There are several different factors that will affect the rate of this reaction they are: Concentration of Sodium thiosulphate (this is what I will be investigating), Temperature of the reaction and Pressure.
What is the rate of reaction?
The rate of a reaction is the speed at which a reaction happens. If a reaction has a low rate that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands of years while other can happen in less than one second. The rate of reaction depends on the type of molecules which are combining.
Concentration: The concentration of the Sodium Thiosulphate affects the rate of reaction because the more molecules there are of sodium thiosulphate, the more often they will collide with one another the faster the rate of reaction because the collisions cause the reactions. A more concentrated solution has more molecules per ml and therefore the faster the rate of reaction.
Temperature: ‘The hotter something is, the quicker the particles in it move.’ (Kinetic theory). This shows that the hotter the solution is, the quicker the particles in it will move, therefore they will collide more often and they react more quickly because they react as they hit. This faster movement is caused because of an increase in energy. Therefore by increasing the temperature of this reaction the amount of energy in the reaction is also being increased which will subsequently make the rate of reaction faster.
Pressure: In gases if the pressure is increased there will be a greater number of particles within a given space this will result in the particles colliding more frequently and much faster. This will then result in an increase in the rate of reaction.
Safety: Sulphur is a choking gas, plenty of ventilation is required. Windows should be left open.
Also safety goggles should be worn as acid is being used.
Apparatus / Chemicals: Pipette, Hydrochloric acid 1 mole and 2 mole, Sodium thiosulphate, water, 3 beakers, measuring cylinder, tile, black marker, conical flask and a stop watch.
Fair Test: In order to make this a fair test I will be using the same equipment and same batch of chemicals throughout my experiment. I will try to keep all of the dependant variables the same. These include keeping the temperature the same throughout the experiment (room temperature). I will keep the overall volume of the solution the same and keep a constant volume of hydrochloric acid throughout the experiment. I will also swirl the mixture for an equal number of times (2) and make sure that I swirl it with a similar force. My independent variable will be the concentration of sodium thiosulphate which gradually diluted.
In order to make this experiment reliable and precise I shall be using equipment such as pipettes to ensure that this is the case. I will also repeat each trial three times in order to make sure that I have not got any anomalous results and that they are all reliable.
The same person should judge whether the cross has disappeared as everyone has a different opinion and different degree of accuracy. The same sized cross should also be used as a larger one may take longer for the sulphur to cover it.
I must also use the same beaker as the bottom of the beaker may have a different surface area and it may take longer to precipitate enough sulphur to cover the cross.
Preliminary Work:
Aim: In my preliminary work I will determine the volume, concentration and best method in order to plan my final experiment. These factors include the volume of: hydrochloric acid and its molarity, volume of sodium thiosulphate.
Apparatus and chemicals: Pipette, Hydrochloric acid 1 molar and 2 molar, Sodium thiosulphate, water, 3 beakers, measuring cylinder, tile, black marker, conical flask and stopwatch
Method: After collecting all of my equipment my first trial was conducted using 25 ml of sodium thiosulphate and 5ml of 1 molar hydrochloric acid. I placed both the sodium thiosulphate and hydrochloric acid inside a conical flask. Underneath the flask was placed a tile with black ‘X’. On adding the two substances into the flask, I gave the solution 2 swirls and began timing on my stop watch. I was able to tell that the reaction was complete when the solution containing the sodium thiosulphate and hydrochloric acid had become cloudy and the cross could no longer be seen due to sulphur being formed. Due to the solution becoming cloudy I was not able to see the black ‘X’ on the tile, when I could no longer see it I stopped timing on my stop watch, this told me that the reaction between the sodium thiosulphate and hydrochloric acid had come to an end.
I repeated the same procedure with the hydrochloric acid and sodium thiosulphate but instead used different amounts of each substance. I did three other experiments including:
- 25 ml of sodium thiosulphate reacting with 5ml of 2 mole hydrochloric acid
- 40 ml of sodium thiosulphate reacting with 5ml of 1 mole hydrochloric acid
- 40 ml of sodium thiosulphate reacting with 5ml of 2 mole hydrochloric acid
Results:
Conclusion: From my results I am able to see what measurements for each substance I am going to be taking for my final experiment. I have decided to use 40 ml of sodium thiosulphate. I have decided to use 40ml of sodium thiosulphate because it gives me a better variety of results and would provide me with a better graph, also when conducting the 25ml of sodium thiosulphate experiment I found that it took too long to react with the hydrochloric acid and this would not be the best way to manage my time.
