Safety: Goggles must be worn at all times when taking part in the experiment so that the eyes are protected.
Keep hydrochloric acid away from skin and try to avoid any spillage because it is corrosive.
Sodium thiosulphate can trigger asthma attacks so asthma sufferers should not be in a lot of contact with sodium thiosulphate when there is no fresh air coming into the room.
Make sure that all long or loose hair is tied back so that they do not go into any of the chemicals.
Make sure that bags are kept under the table in order to avoid tripping over them.
Protective clothing must be worn at all times so that in case there are any spillages, the chemicals would not be in close contact to your skin.
Take care when using glassware to prevent injury.
Method: Using a measuring cylinder, measure 50cm3 of Na2SO4 (Sodium thiosulphate) into a burette. Slowly open the tap at the bottom and allow the Na2SO4 to pour out into the conical flask that you placed below the tap.
Also using a burette, add 5cm3 of 2HCl (Hydrochloric acid) into the conical flask that contains the sodium thiosulphate.
By using a burette, the measurement is more accurate.
Place the conical flask with the sodium thiosulphate onto the paper with a cross on it
Stir the solution with a stirring rod and start the stop clock.
With goggles on, look down through the solution at cross on paper from a bird’s eye view and when the cross is fully obscured, stop the stop the stop clock and rinse the flask out.
This chemical reaction has just taken place:
Na2SO4 (aq)+2HCl (aq) 2NaCl(aq) + H2O (l) + So2 (g) + S(s)
Record the time it took for the cross to be fully obscured reset the stop clock and rinse the flask out.
Repeat the same experiment with the same amount of concentration three times.
Using a clean flask, measure 40cm3 of Na2SO4 into a measuring cylinder then put it into a burette. Slowly open the tap at the bottom and allow the Na2SO4 to pour out.
Add 5cm3 of 2HCl to the conical flask. Make sure it is measured using a burette.
Measure 10cm3 of water to the solution and then place the conical flask onto the paper with the cross on and start the stop clock.
Complete the rest of the experiment by varying the concentrations.
The concentration of hydrochloric acid remains the same (5cm3), the concentration of sodium thiosulphate decreases by 10cm3 and the concentration of water increases by 10cm3. Record results for the six concentrations.
Fair test: For the experiment to be a fair test, all the other variables should be kept constant such as the concentration of hydrochloric acid and the temperature.
I am going to do all my experiments at room temperature, which is about 20 degrees Celsius.
Different apparatus will need to be used each time or it could be rinsed so that it is a fair test because residue from the previous test could alter the reaction in the next.
Also, look at the cross from the same view because different views could also alter the results by delaying the experiment (the time it takes for cross to disappear).
We must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.
We must also 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.
Results:
Analysis: The temperature does not really affect the rate of the reaction but high temperatures such as 60 degrees Celsius and very low temperatures such as –5 degrees Celsius could.
Timing inaccuracies could alter results, but repeating experiment three times and finding the average could avoid that, make the results more accurate, and improve the reliability.
Using a burette or a pipette to measure the concentrations instead of a measuring cylinder would make the results and measurements more accurate.
Using a reasonable range for the concentration such as 0-1.0 mol/cm3 can make the results more reliable and it would be easier to plot the graph as well.
Conclusion: My graph that shows how the concentration affects the reaction time is a bit curved. I drew a line of best fit and saw it has anomalous results. This graph agrees with my prediction because I predicted the higher the concentration, the faster the reaction.
My second graph that shows how the concentration affects the rate of the reaction as almost a straight line.
I drew a line of best fit and it has an anomalous result.
This disagrees with my prediction because as the concentration increases, so does the rate of the reaction while I predicted the higher the concentration, the faster the reaction.
These results also show that the rate of reaction is proportional to the concentration.
Some of the results were as predicted and some were not but the scientific theory backs them up.
The scientific theory about the reaction rates shows that before any reaction between sodium thiosulphate and hydrochloric acid can occur, the molecules must come together and collide.
In gases and liquids, particles are in constant motion.
Millions upon millions of collisions occur every second.
If there were a reaction every time molecules collided, all chemical reactions would be over in a fraction of a millionth of a second. Not all collisions are effective. Only a small fraction of them seems to be successful collisions.
It is not enough for Na2SO4 and 2HCl molecules to collide.
Bonds between the atoms must be broken before new molecules can be made.
Particles in liquid and gases are moving at different ways. Some collisions are ‘head on’, while others are ‘glancing collisions’.
The collisions between particles have a range of energies.
Head on collisions between fast moving particles are the most energetic. If the colliding molecules have enough energy, the collision is ‘successful’ and a reaction occurs.
In chemical reactions, if the activation energy is low, a high proportion of collisions will have more energy and so the reaction is fast. Reactions that have a high activation energy are slow at room temperature because only very small fractions of collisions have enough energy to overcome the activation energy. The ‘success rate’ of collisions is low.
Reaction rates depend on concentration.
In solutions of higher concentration and in gases at higher pressure, particles are closer together. They have a greater chance of colliding, because there are more collisions, the reaction rate is greater.
Evaluation: Most of our results were accurate because when I read the collision theory it told me exactly what my results told me. In addition, we carried out the experiment three times for each concentration and then worked out the average.
By doing this, I think that I have made my results and graph more accurate.
I also know my results are accurate because the graph followed the curve pattern. However, we did have a few anomalous results.
The results highlighted were the anomalous results.
I think there is also a human error factor involved when you are measuring liquids and looking for an end in the reaction.
When measuring liquids the person might have the concentration close to the volume they want it to be but not exactly.
Depending on what view you are looking at the reaction take place, you may not see that cross has been obscured earlier.
My method of carrying out the experiment was fairly accurate.
I made sure that I washed the beakers clean for the other two concentrations and the second experiment, I measured the same amount of solution in each time and I stopped the stop clock when I thought that the cross was fully obscured.
If I were to do the experiment again, I think I should have used fifteen different beakers for the five different concentrations so I could repeat the experiment for the five concentrations three times. By doing it this, I think it would be fair as there would not be any water or precipitate at the bottom.
I think my results were at a suitable range because I had a molar concentration between 0 to 0.15
Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a piece of paper with a cross on, a light beam could be used and when the beam goes out that would indicate the end.
If I had more time, I could do extra experiments to investigate another factor such as the affects of temperature on a rate of reaction.
Another thing we could have done to bring more evidence and improved the method is to have tried to use the hydrochloric acid as the variable substance, and used the sodium thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.
