∙ We must make sure that we add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could affect the results of the experiment.
∙ We must start the stopwatch at the exact time as we put the water and the sodium thiosulphate into the conical flask. This experiment needs two people; one putting the solution in the conical flask while the other person starts the stopwatch.
∙ Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment.
∙ To ensure that the cross is clearly seen in all repetitions of the experiment we will keep the same cross, this keeps all experiments the same and therefore fair.
∙ Me and my work partner agreed we would have one person who looks at the changing solution while the other person times how long it takes for the solution to turn cloudy. This way we know that there is little difference in opinion on the point at which the solution is classed cloudy.
∙ The same place in the classroom was chosen for all experiments because the levels of light were the same and the temperature was more or less similar on each day of testing.
Safety
To make sure this experiment is kept as safe as possible I made a chart showing the risks of what we are doing and what to do if someone becomes sick because of the experiment and the substances used. Firstly you have to realise that this experiment is taking place in a room where there are many risks if suitable precautions aren’t taken. To prevent people falling over and causing injury I am going to measure out all my liquid solutions at the sink and any liquid spilt on the floor will be cleared up immediately.
If someone does hurt they then assess the situation carefully. Do not put yourself at risk check safety. If he/she is unconscious then put the person in the recovery position as shown. Call for help immediately, and call for an ambulance.
Research
Colloids are important in this experiment because the reaction between Sulphur and hydrochloric acid forms a ‘colloid’, which makes the solution go cloudy. When you use a higher concentration the amount of colloids colliding goes up which means that the rate of reaction is speeded up. Sulphur and hydrochloric acid however cannot mix; this triggers the two substances to turn cloudy.
Every colloid has at least two parts; one part is the continuous phase like the water in milk or the air in mist. The other part is split up into minute particles like the fat in milk or the water droplets in mist. These scattered particles make up the disperse phase. Colloids make the light scatter causing the solution to turn cloudy, it does this when light rays pass through a colloid and are reflected or refracted every time it meets one of the droplets or particles in the solution. Therefore when the concentration of thiosulphate solution is increased the amount of particles colliding is increased.
Prediction
I think that the higher the concentration we use of sodium thiosulphate, the faster the reaction will happen. The reason I think that this will happen is because in a reaction particles collide. So if there is a higher concentration of particles in a solution, then there will be a higher number of collisions. If there are a higher number of particles colliding, then chance of the particles reacting increases.
Equation for this experiment is as follows:
Sodium + Hydrochloric Sodium +Sulphur +Sulphur +water
Thiosulphate Acid = Chloride Dioxide
Na S2 O3 (aq) + 2HCL (aq) 2NaCl (aq) + S(s) + SO (g) +H20
This reaction usually produces a white/light yellow precipitate.
Results
Molar Mass = Amount of Thiosulphate
X 0.25
50
Conclusion
The general trends I found when looking at the results from this experiment show that the higher the concentration of sodium thiosulphate the quicker the rate of reaction. Analysis of the graphs shows this. This concludes that my prediction, which was that if you increase the concentration of thiosulphate in the solution it would turn cloudy quicker, was correct. The two graphs line of best-fit show mild deviations. The tables show the full strength solution takes an average time of 36.3 seconds to go cloudy, the most dilute solution's average time is 1445.5 seconds. This is due to the amount of particles in the solution being greater causing the colloids to collide more often. When you double the molar mass of the thiosulphate you should expect to see the rate of reaction to double also. The reaction times are down to probability of colliding so to say that there is a right or wrong set of results is incorrect. I do however feel that my results are quite reliable, there are no clear anomalies shown by the graphs.
Evaluation
I feel that the experiment we did was effective in proving how the rate of reaction is affected by varying levels of sodium thiosulphate when mixed with hydrochloric acid. There were various ‘odd’ results, e.g. the time for the solution to go cloudy was not consistent throughout all the repeats. This might have been due to basic human error of misinterpretation of time, or maybe because we measured the substances inaccurately. The only other possibility is that the apparatus were not cleaned properly in between testing. Generally the results gathered have enabled me to complete the experiment. I found it hard to be able to tell when the cross, had disappeared (having to judge by human eye). I think a more accurate way of doing this experiment would be to use a light gauge. The light gauge would measure accurately when the solution reached the same degree of cloudy-ness in all repetitions. This would make the experiment much more fair because the same would apply throughout. There are a number of ways in which we could have made the results more reliable e.g. by being more thorough in the cleaning of the apparatus and using the light gauge. If I were to redo this experiment, I would change two variables, not only the amount of sodium thisulphate but also the amount of hydrochloric acid. This would give more interesting data to analyse. The reaction times would also be greatly increased making the reaction time shorter so more weak concentrations can be looked at but without taking so long. In general I am pleased with the outcome. I enjoyed doing this investigation as it let me look in depth at how reactions take place. Previous to doing this experiment I lacked knowledge of colloids and found this interesting.
