Acid particle More particles more chance of a successful collision
Powdered reactants react more vigorously than block reactants. As they cover a greater surface area therefore there is more chance of a successful collision.
Small surface area Greater surface area
A catalyst helps to speed up the reaction, but uses less energy, than what the reaction would use without the catalyst. When the reaction has taken place the catalyst remains the same as before the reaction started.
No catalyst With catalyst
Activation energy Activation energy
I have decided to investigate the effect of changing the Sodium Thiosulphate concentration on the rate of reaction.
Fair testing.
In order for this investigation to be a fair test, only one variable can be changed all the others must be kept at a constant. The temperature will be kept at a constant temperature, as will be the concentration of the acid and the volumes of acid and Thiosulphate used. The only substance that will be changed is the concentration of the Sodium Thiosulphate.
The rate of reaction can be determined by timing the rate at which products are formed. I am going to note the time taken for the Sulphur to be produced. I am going to estimate the end of the reaction by placing a piece of paper with a ‘X’ marked on it underneath the conical flask. As the reaction proceeds the solution turns cloudy and eventually yellow, when I can’t see the ‘X’ through the solution, I will take this as the end of the reaction.
Preliminary work.
I have carried out preliminary work, to find out what concentration of Sodium Thiosulphate I am going to use.
First I tried dissolving 5g of Thiosulphate in 10cm. I decided that I would try this out on the highest amount of Sodium Thiosulphate, which has not been added to water and I would try it out on the lowest amount of Sodium Thiosulphate, where the ratio of water is greater than the ratio of the Sodium Thiosulphate.
I am not going to use this amount of Sodium Thiosulphate because it would take too long, so I am going to try 20g/cm of Sodium Thiosulphate, in 30cm of water.
Sodium Thiosulphate concentration.
Na S O : 5H O
Ar (Na) = 23, Ar (S) = 32, Ar (O) = 16, Ar (H) = 1
Mr (Na S O ) = 248
248g in 1000cm gives a 1m solution.
40g in 1000cm gives a 0.16m solution.
I have decided to use five concentrations of Thiosulphate solution. I am going to dilute the original concentration of 0.16m four times by diluting it with water.
e.g
Prediction.
I predict that the more dilute the solution becomes, the longer the reaction will take.
If you double the concentration you double the rate of reaction. This is because there would be more particles in the solution, therefore there are more reactant particles in the solution to react together.
Apparatus.
I need the following apparatus:
- Conical flask,
- Piece of paper with ‘X’ marked on it,
-
50cm3 of Hydrochloric acid,
-
10cm3 and a 50cm3 measuring cylinder,
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300cm3 of Sodium Thiosulphate,
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100cm3 of water,
- Stopwatch and,
- Pipette.
Method.
To start my final experiment, I will measure out 50cm3 of Sodium Thiosulphate into a 50cm3 of measuring cylinder, I will use a pipette for accuracy. I will then measured 10cm3 of Hydrochloric acid using a 10cm3 measuring cylinder, I will once again use a pipette for accuracy. The piece of paper with ‘X’ marked on it will then be placed underneath the conical flask. Then I will pour the two solutions into the conical flask and start the stopwatch. When I can not see the ‘X’ through the solution anymore I will stop the stopwatch, and take this as the end of the reaction. The ‘X’ disappears as the Sulphur is being produced. I will carry out the next four experiments, I will dilute the solution 10cm 3 of water, then 20cm3 of water, 30cm3 of water and finally 40cm3 of water and timed how long the reactions will take.
Results.
I have placed my results in the table below.
I have carried out each experiment twice in case of any error. I then worked out the average times for the reactions. Then I drew graphs of my results, on the next page.
