These were my preliminary results: -
Method
I am going to attempt to calculate the effects of concentration on the acid/sodium thiosulphate reaction by first filling the flask with 5mls of 2M hydrochloric acid. I will then add a mixture of water and sodium thiosulphate to the mixture these ratios of each: -
Water: Sodium Thiosulphate
-
0:50ml ∙ 5:45ml ∙ 10:40ml ∙ 15:35ml ∙ 20:30ml
-
25:25ml ∙ 30:20ml ∙ 35:15ml ∙ 40:10ml ∙ 45:5ml
This makes 50mls of additional liquid to add.
I will then time how long it takes for the reaction to take place once they are mixed.
I will start the stopwatch when the water/sodium thiosulphate is poured totally into the flask with the acid, and stop the timer when the cross is no longer distinguishable from view due to the opacity of the liquid in the flask.
It will be a fair test as I am using the same amount and concentration of acid and only varying the concentration of sodium thiosulphate.
The amount of liquid in the flask will stay the same as I am not changing the volumes and I am also keeping the cross the same thickness and size.
I have decided to apply the same changes to each flask e.g. shaking the flask to encourage colloidal formation.
The temperature during the experiment will be at the current room temperature, which may vary slightly, but so little that this will not effect the experiment (approx. 1-3oc).
The same person will always look at the cross to ensure consistency when observing.
I am choosing to use measuring cylinders to measure liquid volumes in this experiment as I feel beakers are too inaccurate and none of the small size required are available. Measuring cylinders are the right size, and have appropriate intervals between markings to allow precision when using the liquids.
I am also not using pipettes as I do not need the reverse to the above; acutely small amounts. A pipette would rather hinder, than help my experiment by increasing the time required to set up each experiment as I have found with previous experiments involving small measurements.
I shall repeat each experiment three times so as to get an average and make the results fair.
Prediction
I think that as the concentration of sodium increases, the time taken for the cross to become indistinguishable will diminish exponentially as this graph shows: -
I think this will happen because the particles in the liquid will have a larger chance of coming into contact with one another and creating colloidal sulphur that will block the view of the cross by reflecting light.
Colloids are created during the reaction. A colloid is halfway between a solution and a suspension. In a colloid, particles of matter measuring between about one-millionth of a millimetre and one-tenth of a millimetre in diameter are evenly scattered throughout liquid or gas.
An example of a colloid is smoke, which are microscopic particles dispersed in the air.
Other colloids include dairy products like butter, the cytoplasm of cells, and fluids found inside our body tissues.
These colloids absorb light, thereby obstructing a direct line of sight past them. In this way, the colloids created in the reaction, obstruct the view of the cross behind them.
Results
These are the results that I collected:
I worked out the rate by doing the calculation
And also worked out the concentration by
As is possible to see from the chart, as the concentration increases, the time taken decreases exponentially. My prediction was correct. It is a curve because as the concentration increases, the particles of acid are in an increasingly higher ratio. The higher ratio means that collisions ➔ reactions are more likely, and occur more frequently and so the time for the colloids to obscure the cross is less. Eventually the time curve will flatten off, as the maximum reaction speed will be reached for that temperature.
From the graph, it is possible to see that as concentration increases, so does rate. Both increases at equal amounts, creating a straight line of best fit with most of the points on or very close to the line. This means that concentration is directly proportional to rate.
I think this happened because as the colloidal sulphur forms at a steady rate depending on strength of sodium thiosulphate, the steadily increasing concentration brought steadily increasing rate as well. This was because a more concentrated liquid has a larger ratio of particles to water. This meant that the acid molecules were more likely to come into contact with the sodium, and therefore create a greater rate at which colloids were produced.
We had anomalous results as we had to change our cross after the first five experiments, which changed the time scale. Also, the judging by eye technique was inaccurate and crude.
We could not add the liquids together evenly (only by pouring), and the timer was started at small discrepancies with when the liquid was mixed. Also, not all of the acid and mixed liquids came out of the beakers, making it more inaccurate.
We could have improved it greatly by using a photosensitive diode to measure colloidal opacity and a computer-controlled timer would have enhanced our readings. Another way of combing the liquids evenly and without having to use physical means (for instance, a Y tube design with holders at the top would have kept time discrepancies to a minimum. Also, greasing/cleaning the equipment thoroughly would stop droplets of liquid clinging to the Pyrex containers.
We had results to confirm our conclusion, as the points were relatively straight and 80-90% on the line of best fit.
A way to work out reaction time by mathematical means would have been a benefit, cutting out any practical experiment, and eliminating any chance of odd results.
