Investigating a Factor affecting the rate of reaction between Sodium Thiosulphate and dilute Hydrochloric acid

Planning

We have been asked to investigate the rates of a reaction and the effect different changes have on it. The rate of a reaction can be described as the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. This rate is calculated by dividing 1 by the time taken for the reaction to occur. The reaction I have been asked to investigate is:

Sodium Thiosulphate + Hydrochloric Acid → Sodium Chloride + Water + Sulphur Dioxide + Sulphur

Na2 S3 O2 (aq) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + SO2 (aq) + S(s)

The dilute Hydrochloric acid reacts with the Sodium Thiosulphate solution to give a precipitate of sulphur. Both liquids are clear and colourless before they react. The appearance of the insoluble sulphur during the reaction makes the mixture cloudy. The cloudiness increases as the reaction is allowed to proceed.

According to the collision theory of reacting particles, there are five factors that affect the rate of a reaction:

Concentration (of solution)
Temperature
Pressure (in gases)
Surface area (of solids)
Catalysts

For this experiment, I am unable to change the pressure and surface area of any of my reactants. So, I will change the concentration of the Sodium Thiosulphate solution because it is easy to vary and it is hard to go wrong when changing the concentration. I decided not to have catalyst as the independent variable because I could only get two results, with/without catalyst. I could also ruin the experiment by using the wrong catalyst or using it in the wrong way. The dependent variable in this experiment is the time taken for the solution to become completely opaque. To make this a fair test I will have to keep the other factors constant. I will do this by conducting all of the tests at room temperature and making sure that all of the windows and doors are closed in the classroom. A catalyst will not be used so that no other factors will affect my experiment.

The science that is involved in this experiment is the collision theory of reacting particles. Chemicals reactions occur when particles of the reactants collide. They must collide with a certain amount of energy for a reaction to occur, called the activation energy. If this activation energy is not met when particles collide they just bounce off each other harmlessly. The activation energy is needed to break the bonds in particles and get the reaction started.

I predict that, for this experiment, the higher the concentration of the Sodium Thiosulphate, the faster the rate of reaction. In addition to this I predict that as I double the concentration, the rate of reaction will double as well, meaning that they will be in direct proportion to one another. I predict this because when the concentration is higher, there are more particles for the same volume. This means that there will be more particles involved in the reaction, which increases the chance of collision between the reactants. Since there will probably be more collisions, the reaction rate ...

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The equipment that I will use is:

Conical Flask
Burette
Measuring Cylinder
Dilute Hydrochloric acid
Sodium Thiosulphate (six different concentrations)
Splash mat
Goggles
Lab Coat
Paper with X on it
Stopwatch

Diagram

Method

First of all, I will obtain all of the equipment necessary for my experiment. Then I will measure out 50ml of Sodium Thiosulphate using the burette and 5ml of Hydrochloric acid using the measuring cylinder. I will pour the Sodium Thiosulphate into the conical flask first, which will be placed on top of the paper with an X on it. This paper and flask will then be placed on to a splash mat to prevent a mess being made. I have used a conical flask instead of a beaker so that I can swirl the reactants around more easily. Then as soon as I add the Hydrochloric acid to the flask, the stopwatch will be started. Then the solution will turn cloudy and as soon as I can no longer see the cross on the bottom, the stopwatch will be stopped and the time will be recorded. Then this will be repeated for different concentrations of Sodium Thiosulphate. To vary the concentration, I will replace 10ml of the Sodium Thiosulphate with water. I will keep changing the concentration until the solution ends up as just water. This will give me six different solutions, which should be sufficient for me to draw a graph. Then all six tests will be repeated in order to take an average, which will hopefully make my results more reliable and cancel out the inevitable human error. For safety I will wear goggles and a lab coat to protect my eyes and clothes.

Preliminary Experiment

I decided to do a preliminary experiment so that I get a feel for the experiment and I can improve my method before conducting my actual experiment. I didn’t have much time to do the experiment so I was only able to get five out of six results.

From this preliminary I have learnt that I will definitely have to repeat the experiment because some of these results look incorrect. When doing the experiment I found that each time I swirl the conical flask it will speed up the reaction, so it is another factor. Since it is a factor I will have to keep it constant to make it a fair test. I can do this by either swirling it a set number of times or not swirling it at all. I have decided to do the latter of the two, which will be easier and because if I swirled the flask a set number of times, there would be a possibility of swirling it with different forces. The stopwatch I used measured only to seconds and not any decimal places, which means that the results were not accurate enough. For my actual experiment I will use a stopwatch which measures to two decimal places.

