Output variable
Time taken to complete reaction
I will measure the amount of time taken for the reaction to finish using a stop watch. To find the rate from the time simply inverse the time value (i.e. 1/time)
Prediction
I predict as I increase the concentration of sodium thiosulphate you increase the number of reactant molecules per unit volume, increasing the chance of particles colliding successfully and in turn increasing the frequency of collisions thus increasing the rate of reaction. When you double the concentration, you double the chance of a collision and double the frequency thus doubling the rate. The rate doubles when the concentration of the reactants doubles, thus concentration is proportional to rate of reaction.
This shows as you double the concentration of the thio, you double the chance of a collision and therefore doubles the frequency, which in turn doubles the rate. This shows concentration is proportional to rate of reaction.
Apparatus
Hydrochloric acid- reactant in this experiment
Sodium thiosulphate solution- reactant in this experiment
100 ml beaker - To mix the reactants in, 100 ml is chosen as it gives suitable stirrimg space and is the closest size beaker to take the sodium thiosulphate and the HCL as these add to 60 ml.
Two 50 ml measuring cylinders - To pour the water into one, and the sodium thiosulphate into the other. 50ml measuring cylinders are chosen as the sodium thiosulphate will be diluted with water, to change the concentrations, and combine to make 50 ml.
10 ml measuring cylinder - To pour the HCL into. 10 ml is used as it is the most practical size for 10 ml of HCL.
Stopwatch - to measure time taken for the print to be obscured and the reaction to end.
Printed paper - to place the beaker of reactants on and to judge when reaction has finished.
Safety
Show recognition of the safety hazards, for example the HCL is corrosive so handle it with care and when you are not using it place it in the centre of the bench. Tuck ties in as they may dangle into the chemicals and catch on the equipment. Wear safety glasses to protect your eyes from the sulphur dioxide that is given off when the two reactants react. Stand up during the entire experiment so you can easily move out of the way of spillage.
Preliminary Method
This preliminary experiment is designed to find a suitable range of readings and to find any areas of error or difficulty so when the final experiment takes place these problems will have been corrected.
- Take one measuring beaker and pour 50 ml of sodium thiosulphate into it, then pour it into the 100 ml beaker.
- Pour 10 ml of HCL into the measuring cylinder then into the 100 ml beaker.
- Start the stopwatch as soon as you begin pouring the HCL into the beaker.
- Place the beaker upon the printed paper and stop the watch as soon as you can no longer read the print through the reactants.
- Do steps 1-4 but using one measuring cylinder for the sodium thiosulphate and fill it with 5 ml of the reactant. Fill the other measuring cylinder with 45 ml of water.
Preliminary Results
Use of preliminary results to finalise method
- As my results demonstrate, there is a large difference of times between the concentrations of 50 and 10%, showing that there is not enough 'thio molecules to react with all the HCL molecules thus not creating enough of a precipitate to block the print. Therefore in the final method I will do a smallest concentration of 20%.
- I have not done check readings to insure accuracy, so in the final method I will use check readings.
- In the final method I will not give the precise times as the times are subject to human error so I will round the times to the nearest second.
- For precision and accuracy I will measure out all volumes of liquids in measuring cylinders first.
- I will use different measuring cylinders for each liquid as not to contaminate the liquids and begin the reaction too early.
- I will need to take a greater range of readings to get a suitable number of readings to produce a graph line to show the proportionality and to reach a firm conclusion.
Final Method
1) Take one measuring beaker and designate this the sodium thiosulphate measuring cylinder.
2) Pour sodium thiosulphate into the measuring cylinder until it reaches the 50 ml mark.
3) Pour the HCL into the 10 ml measuring cylinder up to the 10 ml mark.
4) Place the 100 ml beaker upon some printed text.
- Pour the HCL and the sodium thiosulphate into the beaker and start timing immediately.
- Observe the beaker constantly and end timing as soon as the text cannot be read.
- Record the time taken for the reaction to complete.
- Wash out the 100 ml beaker as not to contaminate the next reaction.
- This time pour 45 ml of sodium thiosulphate into the designated measuring cylinder.
- Use the other measuring cylinder as the water one and pour 5 ml into it.
- Pour the contents of measuring cylinders into the beaker.
- Pour the 10 ml of HCL into the beaker and begin the timing.
- Use the same print as before to keep a fair test as differing prints could be lighter or darker.
- Repeat steps 9-15 but with concentrations 40 ml of thio and 10 ml of water, 35 ml of thio and 15 ml of water etc. Continue changing the volume of thio by 5 ml smaller each time untill you reach a ratio of 10ml thio and 40 ml of water.
- Do two check readings of each concentration.
Range and Reliabilty
I have chosen to do a range of 8 different concentrations between 100 and 20% sodium thiosulphate as to get the widest set of results, as to make a firm conclusion. I have done two check readings to check reliablity of results and to identify any anomalous results.
Precision and accuracy
I chose to investigate the concentration of sodium thiosulphate as we had greater volumes of this reactant than the Hydrochloric acid. We can use up to 50 ml of thio and when measuring this volume in a 50 ml measuring cylinder we can measure it accurately to a ml which is a margin of error of 2%. When using the HCl ac the changing concentration we can only use up 10 ml. When measuring this out in a 10 ml measuring cylinder you read it accurately to half a ml, or a 5% margin of error. This shows that measuring 50 ml of thio is more accurate than measuring 10 ml of HCl.
