Prediction
I predict that the concentration of Hydrochloric Acid will affect the rate of reaction. I think this because the less Hydrochloric Acid particles there are, the less interaction with the Sodium Thiosulphate particles there will be, this means that the reaction time will be longer the lower the concentration of Hydrochloric Acid.
Equipment
60ml of 2 Molar Hydrochloric Acid
150ml of 0.25 Molar Sodium Thiosulphate Solution
150ml of Water (H2O)
3 x’s 150ml Beakers (large beakers)
6 x’s 50ml Beakers (small beakers)
2 Measuring Cylinders
1 Piece of A5 Paper
1 Black Marker Pen
1 Stop Watch
2 Pipettes
1 China graph Pencil
1 Pair of Safety Glasses
Diagram
Description of the Experiment
When Hydrochloric Acid and Sodium Thiosulphate are reacted together a sulphur precipitate forms. Making the solution cloudy. If therefore a black cross is placed beneath the firstly clear solution the rate of reaction can be timed by how quickly the cross visually disappears.
Sodium Thiosulphate + Hydrochloric acid -> Sodium chloride + Sulphur + Sulphur Dioxide + Water
Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + SO2(g) + H2O(l)
Method: (A preliminary experiment was undertaken to determine the approximate measures to be used)
- Measure 60 ml of 2M Hydrochloric Acid into a large beaker.
- Measure 150ml of 0.25M Sodium Thiosulphate Solution into a large beaker.
- Measure 150ml of tap water into a large beaker.
- Label them with a china graph pencil.
Preparing The Concentrations Of Acid
- Separate the 60ml of 2M Hydrochloric Acid into 2 small beakers so that they contain 30 ml each. Label one beaker as 2M Hydrochloric Acid with china graph pencil and leave one blank.
- Set the labelled beaker aside.
- Add 30ml of water to the unlabelled beaker, this will make 60ml of 1M Hydrochloric Acid.
- Pour 30ml of the 1M acid into a small beaker. Label one of them 1 acid and leave one of them blank.
- Set the labelled beaker aside.
- Add 30ml of water to the unlabelled beaker, this will make 60ml of 0.5M Hydrochloric Acid.
- Pour 30ml of the 0.5M acid into a small beaker. Label one of them as 0.5 acid and leave one of them blank.
- Set the labelled beaker aside.
- Add 30ml of water to the unlabelled beaker, this will make 60ml of 0.25M Hydrochloric Acid.
- Pour 30ml of the 0.25M acid into a small beaker. Label one of them as 0.25M acid and leave on of them blank.
- Set the labelled beaker aside.
- Add 30ml of water to the unlabelled beaker, this will make 60 ml of 0.125M Hydrochloric Acid.
- Pour 30ml of the 0.125M acid into a small beaker. Label one of them as 0.125M acid and discard the excess.
Reacting Hydrochloric Acid and Sodium Thiosulphate Solution
- Set aside are 5 concentrations of Hydrochloric Acid. (2M, 1M, 0.5M, 0.25M and 0.125M) Take the 2M acid and measure with the pipette 10mls of the 30mls into a small beaker. Label it beaker X.
- Draw a cross with lengths equal to the diameter of the Beaker X on the piece of paper with the marker pen.
- Place Beaker X on top of the cross.
- Measure with the second pipette 10mls of Sodium Thiosulphate Solution.
- Add this to Beaker X containing the Hydrochloric Acid.
- Start the stopwatch as soon as the two substances come into contact.
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Observe Beaker X. When the cross is no longer visible, stop the stopwatch. Record the result in this table…
- Repeat this experiment twice more with the other 20mls of 2M acid. Taking multiple results will mean you can take an average, which overall is more accurate.
- After completing the 2M acid, do exactly the same experiment but this time with the 1M acid, 0.5M acid etc… However the same cross must be used to ensure fair testing.
Fair Testing (controls and variables)
Variable
- The concentration of Hydrochloric Acid
Controls
- The temperature
- Volume of Hydrochloric Acid
- Volume of Sodium Thiosulphate Solution
- Concentration of Sodium Thiosulphate Solution
- The amount of agitation.
- Keep the same equipment throughout including the same cross.
The temperature must stay the same because the more heat energy the reaction has the more kinetic energy the particles have and therefore they collide faster and more frequently speeding up the reaction. If the temperature varies so will the results and it will not be known how much the concentration of the hydrochloric acid was affecting the experiment.
The volume of the Sodium Thiosulphate and Hydrochloric acid must stay the same because this could also affect the rate of reaction.
The concentration of the Sodium Thiosulphate must be kept constant because again more than one variable will mean that it will not be known how much the concentration of hydrochloric acid was affecting the experiment.
Agitation will cause the particles to move around quicker and there for collide more frequently speeding up the rate of reaction.
The same equipment must be kept the same because for example if Beaker X is of a different size then the depth of the mixture will vary meaning the cross will disappear either quicker or slower. If the pipettes are mixed up, each substance could be polluted with the other, meaning that the reaction has already started. If this happens it will affect the results. This is why all the equipment used must be clean and not contaminated.
Results
Graph of Averages to show how the time of the reaction varied according to the concentration of Hydrochloric Acid.
Anomalous Results
The graph shows a line of best fit. The line goes through two points on the graph and balances itself between the other results, which do not fit on the line. I would have expected the results to be closer to the line if not on it, than they actually are. However these anomalous results can be explained.
It is possible that human error played a significant role. The concentrations may not have been made as accurately as possible.
Maybe the beakers or pipettes were contaminated and not rinsed out properly.
Conclusion
The experimental results show the prediction to be correct. The graph shows the higher the concentration of the Hydrochloric Acid the quicker the rate of reaction. My theory on the rate of reactions being affected by the concentration of the acid proves to be true. This is backed up by various books and websites such as the NEAB Chemistry Science Revision Guide of 1998 onwards that states: “Increased concentration means an increased number of particles resulting in an increased number of collisions and an increased rate of reaction.” This shows even though my results were not very accurate my prediction and theory behind the experiment was correct.
Evaluation
My experiment was not as successful as it could have been. If the experiment were carried out again I would make sure that all measurements were checked carefully, that all equipment was clean and try to avoid human error as much as possible. Also a factor worth considering would be taking more results using a more exact range between 2 and 0.1 Molar Acid. Also I would take more than 3 multiple results making the chance of inaccuracy minuter. It would also be interesting to see how the results differ if the concentration of Sodium Thiosulphate was varied.
