During the reactions I will record the time taken in seconds using a stopwatch for each concentration to react. I will find the rate of reaction using the equation 1/time taken for cross to disappear. I will find the rate of reaction so that I can draw a graph and compare the concentration of sodium thiosulphate solution against rate of reaction.
When I am measuring all the solutions I will make sure the measuring cylinder is on a flat level surface and measure to the bottom of the meniscus. I will use 10cm3 measuring cylinder when measuring 10cm3 and below and I will use a 50cm3 measuring cylinder when measuring between 10cm3 and 50cm3. I will do this to make sure that experiment is a fair test and I will use equal amounts of reactants in each experiment.
In the preliminary experiment I will be using 4 different concentrations 0.08, 0.06, 0.04, 0.02 mol/dm3. I have chosen these results so that I will have a small range of results to prove my prediction. I will need to try to maintain the room temperature. I will keep checking the temperature with a thermometer and try to maintain the temperature by keeping all windows and doors close.
- First I will set up the apparatus as shown above. I will note down the temperature.
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The first concentration of sodium thiosulphate solution that I will be using is 0.08 mol/dm3. I will need 40cm3 of sodium thiosulphate and 10cm3 of distilled water. I will measure the sodium thiosulphate solution, distilled water and hydrochloric acid into different measuring cylinders.
- Next I will place a conical flask onto a piece of paper with a cross on it. I will need to pour the reactants into the conical flask as I simultaneously start the stopwatch. At the beginning of the reaction the liquid should be clear and the cross should be seen easily.
- I will time the experiment until the reactants have gone opaque. Sulphur is formed in the reaction causing the reactants to become more and more cloudy and I will no longer be able to see the cross. I will then record the time in seconds and dispose of the reactants in a fume cupboard.
- I will then need to repeat the same experiment with the different concentrations.
Preliminary results
Preliminary conclusion
From my preliminary test I found that my prediction was correct and the higher the concentration of sodium thiosulphate solution the faster the rate of reaction but I didn’t get enough results so in my investigation I am going to use more concentrations. The concentrations I will use are 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03 and 0.02 mol/dm3. I think I will get a better range of results and be able to draw a more accurate graph.
In my preliminary experiment I didn’t take repeat readings so my results may not be very accurate. In my investigation I will take 4 repeat readings so I can find an average time taken for the cross to disappear. I will then get more precise results.
Investigation plan
Safety precaution
During my investigation I will need to take into account the following safety procedures:
- I will wear safety glasses at all times,
- Ensure that my hair is tied back,
- Ensure my work area is clear to prevent accidents
- Sodium and hydrochloric acid are irritant to the eyes but they are also slightly irritant to skin, so if I get any on me I will wash it off straight away.
- During the reaction sulphur dioxide is given off, which is poisonous and can also cause bronchitis and conjunctivitis so I will need to put the reactants in a fume cupboard.
Factors to control
The independent variable that I will not control is:
The variables that I will keep the same are:
- Temperature
- Volume of hydrochloric acid
- The concentration of hydrochloric acid
- I will need to use the same cross in each experiment
- The size and depth of the conical flask,
- I will not stir or shake the experiments.
The apparatus I will need to perform my preliminary experiment will be:
- Sodium thiosulphate solution (0.1),
- Hydrochloric acid (1 M),
- Distilled water,
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10cm3 measuring cylinder,
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50cm3 measuring cylinder,
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100cm3 conical flask,
- Stopwatch,
- Thermometer,
- Piece of paper with a cross on it,
- Safety goggles,
- Access to a fume cupboard.
Apparatus
Method
The reactants begin as clear liquids. During the experiment sulphur is produced. This makes the clear reactants turn cloudy until the reactants become opaque. This is how I am going to measure the amount of time the different concentrations take to react. I will measure the amount of time a black cross drawn on a piece of paper underneath the conical flask takes to disappear.
During the reactions I will record the time taken in seconds for each concentration to react. I will find the rate of reaction using the equation 1/time taken for the cross to disappear. I will find the rate of reaction so that I can draw a graph and compare the concentration of sodium thiosulphate solution against rate of reaction.
