Find out the effect of concentration on reaction rates between chemicals.
Introduction
Aim: Find out the effect of concentration on reaction rates between chemicals.
The equation to this is as follows:
Sodium +Hydrochloric Sodium +Sulphur +Sulphur +water
Thiosulphate Acid Chloride Dioxide
Na2S2O3 (aq) + HCL (aq) 2NaCl(aq)+ S(s) +SO2(g) + H2o(l)
This reaction usually produces a yellow precipitate.
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 which happens during a reaction in a single unit of time. The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. It is measured by dividing 1 by the time taken for the reaction to take place. 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
Rate of reaction = I/T
The factor which we are using is Concentration of the reactants.
Plan
Apparatus
· 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 yellow. 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 45 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 5 Cm3 of water (we had to add the sodium thiosulphate and the water at the same time for this to be a fair test also). 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.
Diagram
Fair test
To make this experiment a fair test, we need to make sure we do a number of things.
In this experiment we are trying to find the rate of reaction using concentration as a factor, so there are 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 ...
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We repeated this many times, adding different amounts of sodium thiosulphate and water each time, and recording the time taken each time.
Diagram
Fair test
To make this experiment a fair test, we need to make sure we do a number of things.
In this experiment we are trying to find the rate of reaction using concentration as a factor, so there are 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 chosen to keep hydrochloric acid at a constant concentration. We could have, however, used sodium thiosulphate as a constant, but we had chosen to use hydrochloric acid.
Next 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 effect the results of the experiment.
We must start the stop watch 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 stop watch.
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.
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, one 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. It would also be preferable to wear a scientific apron, however this is not essential.
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 harmfull chemicals, as they can get on to our hands.
Prediction
I predict that the greater the concentration of Sodium Thiosulphate in the solution the faster the chemical reaction will take place. Therefore, the cross will disappear more quickly due to the cloudiness of the solution.
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.
I think that the concentration of a solution effects the rate of reaction because the rate of reaction depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules about, the frequency of successful collisions is greater, and the reactions happen faster.
This is also better shown by this diagrams:
Results
Table of results
st try:-
Sodium Hydrochloric Water Time taken Rate of Reaction Thiosulphate
(CM3) (CM3) (CM3) (S) 1/Time
50
0
0
48.85
0.0205
45
0
5
45.46
0.0202
40
0
0
51.06
0.0196
35
0
5
59.84
0.0168
30
0
20
09.04
0.0092
25
0
25
15.27
0.0087
20
0
30
41.32
0.0071
5
0
35
219.08
0.0046
0
0
40
312.53
0.0032
5
0
45
459.47
0.0022
2nd try :-
Sodium Hydrochloric Water Time taken Rate of Reaction Thiosulphate
(CM3) (CM3) (CM3) (S) 1/Time
50
0
0
45.07
0.0222
45
0
5
46.99
0.0213
40
0
0
48.10
0.0208
35
0
5
57.67
0.0173
30
0
20
89.17
0.0112
25
0
25
07.45
0.0093
20
0
30
34.61
0.0074
5
0
35
79.81
0.0056
0
0
40
234.84
0.0043
5
0
45
347.21
0.0029
3rd try :-
Sodium Hydrochloric Water Time taken Rate of Reaction Thiosulphate
(CM3) (CM3) (CM3) (S) 1/Time
50
0
0
47.299
0.0211
45
0
5
48.04
0.0208
40
0
0
52.17
0.0192
35
0
5
58.76
0.0170
30
0
20
07.88
0.0093
25
0
25
14.09
0.0088
20
0
30
42.52
0.0070
5
0
35
221.26
0.0045
0
0
40
310.33
0.0032
5
0
45
449.27
0.0022
I have also come up with a set of average results for all 3 experiments :-
Sodium Hydrochloric Water Time taken Rate of Reaction Thiosulphate Average
(CM3) (CM3) (CM3) (S) 1/Time
50
0
0
47.07
0.0213
45
0
5
47.10
0.0212
40
0
0
50.77
0.0197
35
0
5
58.76
0.0170
30
0
20
02.63
0.0097
25
0
25
12.27
0.0089
20
0
30
39.48
0.0072
5
0
35
206.71
0.0048
0
0
40
285.03
0.0035
5
0
45
418.65
0.0024
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 and at the graphs with the lines of best fit. In both cases of the graph there is a correlation between the two data.
I have come to this conclusion because of several reasons. Firstly, my results give pretty conclusive evidence that as the amount of Sodium Thiosulphate decreases, and the amount of water in the solution increases there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.
However, not all collisions bring about a reaction. A reaction will only occur if the colliding particles possess more than a certain minimum amount of energy. This is known as the activation energy. (Ea) This activation energy will enable the chemical bonds that already exist between the particles to stretch and eventually break, and atoms, ions and electrons will be rearranged.
I believe I was correct and a secondary source states that 'the reaction time will be faster with a more concentrated solution because, the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up'. - Secondary source Britannia Interactive Encyclopaedia 1998.
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.
Sodium Thiosulphate Hydrochloricacid Water Time Taken
(CM3) (CM3) (CM3) (S)
50
0
0
48.85
0.0205
45
0
5
45.46
0.0202
40
0
0
51.06
0.0196
35
0
5
59.84
0.0168
This is the only anomalous result that I have recorded. The result may have turned out anomalous because of basic human error, or maybe because we measured the substances wrong. It may have even been because we did not clean the apparatus properly.
Apart from this, the accuracy of my experiment has been more or less accurate. Although 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). This excluded the experiment was a success.
