Method
I will need to do five tests, all of different concentrations. The concentration table below will show how much water and Sodium Thiosulphate will be needed per solution:
How Did I Carry Out The Test?
- I did the experiment in a logical order, so I tested the solutions numerically.
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I started with solution one. So, I measured 25cm³ of Sodium Thiosulphate – for this particular solution I will not need to use distilled water (see concentration table above.) I placed the ‘X’ paper underneath the beaker. Then, I mixed the 25cm³ with 5cm³ of hydrochloric acid, and started the stopwatch. I waited for the solution to go so cloudy that it was not possible to see the ‘X’, then I stopped the stopwatch and recorded down the time it took (in seconds).
- I accurately as possible repeated this with the other four solutions, using the Sodium Thiosulphate, hydrochloric acid and distilled water. I carried out the tests in the same way as above, and recorded down four more times, completing my results table.
- I found that my results were very accurate, so I did not have to repeat any of the tests.
- I then took my five results and worked out the rate of reaction, by dividing the time taken in seconds by 1, and I recorded this down to 3 decimal places.
- I will be taking five measurements because it is a wide enough scale and will be practical to plot on a graph.
- I measured the solutions accurately by using two different sized measuring cylinders – one small one to measure anything below 10cm³, and one big one to measure anything above 10cm³. This gave me much more accurate solutions.
Variables
The factors to control are:
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Temperature: if the temperature is increased, the rate of reaction increases too. This is because the particles move faster as they have more energy. This means that they will collide more and are more likely to break the bonds, so the reaction is faster. I will control this by doing the experiment at room temperature, and I will do all of the solutions on the same day so the change of weather doesn’t affect my experiment and make it less accurate. If I don’t control the temperature, it could potentially alter my results (not significantly, but nonetheless it could still alter them) so I have to keep the temperature constant for the best results I can get.
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Surface area: if the surface area is increased, more particles are exposed to the reactant, so they will collide more often, therefore increasing the rate of reaction. Surface area is not an issue, as I will be using the same amount of hydrochloric acid and the solution will always be the same amount too.
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Catalyst: catalysts increase the rate of reactions without getting used up themselves. They lower the activation energy needed. With a catalyst, more collisions occur and the reaction is faster. I will not be using a catalyst so this is not an issue either.
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Concentration: there are more particles in the same volume, so there are more successful collisions and so the reaction is faster. This is the factor I will be varying. I will be varying my experiment by adding increasing amounts of water to my original solution. I will have 5 different values of the variable. (See concentration table above.)
Risk Assessment
Risk assessment is a legal requirement and these are the following steps I will take to try to make my experiment safer and to eliminate any possible hazards:
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Gas produced: sulphur dioxide will be produced in my experiment. This gas is irritating to the lungs and is dangerous to asthmatics. So I should pour the cloudy solution as soon as possible into the fume cupboard.
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Safety goggles: these obviously protect the eyes and are essential to this experiment.
- Tie hair back
- Possibly wear a lab coat
- If any of the acid of the solutions are spilt, inform the teacher and class immediately so no one slips and hurts themselves
Prediction
I think that the higher the concentration of the sodium thiosulphate, the faster the reaction will be. The more distilled water there is in the reaction, the slower it will be. This is because the greater the concentration, the more reactant particles there are in that certain solution. There will be more successful collisions and therefore the rate of reaction is increased. As the concentration of the Sodium Thiosulphate, the rate will double.
Obtaining Evidence Section
Results Table
* Concentration of sodium thiosulphate solution calculated in this way:
Concentration of sodium thiosulphate = 0.15 x volume of sodium thiosulphate
Vol. of sodium thiosulphate + vol. water
** Rate of reaction calculated in this way:
Rate of reaction = 1 ÷ time taken (seconds)
(Note: this will not give me the actual rate of reaction itself, but 1 ÷ time taken is a measure of the rate of reaction.
Analysis Section
The evidence shows that the more Sodium Thiosulphate used in the solution, the faster the reaction took place, and therefore the rate of reaction is smaller. For example solution number one: volume of sodium thiosulphate: 25 cm³, rate of reaction 0.027 (took 37 seconds). The less Sodium Thiosulphate used, the slower the reaction took place, and therefore the rate of reaction was higher – e.g. solution number 5: volume of sodium thiosulphate: 5 cm³, rate of reaction: 0.0027 (took 366 seconds).
Examples:
