- The size and volume of the beaker has to be the same. The beaker is where the reaction is going to take place and the amount of precipitated sulphur will occur. The surface area of the bottom of the beaker must be the same since the larger it is the more time it will take for the reaction to produce the amount needed of precipitated sulphur to cover the visibility of the cross. The material of the beaker must be the same as well, I will use glass because it is highly transparent which is very easy to see through where as plastic is not.
- Another factor to make the test fair is to use the same cross on the paper, as the exact same quantity of sulphur will be required to block the mark (x). The experiment will take place in room temperature so each time a reaction happens it will have the same amount of temperature surrounding it.
- The measuring of the liquids in the measuring cylinders must be done with precision. To do this the solution in the beaker must be on a flat surface and the eye level must be in line with the meniscus level so the proportion of the liquid is measured with high accuracy.
Independent and dependent variables
In the experiment there will only be one Independent variable, sodium thiosulphate, the concentration of this chemical will vary time to time. Diluting proportions of water with the sodium thiosulphate would achieve this but at all point in time the solution will remain 50cm3. By diluting water it will decrease the concentration of Na2S2O3 so in parallel the rate of reaction will also decrease. I will measure the concentration of Na2S2O3 by the unit g/dm3.
The Dependent variables in the experiment would be the temperature and hydrochloric acid. These variables will be the same, as the experiment will be held at room temperature and 5cm3 of hydrochloric acid will be added into the solution at every stage.
Prediction for Independent variable:
In my experiment I will have the sodium thiosulphate as the independent variable. I predict that if I decrease the concentration of Na2S2O3 then more time will be taken for the rate of the reaction to occur as well as the obstruction of the cross. This is because fewer particles will be present in the solution as the water molecules will prevent particles to collide so less chances of collisions taking place, so the reaction will certainly be slower. As there will be fewer particles in lower concentrations of Na2S2O3 then there will be a lower probability of the particles overcoming the activation energy, as it will be more difficult for the particles especially in room temperature to create a successful reaction.
The Preliminary Data
For the preliminary experiment certain apparatus was used such as;
- Safety goggles Cross on a paper
- Conical flasks Stop watch
- Measuring cylinders Hydrochloric acid
- Pipit Sodium thiosulphate
- Tap Water Dry cloth
I will use this equipment for my own experiment as well. The preliminary experiment had been done as a whole class. The preliminary experiment was done so everyone knows how to carry out each stage in the experiment. Firstly we had to layout a detailed and appropriate table to record the results in. This experiment was
held at room temperature and only two concentrations of sodium thiosulphate was used, 40g/dm3 and 8g/dm3, the 40g/dm3 of Na2S2O3 was highly concentrated where as the 8g/dm3 was a very weak concentration. The results were recorded and a huge difference of time can be seen for the cross to disappear. Three trials were tested to increase the reliability for each concentration and also to find the mean average.
In the preliminary experiment only two concentrations of sodium thiosulphate was investigated 40g/dm3 and 8g/dm3. In my experiment I will go further by using 5 different concentrations such as 40, 32, 24, 16 and 8(g/dm3) of sodium thiosulphate. This will give me a much wider range of results that will help me to see a trend. The preliminary experiment let know what, why and how I have to carry out the different stages of my experiment.
Safety precautions during the experiment;
- Ensure all coats and bags are put to the side to prevent accidents (tripping), so the working area is tidy,
- Wear safety goggles at all times during this experiment,
- Be careful when handling chemicals, prevent splashes whilst pouring,
- Clean the surface area if spillage has occurred with a dry cloth,
- Work at a steady pace to avoid accidents.
The Method of the experiment;
Step 1- Measure 50cm3 (depends what stage) of sodium thiosulphate into a measuring cylinder of 100 cm3 make sure the meniscus level is correct to 50cm3 of Na2S2O3 insure this is done on a level surface for better accuracy.
Step 2- Then measure 5cm3 of Hydrochloric acid with a pipit this makes it easier to get the right measurement, in a 10cm3 measuring cylinder, make sure the meniscus level is correct to 5cm3 of Hydrochloric acid but do this on a flat, level surface so the measuring is precise.
Step 3- Measure out the amount of water needed at this stage (0cm3, 10cm3, 20cm3, 30cm3, 40cm3) in a measuring cylinder of 100 cm3. Again clarify the meniscus level to the amounts needed.
Step 4- Get a partner to prepare the stopwatch, make sure the display reads 00.00seconds. Pour in the measured sodium thiosulphate and water into one, clean and dry beaker but also ensure a cross on a piece of paper is underneath the transparent beaker, position the cross mark in the middle of the bottom of the beaker were it can be seen clearly.
Step 5- Then pour in the 5cm3 of hydrochloric acid into the solution insure no splashes are made that will decrease the volume of the solution. Immediately after all of the acid is poured in start the stopwatch.
