Aim
Our aim is to investigate how different concentrations of sodium thiosulphate(Na2S2O3) change the rate of reaction with hydrochloric acid (HCl) which will produce sulphur (S), sodium chloride (NaCl), water (H2O) and sulphur dioxide (SO2)
Na2S2O3(aq) +2HCl(aq) → S(s) + SO2(g) + H2O(l) +2NaCl(aq)
Prediction
I predict that an increase in the concentration of sodium thiosulphate will increase the reaction speed. I have based my prediction on the Collision Theory which is, the more particles in the solution the more collisions, therefore, the faster the reaction speed.
I also predict that an increase in the concentration of hydrochloric acid will also increase the rate of reaction this is also based on The Collision Theory.
An example of this is if you were walking along Ladbroke Grove during the day on an average Sunday you will have less chance of colliding with a member of the public than on a Sunday during the noting hill carnival due to an increase of people (representing particles) in the area.
This is also based on a piece of information I found in the book “Chemistry in Context” it says. During a chemical reaction, reactants are being turned into products. The reaction rate tells us how fast the reaction takes place by indicating how much of a reactant is consumed or how much of a product is formed in a given time. Hence,
Reaction Rate = change in amount or (concentration) of a substance
Time taken
Equipment
- Conical flask
- 4% sodium thiosulphate
- 2M hydrochloric acid
- 2 50ml measuring cylinders
- 1 10ml measuring cylinder
- a tile with an x on it
- stop clock
- pipette
- water
Safety & Fair testing
We are going to be safe by wearing an apron and goggles because we are working with a corrosive solution, hydrochloric acid. We will also be working near a window because one person in our group is asthmatic and the reaction gives off a poisonous gas SO2. We will make this a fair test by having the same people doing the same job; one person watching, one person using the same equipment for measuring, one person timing, one person washing and one person drying. Also washing everything thoroughly as to prevent contamination. We will keep the volume of both solutions constant; hydrochloric acid 10cm3 and sodium thiosulphate 50cm3 and measure from the bottom of the meniscus. We will keep the temperature constant by doing the experiment at room temperature.
To prove that temperature affects the rate of reaction this table is taken from the book “Chemistry in Context Fourth Edition” page 410.
The above table shows the higher the temperature the faster the reaction.
Method
Sodium thiosulphate
- First we put our goggles and aprons on and got our equipment ready.
- Measure out 50 ml of sodium thiosulphate and 10 ml of hydrochloric acid in the measuring cylinders. Take the measurement from the meniscus, that’s the dip in the top of the liquid.
- Put the conical flask on the tile with the ‘x’ on it and pour the sodium thiosulphate in.
- Start the timer when the hydrochloric acid touches the thiosulphate.
- Continue timing until the ‘x’ disappears as a result of the solution turning milky repeat the process three times. Watch for anomalous results.
Sodium thiosulphate and water solution
- Measure out 40 ml of sodium thiosulphate and 10 ml of water in the measuring cylinders. Take the measurement from the meniscus.
- Put the conical flask on the tile with the ‘x’ on it and pour the sodium thiosulphate and water into the flask. This makes the total solution in the conical flask, 50 ml.
- Start the timer when the hydrochloric acid touches the thiosulphate and water solution
- Continue timing until the ‘x’ disappears as a result of the solution turning milky repeat the process three times. Watch for anomalous results.
Repeat process 1-4 sodium thiosulphate and water solution with the following concentrations.
30ml sodium thiosulphate to 20ml water
25ml sodium thiosulphate to 25ml water
20ml sodium thiosulphate to 30ml water
Results
The above table is my results showing the changing in concentration of sodium thiosulphate keeping the hydrochloric acid concentration constant at 10cm3
In all of the experiments when the two solutions mixed, briefly the mixture turned yellow and then went clear and then started to turn milky. When changing the concentration of sodium thiosulphate the rate of reaction changed.
The above table is my results showing the changing in concentration of hydrochloric acid keeping the sodium thiosulphate concentration constant at 50cm3
When changing the concentration of the acid we noticed that the concentration of the acid did not effect the rate of reaction as much as the sodium thiosulphate. This does not support my prediction as I originally thought that they would have the same effect.
Conclusion
My results show that the higher the concentration the faster the rate of reaction. If you lower the concentration of thiosulphate the rate of reaction slows down a lot. If the concentration of thiosulphate is increased then the rate of reaction increases. This is because in a higher concentrated thiosulphate solution there are more particles to collide with hydrochloric acid particles therefore more chances of reaction. This supports my prediction. My prediction was right that increasing the concentration of sodium thiosulphate will increase the reaction speed.
My results also show that if the concentration of hydrochloric acid is lowered the reaction does not slow down as much. This is because it was the sulphur, that is the precipitate, in the sodium thiosulphate which makes the solution turn milky. Therefore the more thiosulphate particles there are the more sulphur particles are produced and the quicker the solution turns milky. This partially supports my prediction. My prediction was that by increasing the concentration of hydrochloric acid it will also increase the reaction speed. It did, but not to the scale I expected. I expected it to be the same as the sodium thiosulphate.
As you can see in the hydrochloric acid graph you can see an anomalous result as labelled this means that you can not completely trust results on deciding which results are anomalous but other than that you can see that the result go in a diagonal line. on the other hand the graph about sodium thiosulphate had a curved line of best fit
Evaluation
The experiment worked because a pattern emerged. This meant that a conclusion could be made.
I think it went quite well but we did have our cross rubbed off once and had to draw it again. This meant the cross that we drew was not the same as the cross before. If it was a darker cross the second time we would have been able to see it for longer. This is a reason why we may have had anomalous results. Another reason could have been that the timing was not accurate. Bad eye sight or tired eyes could have played a part. The solutions may not have been measured up correctly. We also had an unknown liquid in a measuring cylinder when doing one of the experiments so it could have been contaminated. Besides the anomalous results, we still had sufficient evidence to support a firm conclusion.
In an ideal world the experiment could have been done so much better because we would have had unlimited resources and time to do more repeats. We would have clean equipment every time when we came into the room to prevent contamination. We could have had a machine to check when the cross had gone. We could have reduced human error and increase accuracy by using Burettes to measure the liquids rather than measuring cylinders. Burettes measure to the nearest 0.05cm3. As extra work we could see if there is a limit to how fast the rate of reaction can go.