When I am measuring all the solutions I will make sure the measuring cylinder is on a flat level surface and measure to the bottom of the meniscus. I will use 10cm3 measuring cylinder when measuring 10cm3 and below and I will use a 50cm3 measuring cylinder when measuring between 10cm3 and 50cm3. I will do this to make sure that the experiment is a fair test and I will use equal amounts of reactants in each experiment.
- First I will set up the apparatus as shown above. I will note down the temperature.
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I will be investigating 8 different concentrations. The concentrations I will use are 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03 and 0.02 mol/dm3. The first concentration of sodium thiosulphate solution that I will be using is 0.09 mol/dm3. I will need 45cm3 of sodium thiosulphate and 5cm3 of distilled water. I will measure the sodium thiosulphate solution, distilled water and hydrochloric acid into different measuring cylinders.
- Next I will place a conical flask onto a piece of paper with a cross on it.
- Then I will need to pour the reactants into the conical flask as I simultaneously start the stopwatch. At the beginning of the reaction the liquid should be clear and the cross should be seen easily.
- I will time the experiment until the reactants have gone opaque. Sulphur is formed in the reaction causing the reactants to become more and more cloudy and I will no longer be able to see the cross. I will then record the time in seconds and dispose of the reactants in a fume cupboard.
- I will then need to repeat the same experiment 4 times for each concentration. Thus enabling me to find an average time taken for the cross to disappear.
- I will do exactly the same experiment with my 7 other dilutions. Below I have written how I will make my other dilutions of sodium thiosulphate solution.
Range of dilutions of sodium thiosulphate solution
1. 0.09 mol/dm3 = 45cm3 sodium thiosulphate solution, 5cm3 distilled water.
2. 0.08 mol/dm3 = 40cm3 sodium thiosulphate solution, 10cm3 distilled water.
3. 0.07 mol/dm3 = 35cm3 sodium thiosulphate solution, 15cm3 distilled water.
4. 0.06 mol/dm3 = 30cm3 sodium thiosulphate solution, 20cm3 distilled water.
5. 0.05 mol/dm3 = 25cm3 sodium thiosulphate solution, 25cm3 distilled water.
6. 0.04 mol/dm3 = 20cm3 sodium thiosulphate solution, 30cm3 distilled water.
7. 0.03 mol/dm3 = 15cm3 sodium thiosulphate solution, 35cm3 distilled water.
8. 0.02 mol/dm3 = 10cm3 sodium thiosulphate solution, 40cm3 distilled water.
Observations
Conclusion
From my rate of reaction graph I can tell that as the concentration of sodium thiosulphate solution increases the rate of reaction increases. Sodium thiosulphate solution of a concentration of 0.02 mol/dm3 had a rate of reaction of 0.003 l/t (s-1) and sodium thiosulphate of a concentration of 0.03 mol/dm3 had a rate of reaction of 0.006 l/t (s-1). The rate of reaction increases when the concentration of sodium thiosulphate solution increases because the collision theory states that in a higher concentration there is a larger amount of particles in a given volume. This will increase the amount of successful collisions and the rate of reaction will increase.
From my graph I can tell that as the concentration doubles the rate of reaction doesn’t double. Sodium thiosulphate solution of a concentration of 0.02 mol/dm3 had a rate of reaction of 0.003 l/t (s-1) and sodium thiosulphate solution of a concentration 0.04 mol/dm3 had a rate of reaction of 0.0085 l/t (s-1). I think this is incorrect and that as the concentration doubles the rate of reaction should have doubled because there will be double the amount of particles in a given volume so there will be double the amount of successful collisions and therefore the rate of reaction should double.
My results probably didn’t show me this because the way I measured the reaction was not accurate enough and also because there was not a constant temperature in the room which changed the rate of reaction.
From my time graph I can tell that as the concentration of sodium thiosulphate solution decreases the time taken for the cross to disappear increases. Sodium thiosulphate solution of a concentration of 0.08 mol/dm3 took 50 seconds for the cross to disappear and sodium thiosulphate solution of a concentration of 0.07 mol/dm3 took 61 seconds for the cross to disappear. The time taken for the cross to disappear increases as the concentration decreases because a lower concentration has fewer particles in a given volume so there will be less successful collisions and the time taken for the cross to disappear will increase.
