METHOD FOR CONTROL OF VARIABLES
First of all we should get all the materials and chemicals we need in the 200ml beakers so that we don’t have to go and get some for every experiment .For controlling the variables a number of factors have to be kept in mind, which are: -
1 To keep size constant all the reactants should be in solution form.
2 To control temperature all the chemicals, including water, should be allowed to reach room temperature so that all chemicals have the same temperature.
3 We can control the presence of a catalyst by simply not adding a catalyst
4 As the reaction progresses and the chemicals mix, sulphur is formed which creates a precipitate which looks like a yellowish cloudy mixture. This mixture becomes more and more darker as the reaction reaches the end point therefore it is not easy to point out the end point of the reaction. Hence conical flask, in which all the reactants are mixed, should be kept over a white paper having a black cross on it (or anything else that can be used as a mark) so that the person recording the end point of the reaction can do that easily and more accurately.
METHOD FOR THE COLLECTION OF DATA
There are 7 experiments, which are basically the same, except for the amount of water added to the sodium thiosulphate (to decrease its concentration), that increases as the experiment number increases.
Before the experiments it is necessary to check and make sure that all the solutions are at the same temperature, hence the calculations could be inaccurate. Also to have the same room temperature in all experiments all the experiments should be done on the same day, as the room temperatures could change.
Experiment 1(does not involve any water)
1. Pour 40ml of 0.1M sodium thiosulphate solution in the 50-ml measuring cylinder.
2 Using a dropper or a funnel pour 10ml of 2M HCl in a 10ml measuring cylinder
3 On a white sheet of paper draw a black cross or any other mark.
4 put the sheet of paper under a 50-ml max conical flask.
5 Pour the HCl in the conical flask and then add the sodium thiosulphate.
6 Start timing the reaction, using a stopwatch, as soon as you add the sodium thiosulphate
7 Watch clearly as the cross under the conical flask diminishes, stop the watch when the cross is totally out of sight.
8 Note the time in seconds and the concentration of the sodium thiosulphate.
Experiment 2(involves water)
This experiment is basically the same except for the first few steps where you have to add water to the sodium thiosulphate. The steps are: -
1 Take 35 ml of sodium thiosulphate in the 50-ml measuring cylinder and add 5 ml of water to it, to decrease its concentration.
2 Calculate and note the new concentration, and repeat the steps 2 – 8 in experiment 1
All the values have been noted in the table below
OBSERVATIONS
In all the experiments the reactants were the same , hence the observations were also the same. As soon as the Na2S2O3 was added to the HCl in the conical flask , the reaction started, it developed a white-yellowish cloudy mixture, which became more and more dense as time increased . It reached a point where the black cross , was completely diminished .This was because more and more sulphur was precipitating as time went by .
DATA ANALYSIS
We know that m = CV , where c is concentration , m is the no. of moles , v is the volume .
Hence we can work out the no. of moles of Na2S2O3 in each experiment and can get the concentration by simply dividing the no. of moles by the volume of the Na2S2O3 + water solution . As water added increases the volume of Na2S2O3 decrease and hence the concentration of Na2S2O3 also decreases .
The calculated concentrations are listed along with the time taken for the reaction in table 2.
Table 2 Time taken for the reaction when Na2S2O3 solutions with different molarities are used
Experiment No Concentration (M) Time taken for the reaction (Seconds)
1 0.1000 41.45
2 0.0870 48.79
3 0.0750 56.94
4 0.0625 71.24
5 0.0500 93.44
6 0.0375 130.32
7 0.0250 238.99
From the above table we can clearly see that as concentration decreases the time for the reaction increases .The graph, which is a parabola, shows clearly that as concentration decreases the time for the reaction increases or as concentration increases the time taken for the reaction decreases . Hence we can say that time and concentration are inversely proportional .
The curve is getting closer and closer (but never reaching) to an instantaneous reaction. The graph however doesn’t show us how the rate of reaction is being affected by changing concentration. For this we must find the rate of reaction for each of the experiment. For each experiment, this is done by taking the reciprocal of the time taken for the reaction measured in seconds. This is because, for each experiment, whatever the time taken is the time required to reach the same stage in the reaction, when the cross-stopped being visible. So the reciprocal of the time taken is the amount (or the rate) of the reaction occurring each second.
The rate of reaction for each experiment was calculated and presented in table 3 along with concentration:
Table 3
Rate of reaction when Na2S2O3 solutions with different molarities are
Experiment No Concentration (M) Rate of reaction (seconds-1)
- 0.1000 0.024100
- 0.0870 0.020400
- 0.0750 0.017760
- 0.0625 0.014030
- 0.0500 0.010700
- 0.0375 0.007670
7 0.0250 0.004184
Hence from the table and graph we can clearly make out that as the concentration increases the rate of the reaction increases or if the concentration decreases the rate of reaction also decreases. Hence by only looking at the straight line of the concentration vs. time graph we can state that concentration and rate of reaction are directly proportional
EVALUATION
From the results it is evident that lowering the concentration of the thiosulfate ions decreased the rate of formation of sulphur. This was because thiosulfate ions must collide with hydrogen ions for a reaction to occur. When we lowered the concentration we lowered the number of thiosulfate ions in the reactant solution and thereby lowered the chance of collision with hydrogen ions. This resulted in a longer time taken for the reaction to reach the same stage. An increase in the concentration of thiosulfate ions increases the chances of these ions colliding with the hydrogen ions and a quicker reaction occurred.
During the practical there could have been many errors encountered. Which could arise if
1 All the chemicals were not at room temperature or the practical was done on different days for which the room temperature could be different.
2 Instead of measuring the lower meniscus something else was measured.
3 The apparatus was not rinsed properly.
Errors could also arrive due some uncertainties , which are :
Firstly for time , as we can only measure temperature to the nearest minisecond there could be an uncertainty of + or - 0.005 seconds . Hence the actual value for time should be measured value + or – 0.005 seconds .
Secondly and similarly as we can only measure temperature to the closest degree Celsius there could be an uncertainty of + or – 0.5 degrees Celsius. Therefore the actual value should be measured value + or – 0.5 degrees Celsius .
Safety precautions: In order to work in the chemistry lab we have to consider certain safety precautions to avoid health hazards . In this particular experiment we should use the following safety precautions:
1 All the used chemicals should be disposed off carefully, as they could be poisonous, and all the apparatus should be washed and dried with care .
2 Safety goggles and gloves should be used if working with a burner or strong chemical.
3 Other safety utensils like holder, stand, glass rod, etc. should be used wherever appropriate. 4 All the glass utensils should be handled with care in order to avoid breakage and cuts.
There are many other safety precautions used in various other experiments but we are only concerned with this particular practical, therefore the above stated precautions should be enough.
Modifications
In order to improve on this practical I can use some modifications which are:
1 Doing the practical in one day so that there isn’t any change in room temperature.
2