Step 6- One person in the group has to be responsible for the visibility of the cross as the judgment must be unique. Keep a constant watch over the solution to check when the cross is completely covered by the precipitated sulphur.
Step 7- As soon as the cross (x) disappears stop the stopwatch and record the reading to two decimal place for more accuracy.
Step 8- Drain out the beaker containing precipitated sulphur and rinse the flask with water then wipe with a clean dry cloth.
Step 9- Re-do each concentration three times in this exact method. This will increase reliability and the three outcomes for each concentration can be found a mean time.
Table showing the results of my experiment.
Analysis of my table showing the results of my experiment:
This is the table that shows the results I got from my experiment. I decided to choose three trials for each concentration of sodium thiosulphate so the results I receive are more reliable thus giving me a mean time (seconds). The mean time for each of the concentration will help me to plot a graph. The preliminary experiment had only 2 concentrations the strongest (40g/dm3) molarity and the weakest (8g/dm3) molarity but I decided to include 5 different concentrations of sodium thiosulphate so I can see the difference in time for the cross to disappear and also for a better trend. During the process of my experiment I had realised the weaker the solution the slower the rate of reaction so the time increased. In my initial prediction I knew this would happen because of the collision theory. The collision theory can explain these results because the higher the concentration the faster the rate of reaction.
My results show between the concentration of 40g/dm3 and 16 g/dm3 the three trials are very close to each other but the concentration 8g/dm3 the three trials aren’t as close. This may have been caused by the concentration being to low as the water proportion is 4 times as greater than the sodium thiosulphate volume therefore the rate of reaction is very slow. In this case the water molecules are highly dominant than the Na2S2O3 molecules. I think because the time scale taken for the cross to disappear is large the results are less reliable. It is more difficult to assess the situation on a low concentration in this case, but all this said I still think all my results are very reliable that can give me a good conclusion.
This table shows the results for the rate of reaction that will be used to create my second graph:
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Analysis
My first graph, the time taken for the cross to disappear, clarifies the weaker the concentration of sodium thiosulphate the slower the rate of reaction therefore the time increases for the cross to disappear. This can be proven by the collision theory, the higher the concentration of sodium thiosulphate the faster the reaction. This is because when the solution contains high amount of particles the greater the chances of the particles colliding against each other, depending on the force and kinetic energy the particles contain, a reaction will take place. This can only occur if the particles gain enough kinetic energy (in high temperatures) to build a force great enough to create a successful reaction, this is known as a ‘head on’ collision when a particle with force collides with another particle with full contact. The activation energy depends on the temperature and the concentration (pressure). If the activation energy is high then only a small amount of successful collision will occur as it will be harder for particles to overcome the amount of energy to create a successful reaction therefore the reaction will be slow, this is why in higher concentrations of sodium thiosulphate the time for the cross to disappear was quicker. This is because if there is a lot of particles in a solution then there will be a greater probability of those particles overcoming the activation energy. If there is enough input of energy between the particles a reaction will certainly take place.
My first graph, the time taken in seconds for the cross to disappear in varied concentrations of Na2S2O3, displays a smooth curve and also a trend between the concentrations. The five plots on my graph are the mean time that I had done for each concentration. The mean will give me more reliable results as I had done three trials for each of the concentrations therefore a better conclusion. The best-fit curve demonstrates that all my results are very accurate and reliable as there are no anomalous points. The trend shows the stronger the concentration the quicker the time for the cross to be blocked; it also displays the massive drop from the first plot (8g/dm3) to the second plot (16g/dm3). This shows as the concentration of Na2S2O3 becomes more stronger the time becomes much more similar between each plot. It can be seen as less amounts of water diluted with the sodium thiosulphate and hydrochloric acid the faster the time becomes. This is because there are less water molecules to get in-between the two chemical particles colliding, so there are greater chances for the two chemical particles to collide that may create a reaction (precipitate sulphur) to block the cross. The drop at the first point (8g/dm3) is quite significant because clearly the water molecules had dominated the chemical particles in the solution so the probability of the collisions occurring are very grim, the proportion of water is four times greater than the sodium thiosulphate. As the concentrations gets stronger the line flattens out more, 24g/dm3, 32 g/dm3 and 40g/dm3 the flattening of the line is quite rapid. This is because the experiment was held at room temperature that had been constant throughout, so all the concentrations had the same kinetic energy (motion) which means it was down to the difference in concentration that had separated the time scale for the disappearance of the cross. Therefore the line flattens out because there is no difference in kinetic energy as the three strongest concentrations had similar time scale for precipitating the right amount of sulphur needed to cover the cross.
