heating the thiosulphate directly over a Bunsen flame as there will be no gauze to stop the flame from contacting the glass flask directly. Only a medium blue flame will be used.
Preliminary experiment
In order to make sure that the experiment was working and that the procedure was suitable, a preliminary test was carried out and repeated. It only used two concentrations of sodium thiosulphate and the hydrochloric acid concentration was constant for both at 2.00moldm-3. Table 1 shows the results:
Table 1. Results of the preliminary experiment on rate of reaction.
From the preliminary test it was found that the reaction did work and that it was possible to easily measure the time taken for the cross to become invisible after adding the acid. The cloudy suspension of the sulphur developed evenly so it was not necessary to stir or swirl the mixture, so eliminating another variable.
Results
The experiment was set up as shown above and the timings taken for the red cross to disappear using five different concentrations of sodium thiosulphate. Each concentration was repeated three times and an average calculated. The results are shown in Table 2.
Table 2. Results of the experiment on rate of reaction varying concentrations of sodium thiosulphate.
These results are plotted in Figure 2.
As can be seen from the results, and the graph which has been plotted (Figure 2), the average time for the red cross to disappear drops with increasing concentration of sodium thiosulphate. Looking at the results in more detail, it can be seen that as the concentration of sodium thiosulphate doubles, then the time for the cross to disappear drops by half. For example, as the concentration goes from 0.03moldm-3 to 0.06moldm-3, the time drops from 161s to 84s. The same trend can be observed from 0.06moldm-3 to 0.12moldm-3.
In the second part of the investigation, temperature was used as a variable. The sodium thiosulphate was heated to the required temperature using a Bunsen, and the acid was added. The timings were made as above. The results for this section are shown in Table 3.
Table 3. Results of the temperature part of the investigation.
As can be seen from the table and from the graph which follows (Figure 3), the time taken for the cross to become invisible decreases as the temperature increases. The time taken almost drops by half every time the temperature rises by 10°C.
Conclusion
The investigation was designed to determine how two variables, concentration and temperature, affected the rate to the reaction between hydrochloric acid and sodium thiosulphate.
1. Concentration as a variable
The first part of the experiment changed the concentration of the sodium thiosulphate and recorded the time taken for a red cross under the flask to become invisible. The results are shown in Table 2 and the graph in Figure 2. Also from these results it is possible to calculate the rate of reaction using the formula in the introduction. The following table (Table 4) shows the rates of reaction calculated for each of the five concentrations of sodium thiosulphate.
Table 4. Rate of reaction as effected by concentration.
The following graph (Figure 4) shows how the rate of reaction varies with the concentration.
This graph has been plotted as a best fitting straight line because none of the points fall into a curved pattern. Also, it has been drawn through the origin because if there is zero concentration, there will be zero reaction rate. As can be seen from the shape of the graph, the reaction rate approximately doubles when the concentration doubles. According to A-Level Revise Guide (Longmans) for Chemistry, this is a first order reaction. There are two other orders: zero, where the concentration of the reactant has no effect on the rate, and second, where the rate increases in proportion with the square of the concentration.
In this reaction , which we can say is first order with respect to the sodium thiosulphate, the rate can be seen to be directly proportional to the concentration of this reactant. It can therefore be concluded that the hypothesis was correct, in that the reaction rate does indeed increase as the concentration increases, due to the increased frequency of particle collisions. It can also be predicted that if the concentration was to be increased still further, then the reaction rate would continue to increase in proportion. An equation can be written to express the rate in terms of the concentation:
Rate ∝ Conc.
The proportion symbol can be replaced by introducing a constant, k:
Rate = k × Conc.
This equation above is called the rate equation, and is only valid for the sodium thiosulphate, as it was the only reactant whose concentration was varied. However, the graph in Figure 4 gives the information necessary to calculate the rate equation for the sodium thiosulphate and express k as a numerical value with units.
