Rates of Reaction: The effect of concentration on the rate of reaction
Rates of Reaction: The effect of concentration on the rate of reaction
Prediction
I will be investigating the effect different concentrations of sodium thiosulphate has on the rate of the reaction when combined with hydrochloric acid. The given reaction for this experiment is:
Sodium Thiosulphate + Hydrochloric Acid => Sulphur + Water
I presume that when the concentration of the sodium thiosulphate is increased, the rate of reaction will be higher. This is because if there are more molecules, they are more likely to collide and react within a given volume. However, the collision theory says that only a very small percentage of these collisions results in a reaction. This is because of an energy barrier - only those particles with enough energy to overcome the barrier will react when they collide. Thus, if the frequency of collisions is increased, the rate of reaction will increase. However, the percentage of successful collisions remains the same. As the experiment develops, the rate of reaction is expected to slow. This is because sodium thiosulphate and acid particles will be 'used up' after successful collisions (changed into different products). According to collision theory, and inversely to the original prediction, fewer particles in the same volume of a container will cause a decrease in the rate of reaction.
Variables
The concentration of an acid is not the only factor affecting the rate of reactions. Temperature, surface area and catalysts all seem to be able to change rates.
Temperature - When the substances are heated, the particles of the reactants take in energy. This causes them to move faster and collide more often. Since the collisions are occurring with greater force, more are considered successful. The rate of reaction increases.
To control this variable in our experiment, all tests will be carried out at room temperature (~20 C) and the substance will be subject to no external heat sources.
Surface Area - As a reactant is broken down into smaller parts, more atoms become exposed. This means there is a greater chance of successful collisions, and thus the rate of reaction will increase.
This variable is not necessary to take into account in this particular experiment, as sodium thiosulphate is an aqueous solution at room temperature. As such, no surface is present.
Catalysts - In the presence of a catalyst, a collision needs less energy in order to be successful. The result is that more collisions become successful, so the reaction goes faster.
No catalyst will be present in the testing procedure, so this variable is not needed to be taken into account.
Using our particular method to measure the rate of reaction also presents two other possible ...
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This variable is not necessary to take into account in this particular experiment, as sodium thiosulphate is an aqueous solution at room temperature. As such, no surface is present.
Catalysts - In the presence of a catalyst, a collision needs less energy in order to be successful. The result is that more collisions become successful, so the reaction goes faster.
No catalyst will be present in the testing procedure, so this variable is not needed to be taken into account.
Using our particular method to measure the rate of reaction also presents two other possible variables: the cross and the depth of the container.
Cross - The same marked cross should be used for every test. If new crosses are drawn, they may be bolder (prolonging the time it would be able to be seen) or fainter (decreasing the time the cross would be able to be seen).
Depth of container - The depth of the container should remain constant for every test. If a container of a dissimilar depth is used, it may be easier or harder to see the bottom, corrupting the results.
Results and Graphs - see attached sheets
Analysis
After analysing data obtained from my experiment, I can conclude that as the concentration of sodium thiosulphate increases, so does the rate of reaction with hydrochloric acid.
Two different graphs using the data from my results table have been constructed to help prove this statement.
On the first graph I have plotted volume of sodium thiosulphate (cm3) against time (seconds). This graph shows me the time, in seconds, it took for a solution with varying levels of sodium thiosulphate, water and hydrochloric acid to become too cloudy to see a cross drawn below the flask the reaction was occurring within. The graph exhibits a curved negative correlation as the volume of sodium thiosulphate
decreases.
According to my initial predictions, as the volume (and therefore concentration) of sodium thiosulphate increases, the time it takes for the cross to cloud over should decrease. This is because if there are more molecules, the hydrochloric acid and sodium thiosulphate are more likely to collide and react (the 'clouding' effect) within a container of a given volume. The first graph is consistent with this original prediction. As the concentration of sodium thiosulphate was increased, the time it took for the cross to cloud over became incrementally lower, explained above and shown on the graph as a gentle curve.
