So technically the reaction will be:
Na2S2O3 + 2HCl + H2O → 2NaCl (aq) + SO2 (aq) + S (s) + H2O
Sodium Hydrochloric water Sodium Sulphur Sulphur water
Thiosulphate Acid Chloride Dioxide
The solutions I need to make are:
Each set of measurements should be done three times as to ensure the results are accurate and also, other variables must and will be kept constant.
Whilst doing this experiment, safety glasses should be worn as I am working with acid. And other than that, the only other thing to be done in order to ensure safety is that General Laboratory Practice (GLP) should be used.
Variables that should be kept constant to ensure a fair test and accurate results:
Volume of Sodium Thiosulphate
Temperature
Voltage used in circuit
Wattage of Bulb
Distance between beaker and light sensor
Prediction
I believe that in my investigation, the more acid there is (the more concentrated the acid is) in a solution then the quicker the reaction will be. Rates of reaction all centres on collisions. The more successful collisions there are, then the faster the reaction is.
When an acid particle collides with a sodium thiosulphate particle then the two react.
So, if the concentration of the acid is increased, then the reaction is faster. It is easy to see why:
This can also show why reactions slow down over time.
For example the reaction between sodium thiosulphate and hydrochloric acid. At the start of a reaction, there are plenty of sodium thiosulphate particles and hydrochloric acid particles. But they get used up during successful collisions. Therefore, over time, there are less sodium thiosulphate particles and the hydrochloric acid is less concentrated. Thus the reaction slows down.
Start Of Reaction Part Way Through Reaction
Rate of reaction is also effected by temperature and that is why the temperature during my investigation / experiment must be kept constant.
At low temperatures, particles of reacting solutions, or elements, don’t have much energy. But, when these are heated, the particles gain energy. This means they move faster and collide more often. Thus the rate of reaction will speed up. If the collisions have more energy then more of them will be successful.
Results
Analysis
It can be clearly seen from my table of results, that not all the results agree. For example, when the solution with 5ml of Acid was done for the third time, it took 119 seconds, this is a huge difference to the first two times.
It can also be seen that the times taken in the first attempts column (1st time) are generally quicker than the other two repeats. One reason why this could be is that the acid on the first day of doing this experiment was more concentrated to start with, than that used on the second day, the two repeats were done on the same day and so this difference can be easily accounted for.
My graph shows that as you increase the amount of acid in the solution, the quicker the reaction is. In the first solution where there was 30m1 of sodium thiosulphate, 5m1 of hydrochloric acid and 25m1 of water. The reaction took an average time of 87.33 seconds, while in the last solution where there was 30m1 of sodium thiosulphate, 30m1 of hydrochloric acid and 0mI of water. The reaction took an average time of 48 seconds. This shows that the reaction was quicker with more acid in the solution.
But I am unsure that all my points agree, as the result for a solution with 5ml hydrochloric acid is greatly different to the other results and this could be classed as an anomaly. One way to check if this result is correct would to be to do the experiment with a solution containing 7.5ml of hydrochloric acid (the midpoint between the existing results, 5ml and 10ml) and therefore this point could either prove or dis-prove the point at 5ml.
In my prediction I stated that in the investigation, the more acid there is (the more concentrated the acid is) in a solution then the quicker the reaction will be and the less acid there is in the solution, the longer the reaction will take.
My results support my prediction as the general trend of my graph follows the theory of my prediction. For example, when there was 20m1 of acid in the solution, the reaction was faster (56.67 secs) than when there was I 0mI of acid in the solution (60.33 secs).
Sodium Thiosuiphate: 30m1 Sodium Thiosuiphate: 30m1
Hydrochloric Acid: 20m1 Hydrochloric Acid: 10ml
Water: 10ml Water: 20ml
Average Time Taken: 56.67 secs Average Time Taken: 60.33 secs
More concentrated solution reacted the quickest.
Evaluation
I believe that the experiment I carried out did not work well when done in the classroom, using school resources.
Firstly, the experiment practical was done on two separate occasions and therefore there was no guarantee that on both days:
∙ The initial concentration of the hydrochloric acid was the same
∙ The initial concentration of the sodium thiosulphate was the same
∙ The two bulbs used were of the same age and therefore the exact same brightness
∙ The light being let into the transparent beaker from the room’s lights was the same
∙ The light censors had the same accuracy
There are also many improvements that could be made to the investigation to make the experiment more accurate and improve reliability:
∙ Do the experiment all on one day, thus ensuring the solutions (hydrochloric acid and sodium thiosulphate) are exactly the same concentration.
∙ Wrap the sides of the beaker with opaque paper to ensure that the only light being let into the beaker is that from the bulb.
∙ Use the same light censor on each day.
I believe that the evidence shown by my results can support a conclusion that, the rate of reaction between sodium thiosulphate and hydrochloric acid is quicker, the more acid that is used in the reaction and is slower, the less acid that is used in the reaction
But, as in this experiment there is a great room for error, my results will not be entirely accurate and therefore cannot support a firm conclusion.
Overall I believe I carried out this experiment to the best of my abilities, considering all the different factors that could have affected my results.
To extend the investigation I could investigate how temperature would affect the reaction rate between Hydrochloric acid and Sodium thiosulphate. Measuring the reaction times whilst the solutions are at different temperatures could do this. I would predict for this that the warmer the temperature, the quicker the reaction, as in corroboration with my prediction for this experiment, reaction rates are based around collisions and as the reaction is given heat, the particles are given energy and thus there are more successful collisions, therefore speeding up the reaction rate.
