In this Investigation, I am going to investigate what the effect of increasing concentration of sodium thiosulphate g/dm3 has on its reaction with dilute hydrochloric acid.
The apparatus for this experiment is: a mat with a cross marked on it, a beaker, two test tubes, a Bunsen burner, a tripod and gauze, a thermometer and a stopwatch. The chemicals needed are Sodium thiosulphate in different concentrations, and Hydrochloric acid at 36.5 g/dm3 concentration.supplied.
The concentrations of sodium thiosulphate available to us are: 5, 10, 15, 20, 25, 30, 35, and 40 g/dm3.
Preliminary results are done to find out which results we actually need to do, so that we do not waste time, taking reading we do not need to take, to find out the best way to do the experiment, and to know which factors you need to take into account i.e. for a fair and valid test to take place, and to examine the best concentrations and to look at the effect of temperature and to choose the best volumes of HCL and Na2S2O3 to use.
Preliminary Method
Firstly a mat with a cross on it was placed on the desk, and a beaker placed on top of that. I then collected 25 cm3 of 5g/dm3 of sodium thiosulphate, and 15 cm3 of hydrochloric acid in measuring cylinders. I poured them into the beaker and started the stopwatch, when the cross could no longer be seen I stopped the stopwatch and recorded the time, I then did this again for increasing concentrations of sodium thiosulphate, I did it with: 25,40 g/dm of thiosulphate, and recorded all of the results.
Preliminary diagram
Results
I have now found out from my preliminary testing what I need to do to get my, experiment fair, and to inform me to write my full method.
Apparatus
A matt with a cross marked on it, a beaker, two test tubes, a Bunsen burner, a tripod and gauze, a thermometer and a stopwatch. The chemicals needed are Sodium thiosulphate in different concentrations, and Hydrochloric acid at 36.5 g/dm3 concentration.
Method
Firstly, I have discovered that doing the smallest concentration of the sodium thiosulphate (5 g/dm3) would be unnecessary as it takes over10 minutes to complete, I also found that I should keep and monitor the room temperature for the experiments, as I can’t keep the temperature exactly the same throughout the experiment without using other apparatus such as a water bath.
So I decided to start with the concentration of 10 g/dm3, so I collected 25 cm3 of this and also collected 15 cm3 of the hydrochloric acid. I placed a beaker over a mat with a cross drawn underneath, then added the two solutions together, I started timing the reaction straight away, after it had finished, and cross had been completely obscured, I stopped timing it, and rinsed out the equipment. I then went on to using the concentration of 20 g/dm3 as this was double the concentration I used before so therefore the rate would hopefully be doubled, but it does not however matter what order you do the experiments in. I collected 25 cm3 of this, and 15 cm3 of acid, and poured them together into a beaker over a cross, I timed the reaction and when the cross was obscured I stopped timing, I then rinsed out the equipment, I then repeated the experiment but I used the concentration of 40 g/dm3 of sodium thiosulphate as this is double the last concentration I used and will help me when it comes to plotting my results, and will show a clear trend. All experiments were carried out 3 times each and with 6 different concentrations. However it does not matter what order you do the experiments in!
Valid Testing
- Fixed, this is where a substance, amount or concentration stays the same for the whole experiment.
- Varied, this is where a substance, amount or concentration is varied each time.
- Measured, where an amount or time are recorded for later use.
Diagrams
Prediction
I predict that as the concentration of the sodium thiosulphate increases, so will the rate of reaction. This is to do with a variety of things, firstly if the sodium thiosulphate is more concentrated there will be more particles of sodium thiosulphate per unit time, this means that they are more likely to have a successful collision with the hydrochloric acid particles, by successful I mean breaking the old chemical bonds and forming new ones, i.e. exceeding the activation energy, activation energy is . If the Concentration was to say double or even triple then I would expect the rate to go up by twice or three times as much. As Energy is conserved in chemical reactions, most reactions can be generalized into two distinct steps. Firstly, the bonds of initial reactants are broken and secondly the resulting constituents rearrange themselves, forming new bonds. Breaking a bond, pulling a molecule apart, requires a certain amount of energy that will later be released if that same bond reforms. “Strong” bonds take more energy to break them apart. If stronger bonds form in the products than are broken in the reactants, energy is released to the surroundings as heat, and the reaction is termed exothermic. If stronger bonds break than are formed, energy must be absorbed from the surroundings, and the reaction is endothermic. Because strong bonds are more apt to form than weak bonds, spontaneous exothermic reactions are common. For example, the combustion of carbon-containing fuels—that is, the combination of the carbon with oxygen from the air—gives carbon dioxide and water, both of which possess strong bonds. Spontaneous endothermic reactions, however, are also well known; the dissolving of salt in water is one example. Endothermic reactions are always associated with the spreading of energy. Increasing the concentration increase the reaction rate by increasing the rate of molecular collisions.
Results
Analysis
From my graph (on separate sheet) I can see that there are a lot of similarities in my results, and quantitive relationships, for example the rate for 10 is 4.37, and that of 20 is 9.80. This is not exactly doubled but it is very close, and if the results were taken under better conditions that could be kept more constant then they would be more accurate and therefore the doubling would be more exact. I knew this would happen as I have stated it in my prediction beforehand. Also from 20 to 40 g/dm3 of sodium thiosulphate it is almost doubled from 9.80 to 21.10, also this would be more accurate under better conditions, but as you can see from my graph, my line of best fit, goes quite well with the graph and generally fits in meaning there are not many anomalies.
Evaluation
Firstly I am going to start by making a relevant comment on the experiment. I think that the experiment that I did went well, in the preliminary experiments the temperature experiment was a success as it proved that with temperature as a catalyst the speed and therefore rate of the reaction will increase.
On the actual experiment itself I think I could have done it a bit better as the results show,
There are a few anomalies, if you follow the best fit line, for 20 and 30 g/dm3 of sodium thiosulphate the 20 is below the line of best fit, and the 30 is over the best fit line. These could be anomalous for varying reasons: firstly the measuring cylinders I used are not very accurate, as it is hard to judge exactly the measurements, a way to get over this obstacle would to be to use a syringe, or even better a pipette with precise measurements on it, and so you can get it exactly right. Also it is hard to judge exactly when the mixture is cloudy enough, a good, and probably the best way to get around that is to use a light metre, this way you can set it up as follows:
Setting it up this way allows the light metre measurements to go down as the mixture clouds and as it gets to a specific level then the time is recorded, this is the fairest way of testing. From my results I can conclude that when you double the concentration of sodium thiosulphate when it is reacting with acid, then the reaction is doubled, and so on, for example with 10g/dm3 of sodium thiosulphate reacting with hydrochloric acid, the rate is 4.36 and for 20 g/dm3 of sodium thiosulphate reacting without hydrochloric acid the rate is 9.80 these are almost perfectly doubled they are only slightly out due to the fact that we could not get the measurements perfect.
