Collision Theory - The Collision theory builds on the basis that a reaction between two or more chemicals requires (at the molecular level) a collision between two rapidly moving molecules. If the two molecules collide in the right way and with enough kinetic energy, one of the molecules may acquire enough energy to initiate the bond-breaking process. As this occurs, new bonds may begin to form, and ultimately reagent molecules are converted into product molecules. The point of highest energy during bond breaking and bond formation is called the transition state of the molecular process. The difference between the energy of the transition state and that of the reacting molecules is the activation energy that must be exceeded for a reaction to occur.
Reaction
A – The graph is often steepest at the start of the reaction, because there are plenty of reactants and so the reaction is hopping more quickly.
B – The rate of reaction slows down as the reactants begin to get used up.
C – When all of the reactants have been used up no more product are formed and the reaction stops.
Preliminary investigation
In this experiment I can change factors such as temperature, concentration or use a catalyst in the experiment. I will be changing the concentration (varying factor) of the solution (sodium thiosulphate) to observe how the rate of reaction changes when reacting with the hydrochloric acid. The ‘dilute’ sodium thiosulphate solution will always be 50ml (the initial concentration for sodium thiosulphate is 0.1 moles per litre, for the hydrochloric acid it is 0.5 moles per litre). Water will be added and the amount of sodium thiosulphate reduced in 5 ml per interval to maintain the 50ml. This solution will be added to 5ml of hydrochloric acid (which will be constant), (see method for more details).
In this experiment it would be relevant to look at the precipitation instead of the other two possible observations, the volume of gas released and the change in mass. It would be much more simpler to do this and it is relevant in this experiment because I am not sure if any gas gets released during the course of the reaction. I will observe the precipitation by placing a cross directly under the beaker (see method for more details). A syringe will be used for accurate measuring of the solution. I will keep all other factors (except concentration) the same. A catalyst will not be used; I will reduce the Sodium Thiosulphate by 5ml at each interval instead of 10ml. I will also perform the whole experiment three times for accurate results. By this I hope to achieve a fair test and accurate results.
Prediction
I think as the amount of sodium thiosulphate decreases (becomes less concentrated, more diluted in water) the rate of reaction will also decrease. The solution will stay clearer for longer and the cross will be visible. I think this is due to he fact that when any solution gets diluted in water it gets ‘weaker’. There would be less sodium thiosulphate particles and so more collisions would be required with the hydrochloric acid for any reaction to take place thus the rate of reaction would be affected. I also think that the increase in time will not be a constant one, the amount of time required would be similar readings for few measurements and much different for other, e.g. when sodium thiosulphate will be 5ml and water 45ml it would take a great amount of time compared to the earlier measurements.
Hypothesis
I think as the amount of sodium thiosulphate decreases as it becomes less concentrated and more diluted in water because there would be less reacting particles in the solution.
Increase in Concentration (or Pressure) increases the number of collisions.
When there are more particles of around to take part in a chemical reaction, they bump into each other more often, which make collisions between the important particles more likely. In a gas increasing the pressure means the molecules are more squashed up together so there are going to be more collisions.
Apparatus:
Conical Flask
50 ml Measuring Cylinder
10 ml Measuring Cylinder
Stopwatch
Sodium Thiosulphate (0.1m)
Hydrochloric Acid (0.5m)
Thermometer
Syringe
Paper with cross
Safety googols
Method
- Get all the required apparatus.
- Place beaker directly under the cross
- Measure 5 ml of Hydrochloric Acid into the 10 ml measuring cylinder from the Hydrochloric Acid bottle.
- Measure 50 ml of Sodium Thiosulphate into the 50 ml measuring cylinder from the Sodium Thiosulphate bottle (check the temperature of the Sodium Thiosulphate).
- Add both Sodium Thiosulphate and Hydrochloric Acid to the conical flask and start timing.
- Stop timing when cross becomes obscured (viewing from above of the conical flask [birds eye-view])
- Wash the conical flask with water so any chemicals will not remain and repeat experiment by changing the concentration of the Sodium Thiosulphate at each interval (see table or different concentrations).
Results Table (Obtaining Evidence)
After performing the experiment I recorded my results on the following results in the table.
mins – Minutes, secs – Seconds
In the 3rd set of results the last reading (Sodium Thiosulphate = 5ml, Water = 45ml) was not recorded because I did not have enough time to complete timing it, as it does take a very long time to become obscured due to the very low concentration. Therefore I will be considering the previous readings on that row and have worked out the average by the two of them instead of the normal three. This may make the result for this row less accurate there still are two measurements.
I looked at each solution at eye level on a flat bench so that the meniscus that normally occurs with liquids doesn’t affect the actual measurement. There were two people observing the cross to decide weather it had become obscured to reduce the chance of human error. The temperatures for all the liquids used were room temperature; to be certain of this I measured the Sodium Thiosulphate, which was 24ºC.