From the graph, the value of k will be the gradient, which is calculated by dividing the change in y by the change in x. Taking the first an last points on the line to be (0,0) and (0.15, 0.035) respectively, this gives the gradient, and k, to be 0.23. As the units of the y axis are s-1 and the units of the x axis are moldm-3, then the units of k are s-1mol-1dm3. The equation is now:
Rate = 0.23 × Conc.
2. Temperature as a variable
The second part of the experiment varied the temperature of the sodium thiosulphate before the acid was added. The acid was added at room temperature for reasons of safety. The results of this part are shown in Table 3 and the graph in Figure 4. Also from these results it is possible to calculate the rate of reaction using the formula in the introduction. The following table (Table 5) shows the rates of reaction calculated for each of the five temperatures of sodium thiosulphate.
Table 5. Rate of reaction as effected by temperature.
These results are shown in the following graph (Figure 6). As can be seen from the shape of this graph, the temperature has a greater effect on the rate of reaction than the concentration had. This is because the increase in temperature causes both the speed of the particles to increase and also the force or energy with which they are colliding. Therefore, the frequency of the collisions rises due to the speed increase and the resulting kinetic energy that they have means that more of them have enough energy to carry out a successful reaction. In chemical reactions, not all collisions have enough energy to make a successful reaction occur. This energy is called the activation energy and explains why some reactions will not happen at all when cold and require heating. The fact that chemical reactions slow down as the temperature is decreased also explains why we store food in refrigerators and freezers, and why plants grow more slowly in winter. Hence, the reason why the concentration has less effect than temperature is that its increase only causes the frequency of the collisions to increase, and has no effect on the energy of the particles.
Therefore, it can be concluded from this graph that the hypothesis for temperature change was correct in that the rate does indeed increase when the temperature is raised.
From the graph it can be seen that when the temperature is increased from 30°C to 40°C the rate of reaction almost doubles from 0.01 to 0.02s-1. This trend continues as the temperature is further increased. As the line is not straight, the rate is not directly proportional to the temperature, and a formula would be complicated to relate them.
Evaluation
When the results are examined for both parts of the experiments, they are seen to be consistent in that there are never many points which lie far from the graph lines. This means that the experiments have worked to give results which fall into a pattern. As each of the results was the average of two readings, this also means that the experiments were reliable and reproducible. Therefore, the experiment can be concluded to give reliable evidence for the conclusions which were made. However, one possible improvement which could be made, is that the number of repeats could be increased from two to three. As the results are already consistent, then there will be little point in doing more repeats.
Overall, the experiments relied on the observation of the red cross on the paper disappearing. This is prone to several sources of error: light level in the lab, human eyesight and human error of judgement as well as human error in timing. To avoid this, it may be possible to devise a more scientifically accurate way of measuring the appearance of the sulphur. For example, it could be possible to measure the absorbance of a beam of light passing through it. If this was done in the dark, then the above sources of error could be avoided.
In addition, the sodium thiosulphate and the hydrochloric acid were measured out using a 50cm3 measuring cylinder. This is not the most accurate way to do this, and will introduce more errors, especially in the case of the acid volume, which was only 5cm3. A better way to measure the acid would have been in a burette.
When each of the parts of the experiment is examined in further detail, then more detailed evaluation can be made.
- Concentration as a variable
When the results in Table 2 and Figure 2 are examined, they fall into a pattern which is consistent with the hypothesis. However, there are some points which deviate slightly from the curve in the graph. The value for the concentration 0.09moldm-3 appears to be slightly too low, and that for the concentration of 0.12moldm-3 appears to be slightly too high. These points do not fit onto a best fitting curve through the rest of the points. From analysis of the individual times which gave the average for 0.09moldm-3, there is no obvious anomaly which would account for this, so further repeats would not be likely to help. In the case of 0.12moldm-3, one of the times (36s) would fit onto a best fitting curve. In this case it is the other two times (44s and 41s) which cause the average to be higher. This time, further repeats may help.
