Prediction
Having done some background information on rates of reaction, the prediction is that when the reactants are concentrated the particles are crowded and close together and will bash or collide with each other more frequently. This will give an increase number of collisions with sufficient energy, making the rates of reaction go quicker and therefore taking less time.
Based on this knowledge it can be predicted that when the sodium thiosulphate is not diluted at all when it reacts with hydrochloric acid, the rate of reaction will be fasted. It will take an approximate time if say, 35 seconds for the reaction to complete. Next when sodium thiosulpahte is diluted with perhaps say, 50 cm3: 20cm3 of water (5:2), there will be slight less particles, therefore the rate of reaction will decrease to a small extent due to reduced number of collisions. Perhaps the time to complete this reaction will be approximately 30 seconds.
Similarly as the sodium thiosulphate is diluted with 30cm3: 40cm3 of water (3:4), the pattern will follow with a further decrease in time to complete the reaction. This is because once again the particle numbers will be reduced. The time taken for this reaction can be predicted as 60 seconds. The final dilution of 10cm3:60cm3 of water (1:6)- the reaction will be at the slowest pace. The approximate time will be a maximum of 7×30 second, which will be 210 seconds, judging from the time predicted for the reaction to complete in the first place when the dilutions were 60cm3: 10cm3 of water (6:1). Hence it can be said that as the dilution increase of sodium thiosulphate, the particle number will also decrease correspondingly, taking more and more time for collisions to take place.
Preliminary work
Based on the background information and predictions made, preliminary experiments were carried out using three dilutions.
The results of these tests which are carried out three times each are as follows:
Looking at the results it can be concluded that the predictions made at first were accurate. Therefore, the greater the concentration of the reactants of the solutions the faster the rate of reaction. This is because there were more particles. Hence there was a grater chance of collisions occurring. The time therefore was quicker when the concentration increased. The measuring cylinders must be different ones for hydrochloric acid and thiosulphate.
Plan for the Method to be carried out
The following apparatus and materials will be used for the investigation:
Materials Apparatus
- Wear goggles and collect and collect all apparatus and materials and set them out in the middle of the work bench. This is to avoid them being knocked over.
- Measure out 10ml of hydrochloric acid using a labelled measuring cylinder.
- For the first experiment using a large labelled measuring cylinder measure out 70ml of sodium thiosulphate.
- Place the card marked X on the middle of the table an place the conical flask over it. Get the stop clock ready.
- Pour the 70ml of sodium thiosulphate into the conical flask. Then add the 10ml of hydrochloric acid to the sodium thiosulphate in the conical flask. Start the stop clock immediately. Carefully observe the disappearance of the cross on the card, and record the time taken accurately for this to happen.
- Wash out the conical flask thoroughly using distilled water and then place it over the card marked X.
- For the following six experiments organised 10ml of hydrochloric acid as explained above using the small labelled measuring cylinder.
- Decrease the sodium thiosulphate by 10ml using the measuring cylinder.
- Measure out 10ml of distilled water using a third labelled measuring cylinder. Use this to dilute the 60ml of sodium thiosulphate. Pour this into the conical flask.
- Get the stop clock ready.
- Add the 10ml of hydrochloric acid to the diluted sodium thiosulphate and record the time taken for the cross to disappear.
- For the following third, fourth and fifth experiments follow the same procedure. For each test decrease the sodium thiosulphate by 10ml and increase the distilled water by 10ml.
- Therefore the final experiment will have a ratio of 1:6, 1 sodium thiosulphate : 6 distilled water.
- The volume of hydrochloric acid will be kept constant in all experiments to 10ml.
- Record the time taken in all experiments accurately for the cross to disappear.
- Repeat all seven experiments using the same concentrations and volumes of the reactants, at least two or three times. This should be done as a cheque for the results, in order to prove its accuracy.
- Average time taken for the experiments of the same dilution should be calculated and recorded. The rate should also be calculated using the average time as follows:
Rate = 1
Average time
- All results should be recorded accurately in a suitable and clear table.
The experiment
Fair test
- Use the same apparatus and materials for the experiments.
- Hydrochloric acid should be measured out accurately using the labelled measuring cylinder to 10ml for each experiment.
- The concentration of sodium thiosulphate should be worked out accurately as mentioned in the plan giving a total of 70ml for each experiment.
- The conical flask should be washed out thoroughly after each experiment using distilled water.
- The solutions must be used from the same stock bottle.
- The timing should be done accurately, using the same stop clock.
- The same person must be stopping the stop clock when the cross disappears.
Safety
- Wear goggles before collecting the equipment and take them off only when all apparatus are removed from the bench.