Justification of apparatus
The essential apparatus were: The conical Flask, 50 ml measuring cylinder, 10 ml measuring cylinder, the stopwatch, Sodium Thiosulphate (0.1m), Hydrochloric Acid (0.5m) and the paper with cross.
The apparatus listed below were used for extra accuracy or safety.
Thermometer – accuracy (temperature does affect the rate of reaction so we had to check the temperature).
Syringe – accuracy (greatly helps with accurate measuring of the liquids).
Safety googols – safety (sodium thiosulphate and hydrochloric acid could have a serious effect on the eye).
Analysis
My prediction was proved by the results acquired in the experiment. The rate of reaction decreased as the Sodium Thiosulphate became more diluted in water. The rate of reaction was defined by the amount of time it took for the cross to become obscured. The flask containing the solutions was placed directly above the cross. The timer started timing when the Sodium Thiosulphate was added to the Hydrochloric Acid. The temperature for both Sodium Thiosulphate and Hydrochloric Acid was 25° C before the reaction started (the temperature was not measured during or after the reaction). There were two people observing the cross to decide weather it had become obscured to reduce the chance of human error.
The results very clearly show that the water slowed down the rate of reaction (see results table). It also shows that during the range 50 ml – 5 ml of the Sodium Thiosulphate, the rate of reaction decreased in three main groups (see comparison table below). I have compared the relationships between the increasing time at all the individual groups and have calculated an average, (the experiment was performed three times for increased accuracy, in last part of the experiment I did not have enough time to measure the interval where Sodium Thiosulphate is 5 ml and water is 45 ml, however there are still two other measurements for this interval so I can still get a relatively accurate result).
Comparison of Results
A – I calculated the average result for this section by adding all the times and dividing them by the number of reactions: 54 seconds divided by 5 = 10.8 seconds. At this stage the reaction was happening at a very fast rate due to the intensity of the concentration of the Sodium Thiosulphate.
B – The average was calculated using the same method; it was 44.2 seconds slower then the previous section by being 55 seconds. These were the average results where the concentration of Sodium Thiosulphate was starting to decrease from equal concentration with water. (Same method used as A)
C – The increase in time for the cross to become obscured was immense, (same method used as A). The solution had become very dilute in water (look at concentration section in scientific information) and it took a very long time for the reaction to be completed. The increase could be further investigated and be labelled as anomalous. I don’t think it is very wise to put the two measurements together in the same group because the difference in time between them is the greatest out of all the results, however these two measurements are greatly out of bounds of any of the previous measurements and so they were put in one group. The average increase was 623.5 seconds (11.8 minutes). This was approximately an eleven times increase from the previous group. At this stage of the experiment it took many more collisions (look at collision theory section at scientific information) then I predicted. If looked at closely it would be easy to see that the two measurements in this group have a difference that was double the first result, i.e. when Sodium Thiosulphate = 10 ml time = 11 minutes and 1second; when Sodium Thiosulphate = 5 ml time = 25 minutes and 1second, increase by 840 seconds.
The graph shows the separate trends and show how the time increased. The results coincide with my prediction.
Evaluation
The experiment went moderately well. The apparatus used for the experiment was very helpful and relevant. All the required data that needed to be collected had been done except the measurement that I could not do due to the lack of time (3rd set of results, Sodium Thiosulphate 5 ml, Water 45 ml). Only two measurements were used to calculate the average for that interval which would reduce the accuracy of that interval but would not affect the credibility of the whole experiment.
I did observe and conceder the meniscus that was present on each liquid, to overcome this and to make measurements more accurate I placed the measuring cylinder with the liquid inside on a flat bench and looked at it at eye level.
I decided to reduce the concentration of Sodium Thiosulphate by 5 ml at each interval instead of 10 ml to show in further details, any trends and relationships. Using the results I constructed two tables (results table and relationship table) and a line graph, (look at graph and tables).
The initial concentration for Sodium Thiosulphate was 0.1 mole, which was gradually reduced using water. The initial concentration for the Hydrochloric Acid was 0.5 moles, which remained constant throughout the experiment.
I did not observe any specific anomalous data in my results. The massive time increase between the last two calculations (Sodium Thiosulphate 5 ml, Water 45 ml) can be acceptable because of the extremely low concentration of Sodium Thiosulphate. In my opinion I think that the graph should have looked more smother; this may be due to the overall accuracy of the method used or any limitations.
I would recommend changes by:
- Conducting a preliminary investigation to observe and find out if any gasses are released during the course of the experiment, if so then another experiment could be conducted and the results of both experiments compared.
- Calculating the concentration of the solutions, much more accurate results.
- The Water, Sodium Thiosulphate and Hydrochloric Acid could have been weighed; this would be very simple and more accurate then using the measuring cylinders.
- Temperature could have been measured after each reaction to see if any heat had been given off.