I found the volume of acid used to be sufficient for a reaction to occur. I felt I needed to use hydrochloric acid which has a molarity of 2 which reacted with the sodium thiosulphate in a faster time than the one molar. The 1 molar made the reaction too slow and therefore if I used this I would not be able to complete my practical within the given time. By fitting all of my practical work into one lesson would also give me good results as variables such as temperature would be kept the same, whereas over two lessons these could fluctuate drastically causing an error in my results.
Evaluation: In my actual experiment I will be testing the following amounts of hydrochloric acid and sodium thiosulphate; I will also be using distilled water to dilute the solution.
I have decided to dilute the solution by adding 5ml of water each time and I will see how this affects the rate of reaction. By adding 5ml of water each time I will also be decreasing the amount of sodium thiosulphate by 5ml, thus diluting the solution.
However I will keep the overall volume of the solution constant at 40ml in order to ensure a fair test.
In my preliminary work having analysed the results that were obtained for the different volumes and concentration of solutions used I felt that I am now able to plan an accurate and fair method to give me precise results in my actual experiment.
Method: After collecting all of my equipment and setting it up. I measured 35 ml of sodium thiosulphate along with 5 ml of hydrochloric acid, for my first trial I did not use any water. I added both the hydrochloric acid and sodium thiosulphate into a conical flask. On adding the two substances to the conical flask I began timing on my stop watch. I swirled the flask twice with the same force each time and placed it on a white tile on which was a black cross. As the reaction took place the solution containing the hydrochloric acid and sodium thiosulphate became cloudy and a yellow precipitate formed known as sulphur. When I could no longer see the cross on the tile I stopped timing on my stop watch and recorder the time to 1 decimal place. I repeated each trial three times in order to give me accurate and fair results. I used the following volumes of sodium thiosulphate; I decreased the volume of sodium thiosulphate thus decreasing the concentration by diluting the sodium thiosulphate with water.
I used the above volumes of each substance decreasing the volume of sodium thiosulphate and increasing the volume of water proportionally.
Diagram:
Results:
Graphs:
Analysis of results and graph: From my results I can see that as the concentration increases the rate of reaction also increases this can also be seen from my graph, through the curve on the graphs. Graph 1 shows that initially there is a steep decline in the average time taken for the cross to disappear as the volume of sodium thiosulphate is increased. The graph then flattens out between the following values of sodium thiosulphate: 20,25,30,35 ml, showing that the numbers of particles present are sufficient in number to have the reaction commence straight away rather than in a dilute solution (at the start of the graph) where it takes time for the reaction to commence and then continue until it is complete.
Graph 2 shows a more linear form and this can be seen through the line of best fit, this shows that if my experiment had been conducted 100% accurate the points would have fitted along the line of best fit and that as the concentration increases the rate of reaction also increases in proportion.
Conclusion: From my results and graph I can conclude that as the concentration of the sodium thiosulphate decreases the rate of reaction also decreases, I can therefore conclude that my prediction was correct. The graph in my prediction matches the graph I produced using my results, and support the collision theory.
This is because as concentration is increased there are a greater number of molecules within the sodium thiosulphate and hydrochloric acid solution, due to this increased number of molecules they can collide with each other at a much faster rate and more frequently over a given period of time. This in turn results in the rate of reaction being increased and results in the sulphur being precipitated in a faster time, this also means that the black cross on the tile disappears in a faster time when the concentration is increased.
Evaluation: The results in my experiment appear to be accurate as they have provided me with a good graph and results which confirm my prediction. From this I can speculate that my experiment was a success and was very accurate, I kept all of the variables controlled to the best of my ability in order to increase the reliability of my results. The improvements that could have been made to this experiment were keeping the temperature at which the experiment was being conducted more accurate. This was because the room temperature was not as reliable and it fluctuated consistently throughout the time in which the experiment was being conducted due to windows and doors being opened and closed throughout the experiment.
New conical flasks should have been used as a residue amount of hydrochloric acid and sodium thiosulphate may have remained in the flask and could have affected the results. I think that the point at which the cross disappeared was a controversial issue as a persons judgement is different every time, a data logger could have been used to eliminate this problem it could have been attached to a computer and should have been light sensitive to tell when the cross on the tile had disappeared.
If I were to repeat this experiment again, I would use a more accurate way in telling me when the experiment was over e.g. by using a light sensitive data logger, I would also use clean conical flasks or wash them out more thoroughly to help gain more accurate results.
From my results I produced a graph which showed me that I had no anomalous results as all the plotted points fitted into the graph accurately and also the figures obtained in my data seemed to resemble each other.
Further experimentation I would test the other factors that affect the rate of reaction for example temperature, surface area and the use of a catalyst.
Omar Jamshad 10S / B1
