We will be investigating the following reaction:
Na2S2O3 (aq) + Hcl (aq) = 2Nacl (aq) + H 2 O (l) + S (s) + SO 2 (g)
Sodium + Hydrochloric = 2 Sodium + Water + Sulphur + Sulphur
Thiosulphate Acid Chloride Dioxide
Both Sodium Thiosulphate and Hydrochloric acid are clear liquids. When the two react together, a cloudy yellow solution forms, this is due to the sulphur precipitate formed in the reaction.
It is this yellowish precipitate, which will allow us to measure the speed of the reaction.
We will place a conical flask containing the reactants on to a piece of paper with a cross on it. We can then time how long it takes the cross to disappear. In effect, we will be measuring how long it takes for a certain amount (enough to obscure the cross) of the sulphur precipitate (and the other products) to be produced. From this information, we will determine the rate of the reaction.
I didn’t choose to use the speed that water, or sodium dioxide are produced to measure the reaction rate because these two products are difficult to collect and it is difficult to view how fast they are produced.
Hypothesis- As the temperature of the reactants is increased the time for the cross to be obscured will decrease. This means that the rate of the reaction will increase, the higher the temperature -the reaction will speed up as the temperature increases.
(This hypothesis is supported by the collision theory - written above)
Preliminary experiment
It is necessary for us to carry out some preliminary tests in order to determine which values and ranges of variables we use. For example, we need to find out which temperatures it will be possible for us to use. If we use very high temperatures then we will have extremely small times to record. In a small amount of time, small human errors make a large difference so the results would be less reliable than when larger times are recorded.
For the preliminary experiment, we will just do tests of a limited section of temperatures in order to see whether our highest and lowest temperatures will be too high or low.
A table to show which variables we will need to control in order for this to be a fair test and which ones we will measure:
Method for preliminary
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Prepare water bath to required temperature (0-100 oC)
- Place conical flask on white paper with cross on.
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Measure out 10cm3 of HCl into a boiling tube, do the same for Sodium Thiosulphate.
- Place the two boiling tubes into the water bath
- When the reactants have reached the correct temperature, take them out of the water bath and quickly pour them both into the conical flask.
- simultaneously start the timer
- 1 person watches the reaction and stops the timer when the cross is no longer visible
- Record results in table
Here are the results from the preliminary experiment:
From these results, I can see that it would be possible to use this range of temperatures for the final experiment.
Evaluation of preliminary experiment:
Although the temperatures I chose gave me successful results after doing the preliminary experiment, I found that it would be dangerous to heat the reactants to temperatures as high as 100 degrees, since at temperatures above 60 degrees large amounts of Sulphur Dioxide gas are produced. Sulphur dioxide is toxic and harmful. Therefore, for our final experiment, we will range our temperatures from 0-60 degrees but we will use the same interval, we should still have enough results to draw a reliable graph.
As you can see from the table, we didn’t manage to get the reactant temperature as low as 0 degrees. This would be exceedingly hard with the equipment available to us, but it doesn’t really matter since we could still plot a temperature such as 1 degree onto our final graph and get a varied set of results.
After doing the preliminary experiment we can see that it will be very important to keep the actual test as fair as possible. We must avoid cross contamination by using separate pipettes for each chemical and being careful not to mix glassware. We also found that it would be useful for each person in the group to have specific jobs to do, so that we are as organised and efficient as possible. Having specific jobs would also mean that the experiment is more likely to be fair since everyone would know exactly what they are doing.
Actual experiment-we will base our conclusion on this experiment.
For this experiment, we will take three measurements of each temperature in order to avoid having unfair results and to allow for human error.
Equipment needed:
- 2 boiling tubes
- conical flask
- thermometer
- Hydrochloric acid
- Sodium thiosulphate
- 2 measuring cylinders
- A piece of paper with a cross clearly marked on it.
- Stop clock
- 2 pipettes
For setting up the water bath:
- 1 Bunsen burner
-
1 tripod
- 1 heatproof mat
- 1 gauze mat
- Beaker
- Water
For setting up the water bath for around 0 degrees:
- Beaker
- Ice
- A small amount of water
Method:
-
Set up a water bath over the Bunsen to the temperature required (For the temperature of around 0oC place some cubes of ice into a beaker and place a little cold water in).
-
Using a measuring cylinder and a pipette accurately Measure out 10cm3 of acid into a boiling tube. Measure out an equal amount of Sodium thiosulphate into another boiling tube using the other cylinder and the other pipette (always make sure that one pipette and measuring cylinder is used for one reactant and the other ones are used for the other reactant).
- Place the boiling tubes of reactants into the water bath, using a thermometer observe the temperature.
- Place the conical flask onto the paper with the cross directly beneath it.
- When they reach the correct temperature, remove them from the water bath and quickly pour them into the conical flask, simultaneously starting the timer.
- One person watches the reaction and when they can no longer see the cross, they stop the timer.
- Record results in table.
- Wipe out the conical flask – Always wipe out glassware DO NOT rinse the flask or the boiling tubes as this will affect the concentration of the reactants in the next experiment.
- Repeat the experiment twice for each temperature- this gives us three results for each one.
- Draw graphs to analyse and evaluate the experiment.
A table of results from the experiment:
Analysis
Conclusion: The results support our prediction, although further, investigation that is more detailed is needed in order to prove it unquestionably.
From the graphs, you can see the evidence that supports the prediction. My first graph showing time against temperature has a strong negative correlation; this means that as the temperature decreased the amount of time taken for the experiment increased and vice versa, just as I predicted. My second graph has a positive correlation; this shows me that as the temperature of the reactants increased the reaction rate increased also.
Our findings can be explained using the collision theory:
As we gave our reactants more heat, we gave their particles more energy so that more of them had sufficient energy to react when they collided with each other (the activation energy). The hotter the reactants the faster the reaction was because the more atoms had enough energy to react on collision.
Without the activation energy the particles will not react, they will just bounce off each other without breaking their chemical bonds.
Evaluation
The results were overall good, although there are some anomalies. We can see from the results table that the results for 5 degrees were especially unreliable, they are all very different from each other, and this suggests error in the experiment. Other anomalous results are the first time for 30 degrees and all three times for 50 degrees. The problem with the anomalous results could be that:
- The person watching the reaction inaccurately judged when the cross was fully obscured.
- Errors in the co-ordination of the person watching the experiment affected the time.
Using a light sensor would remove these human errors; a light sensor measures the amount of light shining through a substance. It would be wired up to a timer and to the experiment, it would automatically stop the timer at the exact point that no light passes through the liquid, therefore giving us fairer, more reliable results.
Simply repeating the experiment more would give us data that are more accurate, and being extra careful not to affect the concentration by adding water would also help.
To investigate this experiment further we could do more repeats and do a wider range of temperatures with a smaller interval, this would give us more results from which to draw evidence that is more reliable.