Results

To improve the reliability of my results, we used the same person to look at the cross through the solution and decide when they could not see the cross anymore. If we hadn’t done this the experiment would not have been a fair test because my partner and I have very different eye sight. Since my partner had better eye sight than me, he was chosen to look at the cross instead of me. He was also the person to stop the stopwatch because if I had done it, there would have been a slight delay between the moments when he told me to stop the clock and when I actually did. This improved the accuracy of my results. The reaction time for my partner to stop the clock would have remained constant throughout the experiment so it did not pose a problem. We also used distilled water instead of tap water so that the impurities did not affect the experiment, therefore improving the reliability. The burette measured from 0ml at the top to 50ml near the bottom. However, there was a slight gap between the 50ml mark and the place where the solution actually came out. So each time the solution was poured into the conical flask, we had to stop the burette from emptying any more than it was supposed to. This gap also meant that the remaining solution left in the burette had to be emptied every time because if it wasn’t, the concentration of the solution would be affected the next time a solution was poured in.

Conclusion

From looking at my results and graphs, it is easy to deduce that as I increase the concentration of the Sodium Thiosulphate solution, the time taken for the reaction to fully occur decreases. So if the time taken for the reaction to fully occur decreases, the rate of reaction increases. This means that as the concentration increases, so does the number of collisions between the reacting particles per second.

When I plotted the time taken for the reaction to occur against the concentration, I found that I saw a negative correlation, meaning that as the concentration increased the time taken decreased. Therefore, I decided to check if they were inversely proportional to one another by plotting the concentration against 1/time taken. When I did this I got a straight line, which means that they are inversely proportional. Then I realised that 1/time taken is equal to the rate of reaction, which indicates that the rate of reaction is directly proportional to the concentration of the solution because the line of best fit on the graph is a straight line through the origin.

This happens because when the concentration of a solution is increased, the number of reactant particles per unit of volume is also increased. Since reactions occur when the particles of reactants collide, the rate of reaction would increase if there were more collisions. One way to increase the number of collisions would be to increase the number of particles. When there are more particles there is a higher chance that there will be a collision. So when there are more particles, there are more collisions, resulting in a faster reaction.

My results have proven my prediction to be correct because I predicted that the rate of the reaction would be directly proportional to the concentration of the sodium thiosulphate solution. However, I did not predict anything to do with the relationship between the time taken and the concentration of the Sodium Thiosulphate solution, which is an inversely proportional relationship.

Evaluation

I, personally, found the experiment fairly easy to do as I did not have to change the method very much. Nevertheless, there was a problem with the time that it took for the burette to fully empty the Sodium Thiosulphate into the conical flask. If we had waited for this to finish every time, there would not have been enough time to repeat the experiment at all. So to solve this problem, we began to do other things, such as measuring out the hydrochloric acid, while the conical flask filled up with the Sodium Thiosulphate solution.

I think that we took enough results, although I would have liked to repeat the experiment again to improve the reliability of my results, but the time was not ample. I think that the range was also big enough because I was able to draw a good conclusion from the results that I have collected. However, I didn’t take any results with a high concentration of Sodium Thiosulphate. This would have been possible if we had been provided with different concentrations of Sodium Thiosulphate instead of having to mix it with water to vary the concentration.

From looking at my graphs, I think that my results are exceptionally accurate as they lie very close to the line, and I have no anomalous results. If I did the experiment again I would repeat the experiment one more time to improve the reliability. I don’t think the way we measured when the experiment had stopped was very accurate because it was very hard to tell when the cross was impossible to see anymore. So, if I was to do the experiment again I would choose a reaction which results in bubbling, so that it would be easier to tell when the experiment has come to a halt.

My results are reliable as well because when I took the repeats of each concentration, they did not vary much. All of my results are consistent with each other and no results were so different that they should have been ignored. However, I could have repeated the experiment again, but I doubt that they would have changed the average very much unless they were particularly anomalous.

To extend my enquiry I could investigate the rate of a different reaction using solid and a liquid instead of two liquids. This would also give me a chance to change the surface area of the solid reactant, which I could not do in this experiment. I could then look at the graphs that my results will have given me to see if they show the same trends as in this experiment.