- Place all apparatus and materials in the middle of the bench to avoid anything being knocked over.
- There should be no objects under the bench.
- Stand while doing all the experiments.
- Avoid touching your face or any part of your body whilst doing the experiments.
- After everything is cleared away from the bench, wash your hands in water thoroughly.
Result Table
To find the rate of reaction between sodium thiosulphate and hydrochloric acid.
Graph 1: shows that the time increases and the concentration decreases.
Graph 2 : the rate of reaction increases when the rate of reaction increases. It is directly proportional, meaning as you double the reaction the concentration also doubles.
Conclusion
The evidence collected after doing the experiments three times, show as the concentration of the reactant, sodium thiosulphate was at its maximum, i.e. when it was not diluted at all, and the rate of reaction was at its fastest. As sodium thiosulpahte was diluted and when dilutions increase, the rate of reaction slows down, taking more and more time for the reaction.
Therefore, there has been a definite pattern in the evidence obtained. Therefore, according to the results, the pattern was, that the higher the concentration of the reactants the quicker the time it took for the reaction to be completed as explained above. In all three experiments when sodium thiosulphate was not diluted the time taken was the quickest giving an average time of 35.6 seconds. The predictions made of 35 seconds for this result was not far out. This is because the particles of the reactant were at its most as it was a completely concentrated solution. Therefore, collisions of particles as seen in the background information were at its maximum. This gave the quickest time for the reaction to be completed and the fasted reciprocal rate when calculated. As 1
Av.T. As the sodium thiosulphate was diluted to 6:1, the collisions slightly decreased as there were less particles giving a slightly increased average time of 40.95 seconds. Further when the dilution was 5:2, the average time taken in the investigation was 42.20 seconds. The predicted time 30 seconds which was a slightly less than the time obtained. Furthermore, as the dilutions increased the time also increased for the reactions to be completed. As learnt in the scientific knowledge, the particles of the reactions decreased; therefore the number of collisions also decreased which means the rate decreased and the time increased in seconds.
Finally, for the last two experiments when the dilutions were (2:5) the average was 124.33 seconds. The rate decreased to 8.04. For the last experiment when the dilutions were (1:6) the average was time was 230.73 seconds. The predicted was 210 seconds which was only more-or-less correct. The rate for this dilution dropped to 4.33.
All the evidence gathered in all the experiments, for all the dilutions explain the conclusions made so far which is accurate. The predictions made also have been proved some what accurate according to the results collected: in all three experiments, in the average results and the reciprocal rates which were calculated.
Evaluation
All the apparatus used and the procedure after doing the preliminary work was suitable for the investigation. The evidence collected in all three experiments and the average results were as expected. The experiments were conducted three times for each dilution in-order to validate the results for accuracy. The results proved to be correct according to the scientific knowledge that was learnt at the start of the investigation.
The predictions however were not exactly as the results, but trends and patterns were evident. There were no errors anomalies in any of the evidence collected in all the sets of experiments. They were all consistent. The procedure that was used which was based on pilot experiments proved to be highly satisfactory, as accurate results were obtained. However to change or improve the procedure more dilutions can be used, i.e. the distilled water added could vary as follows in the ratio of 5’s – 65 cm3 sodium thiosulphate : 5 cm3 of hydrochloric acid and so on. This will give more experiments and more results for a better conclusion. Also, perhaps beakers could be used instead of conical flasks. The pipettes could have been used instead of measuring with your measuring cylinders, to measure the volume of 10ml of hydrochloric acid.
Besides, if the computer software is available the results can be monitored electronically and seen on a computer screen.
However all the evidence collected proved to be reliable and sufficient to support the conclusion made. The reciprocal rate calculated also seemed to be correct.
As far as further work is concerned other experiments using acids and metals like magnesium, lead, copper etc. can be investigated relating to any other factors which affect the rate of reaction. This further work can also be conducted by physically observing the fizzing of the bubbles of hydrogen gas giving out when different concentrations of acid are added for magnesium. A balloon can be used to collect the gas.
Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen
Mg (s) + 2HCL (aq) MgCL2 (aq) + H2 (g)
For the investigation conducted instead of factor of concentration the temperature can be varied. It can start from 20oC increasing in 10’s up to 60oC. if this is done the following results will be appropriate:
The higher the temperature as seen in the background information provides more energy for more collisions. Hence the rate of reaction is faster.
Factors like particles size and catalyst can also be investigated.
Overall, the investigation conducted has been very interesting and much knowledge has been gained. Factors that affect the rate of reaction has been understood thoroughly.