In my second graph, how the concentration of sodium thiosulphate influences the rate of the reaction, shows a positive correlation and a direct proportional change between the concentration and the rate of reaction. This is also caused by the collision theory; there are more collisions in a concentrated solution so the activation energy is low as the particles collide more frequently. The activation energy has to be kept at a minimum to make it easier for particles especially at room temperature to create a successful collision. If the activation energy is high then a small fraction of collisions have enough energy to overcome the activation energy and the success rate of collisions is low. At higher concentrations of sodium thiosulphate there were more particles present that means greater chances of collisions and greater chances of the particles overcoming the activation energy, this is why the rate of reaction had increased with the concentration of sodium thiosulphate. The graph displays, as the concentration increases so does the rate of reaction both of these factors are parallel to each other therefore the plots make a perfectly straight line going upwards. The plots for each concentration go up each time about the same amount, non of the plots have a similar rate of reaction they always increase with the concentration which is roughly the same amount that they had increased from the previous plot.
The conclusion to my first graph, the time taken for the cross to disappear, is that in higher concentrations if sodium thiosulphate the less time it takes to block the cross. This is because less water molecules are present in the solution to prevent collisions between particles, so the time scale for the reaction to occur was very short. The conclusion to my second graph, how the concentration of sodium thiosulphate influences the rate of reaction is that when the concentration of Na2S2O3 increases so does the rate of reaction, the change between theses two factors are directly proportional so the five plots create a perfect straight line going upwards. This is because there is a greater probability of the particles in higher concentration of sodium thiosulphate to overcome the activation energy therefore the rate of reaction increases dramatically with the concentration of Na2S2O3.
The conclusion to both graphs can certainly support my initial prediction to be correct, my prediction was in higher concentration of sodium thiosulphate the faster the rate of reaction therefore shorter the time will be for the cross to disappear. This can be matched with my conclusion to both of my graphs therefore making my prediction successful.
Evaluation:
The results I received from my experiment I think are very precise, as I have designed two graphs that had been based on these results. The graph, on the concentration of sodium thiosulphate influencing the time for the cross to disappear, displays a perfectly smooth curve that can prove to show my results are reliable. I had also made another graph but this time, how the concentration of sodium thiosulphate influences the rate of reaction again this proves the fact that my experiment was successful as the results were accurate. The way this graph justifies my results to be accurate is because the plots form a perfectly straight line that is directly proportional. There was one factor that had not been as precise as the others; this was the judgment of the visibility of the cross. This is one of the main aspects of the experiment, the judgement may have varied time to time, for example it may have been something to do with the light intensity in the room because the experiment was carried out over a two day period and the weather was not the same for both days therefore the light intensity may have been darker on one of the days. This would have influenced the judgement to differ, as the intensity of light was not constant through out the whole of the experiment. This was slightly improved by carrying out three trials for each concentration. I think to make this situation more accurate, is by closing the blinds in the classroom so no light can enter from outside whilst leaving all the lights in the classroom on, this would differently make the judgement easier and also can be done with more accuracy.
All my evidence that I had collected from my experiment are very accurate, for instance my tables and graphs. I had recorded all the relevant information that I needed from my experiment onto a labelled table which then I had created graphs from. The two graphs approve that my evidence are accurate because neither of my graphs have an anomalous points. My first graph, time taken for the cross to disappear shows a perfectly smooth curve with no anomalous points. My second graph, how the concentration of sodium thiosulphate influences the rate of reaction displays that all five plots create a straight line going upwards which intends again that my evidence is reliable and very accurate. Both my graphs suggests that my evidence is accurate and sufficient to support the conclusion. The conclusion to my experiment is in higher concentrations of sodium thiosulphate the faster the reaction is therefore the quicker the time for the cross to disappear.
I would extend my investigation by carrying out more concentrations of sodium thiosulphate and maybe using other chemicals such as potassium thiosulphate to contrast the rate of reaction between the two. This would let me know how different chemicals influence the rate of reaction and this can also help me to compare the results that would give me a far better conclusion. In order for this to happen I would have to keep everything constant for both chemicals like temperature and the concentration of hydrochloric acid. Another way to extend my investigation is maybe by including a catalyst in the solution of sodium thiosulphate and hydrochloric acid. This would let me go more into depth of the topic, rate of reaction.
The catalyst would let me see exactly how it influences the rate of reaction and how much of the time for the cross to disappear it actually alters. I think the catalyst would speed up the rate of reaction because it would provide a boundary where the particles meet and collide, this should certainly increase the rate of reaction. The catalyst would allow particles to react even if they collide with only low energy, this will make successful collisions more likely. This would happen by the catalyst fixing particles on its surface that will make the chances of collisions more likely. The more collisions there are the faster the rate of reaction. This would certainly precipitate sulphur faster that will make the cross disappear quicker as well.
