For this investigation, I shall be using the most precise and accurate equipment that is required for the results to be accurate and reliable. There may, as one may expect, be a few problems concerning the accuracy of the equipment that might be used. For example, if one was going to use a 100cm3 measuring cylinder to measure –out 10cm3 hydrochloric acid, there may be an accuracy problem. This is because a 100cm3 measuring cylinder does not show a reading of 99.7cm3 (for example), it only shows 99cm3 and 100cm3. Therefore, measuring instruments that are accurate to the volume / mass being measured should only be used. In this case, a 10cm3 measuring cylinder should be used, for its accuracy.
Preliminary Work:
With this experiment, preliminary work should be conducted to find out what temperature values are appropriate when reacting hydrochloric acid and sodium thiosulphate. This is necessary, so that the particles in both substances gain enough energy, and enough velocity from the heat, to collide and react quickly, or slowly enough for the experiment to work, and for myself to be able to judge how fast the reaction takes place, and eventually the rate of reaction.
Furthermore, preliminary work to find out what values for the acid concentration are appropriate to give me results that are spread-out enough, and have a good enough range for me to work out the rate of reaction should be conducted. This is necessary, so that the acid concentrations used are not too high or too low, so that the particles do not react too quickly or too slowly (as concentration is proportional to rate of reaction as discussed previously).
After completing these pieces of preliminary work, I found that the temperature does not alter the rate of reaction too considerably. Therefore, the temperature (that will be controlled) of the reaction will be room temperature, to cut down the amount of work required.
Also, the preliminary work has shown that the most appropriate concentration of acid values are 1.5M, 2.0M, 2.5M, 3.0M, 3.5M and 4.0M. These values allow the reaction to complete not too slowly, and not too quickly, allowing the rate of the reaction to be worked out accurately.
Key factors to vary: Variables:
Prediction:
I predict that as the concentration of the hydrochloric acid is increased, the rate of reaction also increases (they are directly proportional to one-another). Therefore, as the concentration of acid is increased, the time taken for the cross to disappear will decrease.
Prediction Reasoning:
The reasoning for this is that as concentration increases, the amount of hydrochloric acid in a given volume (cm3) of that substance increases. Therefore, more particles of hydrochloric acid will be present in a given volume, and consequently more collisions between hydrochloric acid particles and sodium thiosulphate particles will occur (when the sodium is added to the hydrochloric acid, and the reaction begins). If more collisions between particles occur, the rate of reaction will then increase directly to the concentration of acid increasing. (RESOURCE 2)
Using the above predictions, I have come to the following expectations for the graphed results to be produced:
Apparatus list:
The following apparatus / equipment will be required, to be used in the experiment:
-
2 x 10cm3 measuring cylinder
- 2 x pipettes
- 1 x conical flask
- Supplies of:
- Hydrochloric acid
- Sodium Thiosulphate
- Water
- Stopwatch / time measuring equipment
- Safety goggles
Plan:
Firstly, goggles will be worn, and all apparatus to be used will be gathered, and arranged as the diagram shown below indicates. Next, using a 10cm3 measuring cylinder, 10cm3 sodium thiosulphate will be measured out. Then, using a different 10cm3 measuring cylinder, 10cm3 of 4.0M hydrochloric acid will be measured out. A cross will then be drawn on a piece of white card / paper, and placed under a conical flask. Next, the sodium thiosulphate will be placed into the conical flask on top of the cross on card / paper. Then, a stopwatch will be taken, and the hydrochloric acid added to the sodium thiosulphate in the conical flask. The stopwatch will be started at this point, and kept running until the person judging when the cross has ‘disappeared’ behind the solution says so. At this point, the stopwatch will be stopped, and the time taken for the cross to ‘disappear’ recorded.
The apparatus used will then be thoroughly washed-out in water, and re-used. This process will then be repeated twice more using 4.0Molar hydrochloric acid, and then repeated three times for each of the other acid concentrations (3.5M, 3.0M, 2.5M, 2.0M and 1.5M). In total, 18 runs will be completed. In order to make the other concentrations of acid, the following formula should be used:
Volume of acid x Concentration of acid = Solution volume x Solution Concentration
In this case, we do not know the volume of acid to be used in the first place, to be added to water to make up 10cm3 of the solution. Therefore, using the above formula, I have made the following table to indicate how to make-up the concentrations of acid required:
Diagram of main section of experiment:
Experiment Method:
(Same as plan).
Resources Used:
Results:
Graphic results:
(Please see the two attached sheets).