As the concentration doubles the time taken for the cross to disappear didn’t halve. Sodium thiosulphate solution of a concentration of 0.04 mol/dm3 took 122 seconds for the cross to disappear and sodium thiosulphate solution of a concentration of 0.08 mol/dm3 took 50 seconds to disappear.
My predicted rate of reaction graph is exactly the same as my actual graph. The line of best fit is identical. But my time graph is slightly different. Although the lines of best fit have the same negative correlation. I drew a straight line of best fit on my predicted graph and my actual graph has a curved line of best fit.
Predicted graphs
Actual graphs
Evaluation
My experiment went according to my plan but there were some flaws in my method. I got a good range of results as I expected but I don’t think that my method of measuring the reaction was very accurate. It was very difficult to tell when the cross was completely gone and the reactants completely opaque which could explain my anomalous results. A more accurate and precise way of measuring when the reaction had finished could be using a light sensor connected to a computer underneath the conical flask containing the reactants and measuring the amount of light that can get through the reactants at intervals of one seconds. I could completely surround the light sensor with a piece of card so that only the light that passes through the beaker containing the reactants is detected. I would then put the sodium thiosulphate solution dilution and the hydrochloric acid into a conical flask and start the computer readings. The computer records light levels as a percentage of the percentage of the original level against time. As the reaction progresses more sulphur will be produced and the reactants will become less translucent and form a cloudy solution. I think this would be much more accurate than using a stopwatch and the visual technique that I used.
Diagram of light sensor
On my graph you can see all my results fit my line of best fit except for one. The concentration of 0.06 mol/dm3 was my only anomalous results. The rate of reaction was 0.015 1/t (s-1). From the rest of my results I would have expected it to be about 0.013 1/t (s-1). The rate of reaction increased because at this stage the room temperature increased from 23oC to 27oc as the temperature increases the average kinetic speed of the particles increases, so more pairs of particles will have more energy between them and so there will be more successful collisions. Although my averages produced quite a steady increase in the rate of reaction some of my results were also too high or too low e.g. in the concentration of 0.03 mol/dm3 the first results was 186 seconds. This was much higher than the other 2 results of 164 seconds and 173 seconds. I could reduce this problem by controlling the temperature of the experiment better by placing the reactants into a water bath at a constant temperature of 25oc.
To further my experiment I could use a bigger scale of concentrations. I could use concentrations at smaller intervals. I could go down in 2cm3 intervals of sodium thiosulphate solution instead of 5cm3. This would make my graph more accurate because I would have more points on my graph to plot the line of best fit. I could also perform more repeat readings so I can find a more accurate average.
If I did my investigation again I could do several things to make it more accurate:
- I did my investigation over 2 days and I used different batches of hydrochloric acid and sodium thiosulphate solution. They could have been a slightly different concentration, which would have affected the rate of reaction. I would make sure I used the same batch.
- I used different conical flasks over the two days which could have been of a different thickness of glass and would have made it more difficult to see the cross. If I re did the experiment I could make sure I used identical conical flasks.
- After I had finished experimenting with one concentration I just washed out the conical flask and dried it with paper towels and re used it. This sometimes left residue on the inside of the conical flask which was difficult to clean because of the narrow neck. This residue could have affected my results because it changed the concentration of the reactants. I could try to clean it better or use clean conical flasks for each concentration.
- I would try and measure the volume of the hydrochloric acid, sodium thiosulphate solution and distilled water more accurately by using a burette, metric flask and pipette filler.
- During my investigation it was very difficult to distinguish between the hydrochloric acid, sodium thiosulphate solution and distilled water because they are all clear liquids. If I repeat the investigation I could try dying the liquids different colours but before I did this I would have to make sure it didn’t affect the rate of reaction or time taken for the cross to disappear.
- For safety reasons if I repeated the experiment I could collect the sulphur dioxide that is produced in a boiling tube through a delivery tube. This would prevent any gas being released.
I think my evidence is still quite reliable. Most of my repeat readings are similar. I think I have enough evidence to make a firm conclusion that my prediction was correct.