When the graph in Figure 2 is analysed, it appears as if the curve is starting to level off at concentrations higher than 0.12moldm-3. In order to see whether this is the case, the experiment could be carried out with 0.18 and 0.21moldm-3 sodium thiosulphate. If it does actually level off, then it is the concentration of the hydrochloric acid which is limiting it from going faster.
The next obvious experiment to do would be to alter the concentration of the acid, while keeping the concentration of the sodium thiosulphate constant. The concentration of sodium thiosulphate which would be used would be 0.09moldm-3, as this one gave consistent results and a sufficiently long time to lower the percentage error in measurement. The results of this experiment would tell us the dependence of the overall rate of reaction on the concentration of acid; in other words, the order of reaction with respect to acid. This was not done because of safety considerations.
When the results from the concentration of sodium thiosulphate were calculated to give reaction rates in s-1, the graph in Figure 4 was the result. This graph has points which are derived directly from those in Figure 2, so the anomalies are the same and occur for the same reasons. However, they will affect the position, and therefore also the gradient, of the best fitting straight line. As the gradient gives us the constant, k, then it must be accurate.
Throughout the experiment on concentration, the temperature was kept as constant as possible. This was approximately 22°C. The temperature could have been more accurately controlled by using a water bath and ensuring all glassware and solutions were at the correct temperature before starting. This would also control any variations in temperature caused by the reaction being either exothermic or endothermic. Another option would be to surround the sides of the flask in an insulation material.
- Temperature as a variable
The results for the temperature variable were shown in Figure 3 and Table 3. The thiosulphate concentration was held constant at 0.03moldm-3. This concentration was used as it gave consistent results in the concentration experiment, and the points fell on the best fitting straight line in Figure 2. In this case, only two repeats were made for each temperature, due to shortage of time. However, the results on the graph all lie on a best fitting curve, so perhaps it would not be necessary to carry out many more repeats. One more would probably give an indication of exactly how reliable the results were.
An additional way to see how reliable the results are, is to compare the reaction rate for 0.03moldm-3 sodium thiosulphate in the concentration experiment, which was carried out at 22°C with the 22°C experiment in this section, which used 0.03moldm-3 sodium thiosulphate. They should be the same, and indeed there is only a very small variation which is probably not significant: 0.0061s-1 and 0.0062s-1 were the rates.
The main sources of error in this experiment are the heating arrangements and the fact that the acid was not heated. The sodium thiosulphate was heated directly over a Bunsen flame, using a thermometer to detect when it was at the correct temperature. This method of heating is not accurate as it is uneven and not all the apparatus will be at the same temperature. As this is the case, it will rapidly cool and so the experimental observation will not be at the same temperature as was intended. Instead, it would be better and more accurate, not to mention safer, to warm the flask and its contents in a water bath. Also, a method could be devised to ensure that the desired temperature was maintained throughout the observation. This could be insulation or even doing the experiment in a water bath.
The acid was not heated for safety reasons, because the acid could not have been heated directly over a Bunsen flame without danger of boiling or hot acid splashing out. The fact that the acid was at room temperature would not have affected the results that much because there was only 5cm3 of acid and there was a much larger volume of sodium thiosulphate, 50cm3. However, the effect would be more pronounced at the higher temperatures. It may be predicted that if the acid was also heated that the graph in Figure 5 would look the same shape but would slope more steeply. This is because the point at 22°C would be the same, but those at higher temperatures would have ever increasingly higher reaction rates. For example, that point at 62°C probably represents a reaction occurring at around 55°C as the thiosulphate has cooled after removal from the Bunsen flame and the acid added is also cooler so further reducing the temperature.
In summary it can be stated that the results in this experiment, while not absolutely accurate, show reliable and reproducible trends which provide convincing evidence for the points proposed in the hypothesis.