Sodium thiosulphate (aq) + hydrochloric acid (aq) = sodium chloride (aq)
+ sulphur (s)
+ sulphur dioxide (g)
+ Water (l)
colourless solution turns milky white, then yellow
Prediction
I predict that the more concentrated the hydrochloric acid is, the quicker the rate of reaction will be. I predict this because when there is no acid; there will be no reaction so obviously with the more concentrated acid there will be a quicker rate of reaction. This is because if the concentration of a solution is increased there are more reactant particles per unit volume. This increases the probability of reactant particles colliding with each other. This means that the rate of reaction will increase if the concentration of reactants is increased.
Apparatus
Hydrochloric acid
Sodium thiosulphate
Water
Conical flask
Measuring cylinders
Pipette
Marker pen
Paper
Safety goggles
Stop clock
Diagram
Method
1) On a piece of paper with a marker pen draw a cross the size of the bottom of a conical flask as shown in diagram.
2) Measure out 50mm3 of sodium thiosulphate in a measuring cylinder and pour into a conical flask.
3) Measure out 2mm3 of hydrochloric acid into a measuring cylinder and then 8mm3 of water into the same one to make it 0.2M of acid.
4) Pour the diluted acid into the conical flask (with the sodium thiosulphate in already) and start timing, stop the stop clock when you cannot see the cross anymore.
5) Record 3 sets of results for each concentration of acid and then repeat these steps with 0.4M, 0.6M, 0.8M and 1M. Record results into a table.
Results table
I think I have taken enough results here, and I have found out the average to get them even more accurate.
I think the range of results I got was good as it shows a clear pattern of what the relationship between the concentrations and rate of reaction is.
Conclusion
From our results we can see that as the concentration of acid increases the rate of reaction also increases. We know this because it took much longer for the cross to be obscured form our vision by the reaction, when there was less concentrated hydrochloric acid in the conical flask. We can also see from our graph that as when the acid was less concentrated the difference in time was much greater than the more concentrate acid where it was much shorter time intervals. An example of this is at 0.2M – 0.4M there is a time difference of about 13 seconds, whereas between 0.8M and 1M there is a slight time difference of 2 seconds.
These results support my prediction because I know from scientific knowledge that if the concentration of a solution is increased there are more reactant particles per unit volume which means the probability of reactant particles colliding with each other increases. This proves that the rate of reaction will increase if the concentration of reactants is increased.
Evaluation
We carried out our experiment exactly the way we planned it and followed our method throughout the experiment. Our results were very accurate because we made sure the variables were kept the same which meant we did not get any anomalous results. This was because we found out the average of 3 results, so even if there were any anomalous ones it would hardly make any difference.
My prediction that the time as the concentration of acid increased, the time would decrease was correct but I did not predict that as the concentration was higher the graph would eventually become less steep and gradually fall off towards the end.
I could have extended my investigation even further by going past 1M of acid to see if the graph did level off as the concentration became even higher.