Conclusion:
From the results shown above, and the graph, I conclude that the initial predictions, that ‘as the concentration of the hydrochloric acid is increased, the rate of reaction also increases’, and that ‘as the concentration of acid is increased, the time taken for the cross to disappear will decrease.’ are correct. The predicted graphs are very similar to those produced from the actual experiment, and this shows that the prediction was very precise. From the graphs, it is clear that concentration of acid used is directly proportional to the rate of reaction, as the line of best fit produced has a gradient very close to 45°, which indicates this. The actual line of best-fit gradient was calculated to be 0.00085 / 1.8 = 0.00047. The equation of the line of best fit was then worked out to be ‘y = 0.00047 + 0.0104’, using the equation of a straight line rule, ‘y = m + c’.
The scientific reasoning for this conclusion is that as concentration increases, the amount of hydrochloric acid in a given volume (cm3) of that substance increases. Therefore, more particles of hydrochloric acid will be present in a given volume, and consequently more collisions between hydrochloric acid particles and sodium thiosulphate particles will occur (when the sodium is added to the hydrochloric acid, and the reaction begins). If more collisions between particles occur, the rate of reaction will then increase directly to the concentration of acid increasing.
Evaluation:
Overall, the results obtained were fairly accurate in the experiment conducted. Inaccuracies with the conduction of the experiment may have been brought about by the following problems:
- All time measurements were gained using manually controlled equipment, which relies on the accuracy of the human eye (such as in the use of a stopwatch).
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All measurements of liquids, such as hydrochloric acid were measured using manual equipment, which can be inaccurate (as they relies on the accuracy of the human eye). Also, the measuring cylinders can sometimes be slightly inaccurate, due to the fact that the ones used could only measure to one decimal place (8.7cm3 for example).
- Judging when the cross has ‘disappeared’ behind the sulphur in the solution produced can often be inaccurate, as the lighting present often meant that a shadow was cast over the conical flask in which the reaction was taking place. This meant that often, one could not judge accurately when the cross had in fact ‘disappeared’.
Furthermore, the procedure had a few problematic areas, mainly concerning the controlled variables:
- Three of the four controlled variables were actually controlled with accuracy, being ‘presence of a catalyst’, ‘light intensity’ and ‘the person judging when the cross had ‘disappeared’’. These three variables were always the same, as the experiment was conducted in the same laboratory every time, and by the same person (myself). Also, no catalysts were used in the experiment, and therefore the reaction ‘activation energy’ was as it should be (according to the acid and sodium thiosulphate used)
- The fourth control variable could not be controlled whatsoever under the circumstances. The temperature of the laboratory in which the experiment was conducted could have been changing slightly, during the conduction of the experiment. This can, as previously explained, have an effect on the rate of a reaction, because when temperature is increased, rate of reaction also increases.
As explained, the results are fairly accurate, but are not particularly reliable. This is because there are so many factors that can affect the time taken, and consequently the rate of reaction obtained and recorded. For example, human error, the equipment used and contamination of reactants used can all have an effect on the results (rate of reaction) obtained, but not considerably. Therefore, I feel that the evidence gained, in conjunction with the graphed results found is sufficient to support a firm conclusion as shown.
On the graphs, certain anomalous results were identified, as they were clearly not following the same ‘pattern’ as the other results obtained. With the error bars shown, only two of the four anomalous results (incorporating both graphs) were actually found to be anomalous, these are those highlighted in bold. This is because when incorporating error from when obtaining results, two of the previously ‘anomalous results’ actually could have been following the ‘pattern’ indicated by the other results, and the results predicted. The reasons for including error bars of the certain heights shown were due to the fact that the ‘time taken’ results obtained could have been ±2 seconds. This then affected the rate of reaction directly, because rate of reaction was calculated by 1 / seconds. The error bars on the graph showing the affect of concentration on the rate of reaction were then worked out by calculating the rate of reaction of the values shown on the error bars from the other graph.
In order to improve the accuracy of the evidence further, the following changes could be made to the overall experiment:
- Electronic measuring devices, and precise data logging could have been used to measure when the sulphur produced in the precipitate reaction had completely blocked-out all light coming through the solution produced.
- Electronic measuring devices could also have been used to measure-out precisely the hydrochloric acid and sodium thiosulphate used in each reaction.
- New measuring cylinders could have been used to measure-out each new amount of hydrochloric acid or sodium thiosulphate used, reducing the effect of contamination on rate of reaction obtained.
- To overcome the temperature differences present with the experiment, the reactions could have taken place in a temperature-controlled water bath, or by using a Bunsen burner to heat the hydrochloric acid and sodium thiosulphate before the reaction had begun.
To extend the experiment with new lines of enquiry, I could change the concentration of sodium thiosulphate instead of the hydrochloric acid, to analyse and compare the results obtained with the results obtained from this experiment. Also, different variables such as temperature or presence of a catalyst could be used as input variables, other than concentration of the acid used.