Hydrochloric acid + sodium thiosulphate → Sodium chloride + sulphur + sulphur dioxide + hydrogen oxide (water)
Na2S2O3 + 2HCl → 2NaCl + SO2 + S + H2O
I predict that the amount of acid will negatively correlate with the rate of reaction – that is, the more hydrochloric acid, the faster it would take for the cross to disappear. I think this because the more acid within the solution, the more meaningful collisions would take place. The displacement would hence occur at a faster speed and the products would be produced in a small amount of time.
- Conical flask
- 2x measuring cylinder
- 300ml of 0.1 Molar Hydrochloric acid
- 500ml of 0.1 Molar Sodium thiosulphate
- Black marker pen
- 2x Pipette
In order to verify how much acid we should use without the reactants taking a ridiculously long or short time to react, I conducted a preliminary experiment. In this experiment, I tested reacting 100ml of sodium thiosulphate with 100ml of hydrochloric acid 20ml of hydrochloric acid, which were the two extremes of my experiment. I found that the time taken by the reactants to react was reasonable and did not occupy too long or short a time. If the time taken to react was too short, the human reaction time in pressing the stop button of the chronometer will cause a large error margin of approximately ±0.5 seconds which will make our results very inaccurate. If the reaction time was too long, it would waste time in which we could be conducting our next experiment without using a large amount of extra equipment. This table shows the results of our preliminary experiments:
- Put goggles and apron on.
- Measure 100ml of sodium thiosulphate using a measuring cylinder. I will pour most of the sodium thiosulphate straight from the bottle into the measuring cylinder. I will add the last few drops with the aid of a pipette to ensure that I don’t accidentally pour the sodium thiosulphate above the 100ml mark. Another way in which I will make sure the amount of sodium thiosulphate is exactly 100ml is to have two people at the measuring cylinder – one doing the pouring and the other kneeling down with his eyes level with the 100ml mark. The latter person will tell the former person to stop when the mark is reached. I am taking this precaution to prevent parallax. If the person who is doing the pouring is standing up and therefore has his eye level above the 100ml mark, he might think he has poured the correct amount but due to parallax, he will have poured less.
- Measure 100ml of hydrochloric acid with another measuring cylinder. To prevent parallax and ensure that it is a fair test, I will take the same precautions as I did when I pour the sodium thiosulphate.
- I will then get a conical flask and a put it on a piece of paper with black cross on it. The two lines will be perpendicular to each other and the arms will all be 4 cm in length.
- I will pour the sodium thiosulphate into the conical flask.
- After this, I will pour the hydrochloric acid into the flask. Once all the acid is inside, I will start the chronometer.
- In my preliminary experiment, I established the time taken for the cross to get clouded over is usually more than 50 seconds. That is ample time for me to set up my next experiment where we use 100ml of thiosulphate and 80ml of acid.
- When the cross is invisible, I will stop the chronometer and record the time in my notebook.
- I then repeat the experiment using the 100ml of sodium thiosulphate and 80ml of hydrochloric acid.
- I will do the same experiment using different amounts of hydrochloric acid each time: 100ml, 80ml, 60ml, 40ml and 20ml.
- I will repeat the experiment for each amount of acid three times so I have three readings for each amount of acid.
I can see from the graph that 4 of the points match my prediction i.e. the amount of hydrochloric acid negatively correlates with the rate of reaction. However there is one point which does not seemingly fit in. When there is 100ml of acid, it takes a longer time for the cross to disappear than with 80ml of acid. The blue line on the graph shows the line of best fit when the seemingly anomalous result is included. However, I do not think the inclusion of an anomaly gives a fair representation of the experiment. The red line shows the line of best fit without the anomaly included.