Amount of times for experiment to be conducted: Three times
Apparatus List:
Hypothesis/Prediction:-
Increase in the concentration of Hydrochloric Acid (HCl) is directly proportional to the increase in the rate of reaction between HCl and Sodium Thiosulphate.
Concentration is the measurement of the amount/number of particles in a given volume; therefore an increase in the concentration of a substance implies an increase in the number of particles of that particular substance in any given volume. The higher the amount of particles in a closed volume indicates that there will be a higher number of random collisions between particles, therefore there will be higher number of effective collision, thus resulting in a faster reaction. This theory is partially backed up by the preliminary experiment that was conducted, in the one molar HCl experiment when it was diluted with distilled water the reaction time decreased. Also the rate of reaction increased as the concentration (molar of acid) increased. In the one molar test the average time was 01:29 mins while in the two molar the average time was 01:16 mins. There is 13 second difference. However for the three molar tests the average time was 01:14 mins, it is still faster however the margin of time difference is only 2 seconds. This leads me to another hypothesis; when the concentration of the Hydrochloric acid is roughly 70% the difference in reaction time will be small compared to the reaction time when the HCl is at 100% concentration. However when the concentration of the reactant is at approximately 20 – 30% the difference in reaction time will be large compared to reaction time of the reactant when it is approximately 50 – 60%. In short the reaction time of reactants in the lower bound of concentration differ greatly when compared to reactants with concentration of 50 – 60%, however when the upper bound reaction times are compared with the reaction times of reactants with 100% concentration the difference is small even though the difference in concentration for both is 30%. This may be because when for example the concentration of a reactant is 30% the probability of collision (Not effective collision) is 3/10 and the probability of effective collision is the same or lower. However when the concentration of the reactant is 60% the probability of collision is 6/10 which is double 3/10 so it is twice as likely to collide, and also because it is 60% concentrated the reactant is a majority in the solution so it will have a higher chance of effective collision with less obstruction. Now let’s consider the example with reactant with 70% concentration, the probability of collision is 7/10 while a reactant with 100% concentration has a 10/10 probability of collision however this may be lower than this and the rate of effective collision is most likely lower than that. Also the 100% concentrate reactant is 1.4 times more likely to collide than the 70% concentrate. While the 60% is twice as likely to collide as the 30% concentrate, I believe these probabilities are why there is such a different in reaction times of 30% concentrate and 60% concentrate, on the other hand there is a small difference between the reaction times of 70% concentrate and 100% concentrate. (Note: These numbers are merely hypothetical and were inspired by the results from the preliminary test and when I refer to the concentration values like this e.g. lower bound of concentration I am merely hypothesising using values from the preliminary experiment.)
Variables:
The concentration of the acid in this experiment shall be changed but the amount of Sodium Thio Sulphate shall remain the same. Also the HCl will always be two molar.
Method:-
- Put 50cm³ of Hydrochloric Acid into a measuring cylinder.
- Afterwards measure out 25cm³ of Sodium Thiosulphate into a measuring cylinder (watch out for cross contamination make sure that you find a way to distinguish between the two.)
- Set up the conical flask, make sure it is dry. Then place it over a laminated mat which has a cross in the centre.
- Then pour both solutions into the conical flask and have someone start the timer when this is done.
- Then assign someone to watch the conical flask from overhead and to stop the timer when they can no longer see the ‘X’ cross on the laminated mat beneath. (remember to wear safety goggles, this applies to everyone in the group.), Also it must be the same person each time the experiment is done because people have different quality of vision and also it is slightly subjective to your viewpoint.
- After this you then measure out 45cm³ of Hydrochloric Acid and 25cm³ of Sodium Thiosulphate (This will remain the same throughout the experiment) and also measure out 5cm³ of distilled water (so the total volume of the reactant is 50cm³).
- You then repeat steps (3), (4), (5).
- You will then Measure out 40cm³ of HCl and 10cm³ of distilled water (each time you do it you will take away 5cm³ of HCl and in its place add another 5cm³ of distilled water. You will keep doing this until you reach 0cm³ of HCl and 50cm³ of distilled water.) Measure out 25cm³ Sodium Thiosulphate.
- You then repeat steps (3), (4), (5).
- You then repeat this experiment three times, and calculate an average of your data.
Table 1: Reaction between Sodium Thiosulphate and Hydrochloric Acid
Conclusion:-
These results indicate a steady increase in reaction time as the concentration of Hydrochloric Acid decreases. However there are a few anomalies marked in yellow which do not fit the pattern and these anomalies have affected the average. This is because the arithmetic mean is affected by extreme values (anomaly). If we look at the averages we see that the difference between the reaction rate of the solution containing 90% HCl and 10% water and the solution containing 100% HCl is a mere 3 seconds, however in the solution containing 70% there is a jump in reaction times, the difference in reaction times between it and the solution containing 90% HCl (I am not comparing it with the solution with 80% HCl because there is an anomaly (extreme value) affecting the total average.) is 10 seconds. Between the experiments with 70% concentration of HCl and 20% concentrate there is a small difference in reaction times. These results partially back my hypothesis, as we can see between 30% HCl and 10% HCl concentrate there is a 32 second difference, while the difference between 70% concentrate and 100% concentrate is only 10 seconds. More extensive experiments would be needed to determine whether there is some truth to my hypothesis. However it seems my first prediction was true; there is a general trend in my results. The reaction time increases as the concentration decreases. However there are still a few anomalies within the results.
Evaluation:-
The experiments conducted had few mistakes and have been identified. The mistakes have not had a great overall effect on the results except in some cases. To improve results it would be best to have the same person timing THROUGHOUT the experiment. Also the person watching the reaction must be present at all times during the experiment. No cross contamination could have taken place during the experiment because the conical flask was thoroughly rinsed and dried. If the experiment were to be conducted again these controls will be observed and also the experiment should be conducted ‘at least’ 5 times. Also it would also be good if we could do the same for one molar HCl and three molar HCl. However everyone wore safety glasses while experiments were conducted and also there was a fair amount of accuracy in the results. Further experiments would need to be conducted to get a more accurate result.
