Diagram 1- showing the effect of different concentrations of Sodium Thiosulphate on the rate of reaction using the collision theory explained in diagrams.
In this solution of Sodium Thiosulphate and Hydrochloric
acid, there are only a few Sodium Thiosulpahte molecules
to collide with the acid molecules. This means there is less
chance of collisions occurring and a less probability of
effective collisions occurring to form a product. This
reduces the rate of reaction.
In this solution of Sodium Thiosulphate and Hydrochloric
acid, there are a lot of Sodium Thiosulpahte molecules
to collide with the acid molecules. This means there is more
chance of collisions occurring and a higher probability of
effective collisions occurring to form a product. This
increases the rate of reaction.
Sodium Thiosulphate molecules
Hydrochloric acid molecules
This diagram illustrates that as the concentration of Sodium Thiosulphate increases, so will the rate of reaction. A reason for this is the increase in frequency of effective collisions, which means a higher rate of reaction. The fact that Sodium Thiosulphate lowers the required activation level for a reaction to proceed also justifies an increase in concentration means an increase in rate of reaction. This is because, if there are more molecules of Sodium Thiosulphate in the solution, there will be an increase in the weakening of the bonds of Hydrochloric acid. This means there will be an easier reaction between the reactants, and a product will form quicker. The rate of reaction is the time taken for a product to form or be lost.
I predict that the graphs from my results will have a positive correlation, and will be curved, as the increase in the rate of reaction will not be equivalent in value to the increase of concentration this is because as the concentration increases, the initial reaction rate increases. However, as the reaction continues, the concentration gradually decreases in the solution, as there will be fewer moles in the solution.
Graph 1- a graph that reflects my prediction, that the rate of reaction will decreases with time and as the concentration decreases and shows the relationship between the two
1.0
0.5
concentration
of reactant
0.25
0
2 4 6 8 10 12 14 16
time in seconds
I expect there to be a mathematical relationship between the concentration of the solution and the rate of reaction. I think that the speed of initial reaction is proportional to the concentration of the acid. I predict that if the acid concentration is doubled the speed doubles.
Only the initial reaction rate could be predicted. This was because as the reaction in the solution progresses the concentration of the solution will decrease, as there will be fewer particles to collide as the reaction continues. This means I will not be able to accurately record the concentration as the time of the reaction progresses. This the reason that the curve for the graph of results reduces its steepness at the end, because the rate of reaction decreases.
The exact relationship between the rates of reaction is dependant on the reaction ‘mechanism’. The nature of the reaction mechanism that controls the rate of reaction can only be revealed by experimental observation. This means that my mathematical relationship can only be an educated guess or estimation.
Apparatus
eye observing reaction
stop clock
conical flask containing reactants in a solution
sheet with ‘X’ on it
Method
The reaction in the experiment is between Sodium Thiosulpahte solution and Hydrochloric acid. The reaction is used to investigate the relationship between concentration and rate of reaction. When dilute Hydrochloric acid is added to Sodium Thiosulphate solution, a pale, cloudy yellow precipitate of sulphur is formed. This means I will be using the turbidity of the combined chemicals as a measure of how much they have reacted. This means that when the solution becomes opaque, the assumption I can make is that the reaction is complete; therefore, I will stop the stop clock. As the Sodium Thiosulphate solution is diluted, the precipitate took longer to form.
Both the Sodium Thiosulphate and hydrochloric acid are soluble in water, so either of the solutions can be altered. I chose to vary the concentration so Sodium Thiosulphate. I altered the concentration by diluting the Sodium Thiosulphate, but raised the concentration. The volume of Sodium Thiosulphate was always a total of 40cm³. I ensured that the acid concentration was always 1 mol/dm³ and the volume was 10cm³.
10cm³ of Hydrochloric acid with concentration 1m0l/dm³ and 40cm³ solution of S0dium Thiosulphate with a certain concentration are measured with measuring cylinders and the use of pipettes for accuracy. The Sodium Thiosulphate solution is poured into the conical flask; the stopwatch is automatically started when the last drop is added. When the mixture in the flask has turned sufficiently cloudy to the extent, where the letter ‘X’ can longer be noticed. At this point, the stopwatch is stopped and the time is recorded. The experiment is repeated with all the concentrations.
My findings would obviously have a degree of error and there may be anomalies. However to have valid results, I had to ensure the experiment was fair. I used the same standard for judging when the ‘X’ has disappeared. I will try to measure at the same distance from ‘X’, using the same eye so there is a reduction of inaccuracy. I will also ensure the measuring cylinders for pipettes for the Hydrochloric acid and the Sodium Thiosulphate solution will not be contaminated.
I will use fixed volumes, temperatures and any other variables that may affect the outcome of this experiment. I will keep this variable constant by avoiding excess heat so I plan to keep the Sodium Thiosulphate at room temperature. The temperature should be kept fixed because the higher the temperature, the faster the rate of reaction. This is due to the fact when a substance is heated the molecules gain kinetic energy. This consequently results in more collisions between the free moving particles in a solution, causing a reaction between them, which would alter the overall initial reaction rate. This would also mean that the probability of the particles reacting.
