In this experiment I am going to find out how the rate of reaction between sodium thiosulphate and hydrochloric acid is affected by changing the concentration.
Introduction
In this experiment I am going to find out how the rate of reaction between sodium thiosulphate and hydrochloric acid is affected by changing the concentration.
We place the reaction mixture on a paper with a black cross drawn on it. When the cross is completely obscured, the reaction will have finished. The time taken for this to happen is the measure of the rate of reaction. We must do this several times, and change the concentration of sodium thiosulphate.
The rate of reaction is a measure of the change that happens during a reaction in a single unit of time. The things that affect the rate of reaction are as follows:
* Surface area of the reactants
* Concentration of the reactants
* The temperature at which the reaction is carried out
* Light
* Use of a catalyst
The factor that we are using is Concentration of the reactants.
Plan
The equation for our reaction is:
2HCl (aq) + Na2S2O3 (aq) -> S (s) + 2NaCl (aq) + SO2 (g) + H2O (l)
Which simply means: hydrochloric acid + sodium thiosulphate -> sulphur + sodium chloride + sodium dioxide + water.
Apparatus used:
* Conical flask
* Sodium thiosulphate (different concentrations)
* Hydrochloric acid (same concentration each time)
* Water (different concentrations)
* Stop watch
* Paper with black cross on it
Method
Firstly, we drew a black cross on a white piece of paper. Then, we put 10 cm3 of hydrochloric acid in a conical flask and then put the conical flask on the black cross on the paper. We then added 50 cm3 of sodium thiosulphate to the hydrochloric acid, and at this time we do not need to add any water. Immediately, we started the stopwatch.
We then waited for the black cross to be completely obscured by the mixture. When this happened we stopped the stopwatch. By this time the mixture was completely opaque. Then we recorded what the time was.
We then emptied out the contents of the conical flask, and cleaned it out, and dried it. Next, we added 40 cm3 of sodium thiosulphate; we kept the HCl acid constantly at 10 cm3. Now, because we have to keep the volume the same for it to be a fair test, we added 10 cm3 of water. We then recorded the ...
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We then waited for the black cross to be completely obscured by the mixture. When this happened we stopped the stopwatch. By this time the mixture was completely opaque. Then we recorded what the time was.
We then emptied out the contents of the conical flask, and cleaned it out, and dried it. Next, we added 40 cm3 of sodium thiosulphate; we kept the HCl acid constantly at 10 cm3. Now, because we have to keep the volume the same for it to be a fair test, we added 10 cm3 of water. We then recorded the time for the cross to become completely obscured. We repeated this many times, adding different amounts of sodium thiosulphate and water each time, and recording the time taken each time.
Fair test
In this experiment we are trying to find the rate of reaction using concentration as a factor, so there is a number of things we need to make sure we do to keep it a fair test.
Firstly, we need to keep a chemical at a constant concentration. So, in this experiment we have chose to keep hydrochloric acid at a constant concentration. We could have however, used sodium thiosulphate as a constant, but we had chose to use hydrochloric acid.
We must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.
We must also make sure that we add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could affect the results of the experiment.
We must start the stopwatch at the exact time as we put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in the HCl, and one person can start the stopwatch.
Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment.
Finally, we should do the experiment 3 times and average our results.
Safety
There are a lot of safety issues we must abide by in this experiment also.
We must remember that the substances which we use in this experiment can be very harmful if used the wrong way.
When we do this experiment, it may be necessary to wear safety goggles, as things are very unpredictable, and even though it is very unlikely that the solution would come out of the conical flask during the experiment, we must still be cautious of spills.
We must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals.
We should also try our best not to spill any chemicals, and we must not eat or drink in the lab while dealing with these harmful chemicals, as they can get on to our hands.
Prediction
I think that the higher the concentration we use of sodium thiosulphate, the faster the reaction will happen.
The reason I think that this will happen is because, in a reaction particles collide to give the end results of the reaction. So if there is a higher concentration of particles on a solution, then there will be a high number of collisions. If there are a high number of particles colliding, then there is a higher chance of them reacting.
Results
Below I have shown my three tables of results from the experiment and the table of average results.
Experiment1
Sodium Thiosulphate (cm3)
Water (cm3)
Hydrochloric Acid (cm3)
Time taken for reaction (sec)
50
0
0
31.63
40
0
0
42.53
30
20
0
54.50
20
30
0
81.90
0
40
0
233.31
Experiment 2
Sodium Thiosulphate (cm3)
Water (cm3)
Hydrochloric Acid (cm3)
Time taken for reaction (sec)
50
0
0
34.57
40
0
0
45.19
30
20
0
57.15
20
30
0
78.12
0
40
0
202.10
Experiment 3
Sodium Thiosulphate (cm3)
Water (cm3)
Hydrochloric Acid (cm3)
Time taken for reaction (sec)
50
0
0
36.93
40
0
0
43.47
30
20
0
57.85
20
30
0
86.44
0
40
0
206.32
Average
Sodium Thiosulphate (cm3)
Water (cm3)
Hydrochloric Acid (cm3)
Time taken for reaction (sec)
50
0
0
34.37
40
0
0
43.73
30
20
0
56.50
20
30
0
82.15
0
40
0
214.57
Also, from my average result table, I have constructed a graph to show my results.
From my graph...
Conclusion
In conclusion I have found that my prediction was correct. The rate of reaction was quicker when there was a higher concentration of sodium thiosulphate. This can easily be seen when we look at the results table.
This may have also have been the same if I had used the hydrochloric acid as a variable instead of the sodium thiosulphate. It may have been faster or slower at reacting than the sodium thiosulphate.
Evaluation
I think that most of the experiment I have done has been fine, with as little anomalous results as possible. However, there are still a few anomalous results, which I will now point out.
There are a number of ways in which we could have made the results more reliable. For instance, we could have used better measuring equipment, because the apparatus we used was mainly basic equipment.
Another thing we could have done to bring more evidence is to have tried to use the hydrochloric acid as the variable substance, and used the sodium thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.
I think that the evidence, which I have received, is enough to reach a suitable conclusion, but there are a few flaws to the experiment (which I have mentioned). Apart from them, the experiment is fine.