Results
I have identified one anomalous result and it has been circled. I did not use this in the average for that concentration. The times for the reaction to complete increase as the concentration is increased, the times increasingly increase producing the curve, The rate increases uniformly with a difference of 5 or 6 between each concentration showing a proportional trend.
The graph of time against concentration shows a positive correlation that has a trend of a curve. As the concentration increases the time increases.
The graph of rate against concentration had a straight-line trend showing proportionality. However the graph line did not go through the origin as predicted, because I did not take concentrations of 0 to 15% of thio.
I have drawn a sketch of the graph obtained. In my prediction I stated that concentration would be proportionate to rate of reaction in the form x α y or 2x = 2y
I am going to demonstrate this : -
40% = 12.5 so if I were to take double the concentration I should produce double the rate.
80% should equal 12.5 x 2.
80% = 25
My results were 80% = 27
25 ≅ 27
My prediction was correct.
I am going to check this by doing one more calculation: -
20% = 6 so if I were to double the concentration I should double the rate.
40% should equal 6 x 2.
40% = 12
My results were 40% = 12.5
12 ≅ 12.5
As I predict as I increase the concentration of sodium thiosulphate you increase the number of reactant molecules per unit volume, increasing the chance of particles colliding successfully and in turn increasing the frequency of collisions thus increasing the rate of reaction. When you double the concentration, you double the chance of a collision and double the frequency thus doubling the rate. The rate doubles when the concentration of the reactants doubles, thus concentration is proportional to rate of reaction.
Therefore I believe I have I have sufficient evidence to support my prediction due to my processed evidence which satisfies my hypothesis, also my scientific description of how concentration is proportional to rate of reaction supports my prediction.
Evaluation
The results obtained showed a steady pattern and the apparatus was simple to set up. All my points were close to the best fit line, this was due to the precision of the measuring cylinders used thus creating volumes to 2% desired. Also I maid sure that none of the measuring cylinders were contaminated by using separate cylinders for the water and sodium thiosulphate. Also by maintaining the other variables constant all my points were close to the best-fit line.
I chose to investigate the concentration of sodium thiosulphate as we had greater volumes of this reactant than the Hydrochloric acid. We can use up to 50 ml of thio and when measuring this volume in a 50 ml measuring cylinder we can measure it accurately to a ml which is a margin of error of 2%. When using the HCl ac the changing concentration we can only use up 10 ml. When measuring this out in a 10 ml measuring cylinder you read it accurately to half a ml, or a 5% margin of error. This shows that measuring 50 ml of thio is more accurate than measuring 10 ml of HCl.
I had no anomalous results, as all my points lied close to the best-fit line. This was due to me keeping all the variables constant, and timing the length of reaction precisely. Also by measuring the volumes within the smallest suitable measuring cylinders I therefore had the smallest possible error, being able to measure accurately to the nearest 2% of the volume. All these factors helped me to obtain no anomalous result.
Criticisms and Limitations
I used Sodium thiosulphate from a different bottle, the solution was not allowed to cool, due to it having an endothermic reaction with the water.
- Use Sodium Thiosulphate from the same bottle or allow cooling to the same temperature as the original bottle temperature.
I used a different printed sheet when doing check readings, the sheets had a different shade of black, one lighter than the other. This effected the judgement for the end of the reaction, obviously the beaker under the lighter print would be considered to have finished the reaction earlier than the one with darker print.
- Use the same printed paper to use to judge when the reaction is over.
It was difficult to be consistent when judging when the reaction was over.
- Using a complex laser beam to direct through the solution could consistently judge when the reaction is over.
I feel the that evidence is not enough to support a firm conclusion, as although the results were as predicted, to reach a firm conclusion I would have to investigate reactions between different reactants such as Magnesium and HCL and collect the volume hydrogen given off. Also to reach a firm conclusion, I would investigate concentrations smaller than 20%, to prove proportionality at 10% concentration and below. I would also study reactions that took considerably longer and still see if rate is proportional to concentration.
Further work would be to investigate the other variables: -
Temperature
I could use the same reactants as used for this experiment, but maintaining a concentration of 40% sodium thiosulphate. I would investigate temperatures between 0° and 100°. I would investigate how temperature effected the rate of reaction.
Catalysts
I would investigate which transitional metals lowered the activation energy of a successful collision the most. One experiment I could do would be to leave hydrogen peroxide in a conical flask with a collecting syringe to collect the oxygen, and leave it to decompose naturally. This would be the control. I would then set up another flask and collecting syringe but with a gram of manganese 3 oxide and see how the transitional compound effected the rate of decay. I would then try different catalysts such as platinum wire and see which is the better catalysts and what properties are needed for a good catalyst.
Surface Area
I would also investigate different surface areas of reactants, or different pressures of gases and see how it effected rate of reaction. For this experiment I would react calcium carbonate with HCl. I would place the chips in a conical flask with a division. I would attach this flask to collecting syringe. I would start the experiment by shaking the flask and allowing the two reactants to mix. I would study the rate of which the hydrogen is given off. I would the repeat this experiment using the same volume of calcium carbonate but this time use a powder form and study if it gave off hydrogen faster than the chips.
