In this case the concentration of the sodium thiosulphate will be the independent variable. That will be changed in order to provoke a reaction from the dependant variable (time taken for the substrate to cover cross completely).
A Catalyst is a substance which increases the speed of a reaction without being changed or ‘used up’ in the reaction. A Catalyst works by giving the reacting particles a surface to stick to where they can collide with each other. This increases the number of collisions too.
But this investigation will have no catalyst.
- The Surface Area (Size Of Solid Particles)
If one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area. This means the particles around it in the solution will have more are to work on so there will be more useful collisions. The more finely divided the solid, the faster the reaction, because the surface area in contact with the gas or liquid is much greater.
This investigation does not deal with solids; however, surface area is to solids as volume is to liquids. The volume of solution will stay the same; rather the concentration will be changed. This is yet another controlled variable.
Another variable is time, the time taken for the reaction. This will be the dependant variable. It is the variable that will be measured and investigated.
Prediction
I predict that the more diluted the Sodium thiosulphate the slower the reaction between it and the dilute hydrochloric acid will be. I predict this because from my knowledge of the particle theory I know that the more dilute the solution the less sodium thiosulphate particles present. This will then mean the chance of a favourable collision between a sodium thiosulphate particle and a dilute hydrochloric acid particle will be reduced. Hence the time taken for all the particles to collide will be reduced. This is a qualitative prediction, as a quantitative prediction, I predict that halving the Sodium thiosulphate concentration will double the time taken for the substances to react.
Equipment
The equipment needed for the investigation:
- Sodium thiosulphate solution (40 grams per litre)
- Dilute hydrochloric acid (2M)
- Measuring cylinder x 2
- Conical flask
- Stopwatch
In order to keep it a fair test the measuring cylinders and conical flask must be cleaned properly as so there is no solution left so it cannot affect the measuring in the next experiment.
Procedure
When dilute hydrochloric acid and sodium thiosulphate are mixed, a yellow precipitate of sulphur gradually appears. A cross will be drawn on a piece of paper in pen. This cross will be placed under a conical flask, in which the sodium thiosulphate and dilute hydrochloric acid will be mixed. In one of the measuring cylinders a certain amount of sodium thiosulphate will be measured together with a certain amount of water, to dilute the solution. In the other a constant amount of dilute hydrochloric acid. Both will then be mixed into the conical flask and timed. The stopwatch will be stopped when the precipitate formed completely covers the cross so that it has disappeared from view. The time will then be recorded and the experiment repeated with different concentrations of sodium thiosulphate.
The results will then be presented in a table such as this:
This data will then be presented as a graph.
A minimum of five different results will be taken each repeated at least twice. This will ensure the accuracy of the results.
To keep it a fair test the experiment will be carried out in the same place, so the temperature will remain on average about the same. As earlier explained the temperature cannot be controlled due to practical limitations. Also the equipment will be cleaned thoroughly and the volumes kept constant.
To ensure safety goggles will be worn at all times and great care will be taken when handling the chemicals.
A diagram of the equipment which will be used:
Preliminary Readings
From the preliminary readings a range of 20-0 was decided for both the sodium thiosulphate and water, both made up the same volume. But just caused the concentration of sodium thiosulphate to change. This range was chosen as it gave a wide range of tangible results. Also an extra reading of 0 sodium thiosulphate was decided to be added. This shows that the measuring cylinders and conical flask was cleaned properly and hence showing it as a fair test. These readings also showed the procedure chosen was the correct method of carrying out such an investigation.
Observations
The results were as follows:
The main observation noted was the colour change. As the two solutions were mixed gradually a white precipitate was formed. After a while (depending on concentration) the precipitate became opaque and completely made the cross underneath disappear from view.
Analysis
For the purposes of the graph an average of the two times was calculated and used against concentration of sodium thiosulphate.
From the graph the following conclusions can be made. Firstly, the rate of reaction does decrease as the concentration of sodium thiosulphate decreases, thus proving my prediction. I predicted that the more diluted the Sodium thiosulphate the slower the reaction between it and the dilute hydrochloric acid will be. From my graph it can be seen to be true. This is because the curve becomes less steep as the time taken increases. My quantitative prediction that, halving the Sodium thiosulphate concentration will double the time taken for the substances to react, is also proved right by the graph. But not exactly, this could be down to errors in timing or other external factors. E.g. at 20cm³ (sodium thiosulphate volume) the average time taken is 51 seconds. At 10cm³ (sodium thiosulphate volume) the average time taken is 98 seconds. Even though not exactly half, it is quite close, only a four second difference. There are no anomalous readings.
The more dilute the solution the less sodium thiosulphate particles present. This will then mean the chance of a favourable collision between a sodium thiosulphate particle and a dilute hydrochloric acid particle will be reduced. Hence the time taken for all the particles to have collided with each other will be reduced. This is the scientific reasoning behind the results obtained.
Evaluation
Overall I believe the investigation went quite well and the procedure was an accurate and an appropriate way to carry out such an investigation.
Even though there were no anomalous readings the test was not completely accurate. The procedure was very susceptible to human error. The stopwatch could be stopped late or the cross could be seen by one person but have disappeared to another.
The investigation was however fair and quite reliable, as the instruments were cleaned out properly and the same conditions were used for all the experiments.
One obvious improvement is the variable of temperature; it was neither checked nor controlled in any way. Controlling the temperature would allow for the investigation to be truly fair but due to practical limitations this was not possible so it, in this case, is appropriate to assume it as room temperature and as not changing much.
Overall I think the test went well, tangible results were obtained, the prediction proved and, surprisingly for an investigation like this, no anomalous readings were formed.