On the second graph I have plotted 1000/time (secs-1 to 1dp) against the volume of sodium thiosulphate (cm3). This graph shows me a relative rate of reaction for the different concentrations of sodium thiosulphate as a reciprocal of the time it took for the cross to cloud over. The graph exhibits a linear positive correlation between the increasing volume of sodium thiosulphate and the reciprocated time.
According with my initial predictions, I would expect this graph to take the form of a straight line because the success rate of collisions (the 'clouding' effect) is based on a constant percentage. This would denote the rate of reaction in the tests of different concentrations of sodium thiosulphate remaining constant relative to the volume used. This is due to the concept of collision theory - if there are a greater number of molecules of acid and thiosulphate within a container of a fixed volume, the rate of reaction will increase. Of this increase, however, the percentage of successful collisions remains the same as only those particles with enough energy to overcome the given barrier will react when they collide. As expected, the graph turned out to be a generally straight line, verifying my prediction due to the reasoning above.
Evaluation
After evaluation of my experimental procedure, I have recognized some grounds for unreliability of which I have identified three in detail.
* the human eye was relied upon to measure both the amount of solution required and the time at which the cross clouded over. When measuring the amount of certain solutions, there may have been a small discrepancy as I was not able to tell if the solution was at an exact value due to my vision. In deciding whether the cross had clouded over, my perception may have varied slightly throughout the different tests as I could not remember exactly at what point I had first decided the cross was completely obscured.
* the apparatus used presented a problem in that I could only measure the amount of solution to the nearest cm3, even if my vision was perfect at the time. As such, any given volume may show a discrepancy to half a cm3 either side of the actual value.
* the cross that I used to determine when the solution had reached a certain point of obscurity got wet, and therefore smudged, half way through the experiment. This may have effected the point at which I claimed to not be able to see the cross.
Although due to the reasoning above some of the tests may not have been absolutely fair, I believe the level of precision I employed for this experiment was sufficient to obtain generally accurate results. There was, however, one anomalous reading when I repeated the test involving 35cm3 of sodium thiosulphate. In my first test at this volume, I believed the cross had been obscured after 73 seconds had passed. The repeat claimed this process only took 48 seconds. Because the first result fitted in better with my other readings, I chose to disregard this test and primarily consider my original reading. The grounds for the anomalous result could have been any of the experimental problems mentioned above, the most likely of which I think the result was affected by being the discrepancies of human vision.
I took 11 test readings in all, using decently spread values when deciding upon the volume of the sodium thiosulphate. Of these 11 tests, 4 were repeats. Minus one anomalous result, I have 3 secondary test results that support the evidence of their similar primary tests. This common experimental practice of repeating a test to determine if its first result was accurate leads me to further believe that this experiment provided generally correct results.
There are several improvements I would like to make to this experiment if repeating it another time.
* instead of relying upon the human eye to determine when the cross was clouded over, a computer controlled light intensity monitor could be set up above the flask holding the reactants to provide a more consistent measure of the point at which the solution clouded over.
* more accurate apparatus could be used in a repeat experiment, for example, a measuring cylinder marked to the nearest 1/10 of a cm3. This would decrease the grounds for measurement discrepancy to one-hundredth of a cm3.
* if the experiment was still to be performed using a cross, next time I would make sure it was covered in a waterproof material. This would make sure that no smudging would occur and the cross would maintain a consistent boldness throughout the experiment.
* more repetitions and a greater amount of values could be tested to provide more data for analysis
Each of these possible improvements to the experiment would increase its level of accuracy and therefore the overall reliability of the tests. With a greater timescale, I would also be interested to find out how temperature effects the rate of reaction in combination with the concentration of sodium thiosulphate.
All in all, I think this was a well performed and interesting experiment performed to the highest level possible considering the time and resources available. The results supported my predictions with a fair degree of reliability, and I am happy with the conclusions made.