The concentration of Hydrochloric acid will be kept constant, by using the acid from the same bottle each time and keeping the concentration constant, by using the acid from the same bottle each time and keeping the concentration fixed. This must be done because the more particles of Hydrochloric acid, the higher the probability of collision, which will lead to a higher rate of reaction.
I am going to ensure I use the same conical flask for each experiment; this is because if the conical flask is wider then the cloudiness will appear less concentrated as it spread across the larger area of the bottom of the flask, so it will take longer for the cross to disappear and the result to alter.
I will make sure that there are no other materials present apart from the reactants; this is so that no catalysts are present. Catalysts speed up reactions, which mean that other variables will influence the reaction. The material on the cross below the flask, which is used determine the finishing of the reaction, easily influences how the cross is seen. This will influence the results; therefore, I must use the same marker or same size and font of the ‘X’ and even using the same sheet time. If I shake or agitate the solution in the flask, then the reaction nay speed up and the rate of reaction will increase. The same effect will be achieved by stirring. Stirring increases the chance of collision and provides more kinetic energy; this consequently increases the rate of reaction.
Table 1- table showing the time in seconds it take for a reaction whilst varying the concentrations
Results
Conclusion
I predicted that as the concentration of Sodium Thiosulphate increases, the rate of reaction also increases. This is reflected in the graph. The graph as I predicted has a positive correlation and curved. This graph is curved because as there is an increase in the rate of reaction and is not equivalent to the value of increase in concentration.
I was partly correct in my prediction, as the concentration increased, so did the rate of reaction. I was correct in saying as the concentration progressed the steepness of the curve decreases. This is because there were fewer moles as the reaction continued, so the rate of reaction decreased.
However, I as prepared for this since it is not possible to find the exact relationship between the rate of reaction and the concentration and concentration without doing the experiment. From the graph, I learnt that the reaction is fastest at the start when the reactants are at a maximum. The gradient becomes progressively less as reactants are used up and the reaction slows down. Finally the graph levels out when one of the reactants is used up and the reaction stops. I predicted that doubling the concentration would double the rate of reaction. That would mean that if I had 05M, I would have an initial reaction finishing in 54 seconds, since 1.0M took 27 seconds. However instead I got 75 seconds. This meant that my prediction was not what I expected.
It appears that as the concentration decreases due to the reaction ‘mechanism’, there are less Sodium Thiosulphate molecules to react with, which means the rate of reaction takes longer as collision has a less probability of occurring. My results and graph both prove this theory, in the sense that the relationship between the concentration and rate of reaction. This also supports the idea that it takes longer for the molecules of Sodium Thiosulphate molecules to find each other so they can react. It also supports the fact that if the molecules move around in solutions at the same speed each time, at a constant temperature, then the further the distance they have to travel to reach its neighbouring molecules, the longer it will take for them to reach the neighbouring molecule and then react.
Evaluation
The results from the experiment appear to be accurate and my conclusion was definite. From this, I can conclude that my experiment was conducted well and the results have one anomalous result, which does not follow the general trend, and show a higher rate of reaction than was expected at that point. This was probably caused by the influence of another variable. In the experiment, there was slight shaking and agitation to the solution. As we know shaking would provide more collisions in the solution, which means there will be an increase in the rate of reaction.
There were only few flaws in the experiment. In the measuring of concentrations and in the judging of when the reaction had completed. This was because it was difficult to get the exact volume of water or Sodium Thiosulphate gets the right concentration. However, these results could have been more accurate by collecting more data. This would mean that if some of the results may be wrong, the results would have been more accurate.
In the experiment, there was no real method for me to realise when the reaction was complete. The method I used in the experiment for my turbidity of the solution was very inaccurate. If I had the opportunity, I would have used a colorimeter. A colorimeter is an electronic device that could detect the turbidity more precisely. It shines a laser through the solution as soon as Sodium Thiosulphate is added. The laser is received by a very sensitive LED on the other side. This LED could be connected to a stop clock, which would stop once the LED stopped receiving direct light.
One area, which could have been more controlled, was the temperature. Temperature increases the kinetic energy and probability of collision. This consequently increases the rate of reaction. Therefore, the fact that the experiment was conducted on two different days and the temperatures were probably different my results are less reliable. However, I did use the same conical flask so that variable did not affect the results, I also had no other material present in the experiment and used the same sheet with ‘X’ on it.
To extend the experiment I could use a bigger volume of Sodium Thiosulphate. This would enable the concentration to remain the same for longer, which consequently means the results will be more reliable and the rate of reaction is more accurate. This is because as the experiment progresses the concentration decreases due to the fact the molecules in the Sodium Thiosulphate react so less of the molecules remain. Observing the other variables in the experiment such as temperature; it would enable me to find out what other variables affect the rate of reaction.
Sources
www.danyey.co.uk
www.courseworkbank.com
www.wpbschoolhouse.btinternet.co.uk
