The five main factors that increase the rate of reaction are:
- Temperature
- Concentration
- Surface Area
- Catalyst
- Pressure
I intend to investigate how changing the concentration of Sodium Thiosulphate has an effect on the rate of reaction.
Key factors:
- I’m going to use no sodium thiosulphate in one reaction as a control to see if the acid is reacting with the sodium thiosulphate and how it affects the rate.
- The temperature will remain constant at room temperature as an increase in temperature can increase the rate of reaction as it speeds up the movement of the particles, increasing the amount of collisions.
- I’m only changing the concentration to keep the experiment fair, as other things such as pressure, surface area and using a catalyst increases the rate of reaction.
Prediction:
I predict that the solutions with the higher concentration of Sodium Thiosulphate will have a faster rate of reaction then those with a lower concentration of Sodium Thiosulphate. The reason for this is that if the solution is more concentrated there is a higher probability of collisions between the reactant particles as there are more reactant particles present in the same volume. These frequent collisions will then increase the rate of the product formation. Therefore the solutions that contain 50ml, 40ml and 30ml of Sodium Thiosulphate will react with the Hydrochloric acid, faster then the solutions with 20ml and 10ml of Sodium Thiosulphate. I also predict the solution, which has no Sodium Thiosulphate present will not react at all.
The equipment that I intend to use will be accurate. The measuring cylinders have very precise markings of the volume in ml³; I will the use the same measuring cylinder for each substance, i.e. one for the hydrochloric Acid, one for the Sodium Thiosulphate and another for the water. However I will rinse each measuring cylinder and make sure it is dry for the next experiment, because if a bit of the substances are left in the cylinders it can add to the rate of reaction and lead to anomalous results. I will place the measuring cylinders on a flat surface when measuring out the substance and carefully make sure that the meniscus lies on the measuring line.
I will also use the same, conical flask for each experiment so that the solutions have the same area and volume to react in. The conical flask will also be rinsed and dried out for each experiment. I am going to observe the experiment throughout myself, because if other people observe it as well, then the results may become anomalous as the observation of two different people will not always be exactly the same.
Preliminary Work
For my preliminary work I decided which experiment would be the best to investigate.
I had the option of:
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Measuring the mass loss, as a gas is produced, using a balance.
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Measuring the maximum volume of gas and the time at which the reaction stops using a gas syringe.
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Measure how long it takes for a certain amount of sulphur precipitate to form, by observing the reaction through a conical flask until the cross from underneath isn’t viewable.
I decided not to measure the mass loss of gas produced because the experiment can become inaccurate if a low mass gas is produced, e.g. Hydrogen gas. This would make the results harder to plot. I had however intended to measure the maximum volume of gas produced using a gas syringe, but due to limited equipment, I was unable to continue with the experiment. I then decided to time the rate at which the Sulphur precipitate was formed because it would give me accurate results. Also because I’m not mixing a solid and a liquid I would not have to stir the solution. This will allow more accurate results as stirring is a factor, and if one solution is stirred more then the other, inaccuracies in the results would follow.
Obtaining Evidence
Table of Results _
First Set of Results
Experiment Hydrochloric Acid Sodium Thiosulphate Water Time (ml³) (ml³) (ml³) (s)
- 5 50 0 37
- 5 40 10 45
- 5 30 20 62
- 5 20 30 109
- 5 10 40 260
- 5 0 50 No Effect
Repeated Experiment
Experiment Hydrochloric Acid Sodium Thiosulphate Water Time (ml³) (ml³) (ml³) (s)
- 5 50 0 38
- 5 40 10 49
- 5 30 20 75
- 5 20 30 115
- 5 10 40 260
- 5 0 50 No Effect
Averages of Both Experiments
Experiment Hydrochloric Acid Sodium Thiosulphate Water Time (ml³) (ml³) (ml³) (s)
- 5 50 0 37.5
- 5 40 10 47
- 5 30 20 68.5
- 5 20 30 112
- 5 10 40 280
- 5 0 50 No Effect
Analysing and considering Evidence.
After looking at the results, I have found out that the more the concentration of Sodium Thiosulphate the less time it takes to form the Sulphur precipitate.
I have produced two graphs, one showing both the experiments and the other showing the averages of the experiment. The graphs I have produced are curves with negative gradients. They have been plotted against the percentage concentration of Sodium Thiosulphate. The trend they follow also shows that, as the percentage concentration of Sodium Thiosulphate increases the rate of reaction (the time it takes for the Sulphur precipitate to form) decreases.
The gradients of the graphs are negative because it shows that as the concentration of Sodium Thiosulphate increases, there are more collisions occurring so the reaction is taking less time to form the precipitate of Sulphur. So because the graph starts with a low percentage concentration of Sodium Thiosulphate, the rate is very high and as the percentage Sodium Thiosulphate increase, the rate of the reaction decreases.
The graphs and results tables make it clearer to see the trend the results and my prediction both follow. My results show that my prediction was correct because, as I had predicted the more concentrated solutions of Sodium Thiosulphate, such as the 60%, 80% and 100% took less time to form the Sulphur precipitate, then the solutions 20% and 40% which had a more diluted Sodium Thiosulphate. This is due to the increased amount of collisions in the more concentrated solutions.
Evaluating
My experiment was successful because I took great precautions when undertaking the experiment. I used accurate equipment, and observed the reaction throughout so that the results had been recorded under the conditions of the same observation.
The method I used was suitable as it goes through the experiment systematically. Nevertheless, there is always room for improvements; such as I could have used a digital stopwatch to time the reaction as a digital stopwatch measures the time in seconds up to two digital places (0.00 seconds) This would automatically improve the accuracy of the results. I could also use another person to stop the stopwatch when the reaction comes to its end as a second could be added in stopping the stopwatch.
Although there is room for minor adjustments, the results seem accurate and reliable because they follow the same pattern. Thus I believe they support my prediction and collusion as both show that the rate of reaction decreases as the concentration of Sodium Thiosulphate increases. Also they both show that the Sodium Thiosulphate reacts with the Hydrochloric Acid to form the reaction and produced the Sulphur precipitate, as when there was none present the reaction had no effect. The following reaction produces the Sulphur precipitate,
Sodium + Hydrochloric Sodium + Water + Sulphur + Sulphur Thiosulphate Acid Chloride Dioxide
Na S O (aq) + 2HCl (aq) 2NaCl (aq) + H O (l) + S (s) + SO (g)