Also, a trial experiment was carried out using sodium thiosulphate and HCl and it was proven that the temperature affected the rate of reaction. This trial also helped plan the method used for this investigation. It showed that as the temperature was increased, the time for a cross to disappear was shortened. This means when the temperature is increased the particles all move quicker. So, if they’re moving faster, they are going to have more speed to overcome the activation energy so they can have more collisions. This graph shows this:
There are some other factors that affect the rate of a chemical experiment. Below are all four of the important factors explained using the collision theory:
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Temperature increases the number of collisions. When the temperature is increased the particles all move quicker. If they are moving quicker, they are going to have more collisions.
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Concentration increases the number of collisions. If the solution is more concentrated it means that there are more particles of reactant between the solution molecules, which makes collisions between important particles more likely.
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Size of solid particles increases collisions. If one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area. This means the particles around it in the solution will have more area to work on so there will be more useful collisions.
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Catalyst increases the numbers of collisions. A catalyst works by giving the reactant particles a surface to stick to where they can bump into each other. This obviously increases the number of collisions.
In class an experiment was undertaken to see how the catalyst affects the rate of reaction. The decomposition of hydrogen peroxide was used. This reaction happens very slowly at room temperature unless we use a catalyst. 50 cm3 of hydrogen peroxide solution was added to a conical flask. Two spatulas of the catalyst manganese oxide was added to the flask and a gas syringe connected. A stop clock was started and the volume of oxygen collected recorded every 10 seconds. The syringe readings were entered in the table of results over the page.
A graph of Volume of oxygen against time was plotted:
This experiment proved that a catalyst increases the number of collisions.
Here are the results that will be taken over a certain range for this investigation:
5cm2 of sodium thiosulphate + 45cm2 of water
10cm2 of sodium thiosulphate + 40cm2 of water
15cm2 of sodium thiosulphate + 35cm2 of water
20cm2 of sodium thiosulphate + 30cm2 of water
25cm2 of sodium thiosulphate + 25cm2 of water
30cm2 of sodium thiosulphate + 20cm2 of water
35cm2 of sodium thiosulphate + 15cm2 of water
40cm2 of sodium thiosulphate + 10cm2 of water
45cm2 of sodium thiosulphate + 5cm2 of water
50cm2 of sodium thiosulphate + 0cm2 of water
As you can see 10 different concentrations will be used. This will be the range.
Each result will be repeated twice then repeated a third time if any results don’t fit the pattern and leave the odd result out of the average. Repeating these results will increase the reliability and help results to be more accurate and will eliminate any anomalous results.
References
EDEXCEL revision guide
Class notes
Class worksheets
Prediction
My prediction for this practical work is simply; the faster the reaction takes place, the shorter the time will be for the reaction to finish, and that if the concentration is doubled then the reaction time is halved. In scientific terms, the speed of the reaction is inversely proportional to the time taken for the reaction to finish,
Speed of reaction = 1/Time
You can see this from the diagram below, 1/Time = Concentration:
Method
For the experiment to be carried out the equipment will have to be set out as below:
- Draw a cross (using a HB pencil) onto a piece of paper,
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Measure out 5cm2 of Sodium thiosulphate from a burette and then pore into conical flask,
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Measure out 45cm2 of distilled water and place it into the conical flask,
- Next swirl once to the mix solution,
- Place the conical flask on the crossed paper and take the temperature,
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Then add 10 cm2 of hydrochloric acid,
- Again swirl the solution and start the stop clock,
- Now time how long it takes for the cross to be blocked from view.
After that is completed the experiment will be repeated before going onto the next concentration.
Goggles must be worn to stop liquid splashing into person’s eyes.
Apparatus
The equipment that will be used for the practical is below in the form of a list:
- Conical flask
- Stop clock
- Burette
- Measuring cylinders
- Piece of paper with a drawn cross in the middle of it
- HB pencil
- Ruler
- Thermometer
Fair test
To make sure that the practical is a fair test:
- Only the concentration of thiosulphate will be changed,
- The distance that the cross is viewed from will be kept the same,
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As will the total volume (50cm2),
- The same cross will be used,
- A HB pencil will be used to draw the cross,
- The same person will be viewing the cross and the same person taking the reaction time,
- The same concentration of HCl (2m) will be used though out the practical,
- The conical flask will be washed after each experiment with distilled water,
- The temperatures of the solutions will be the same when doing the practical,
- Lastly, the contents of the conical flask will only be swirled once.
Analysing evidence
Once the planning was complete the investigation was carried out. Here are the results:
The average time, rate of reaction and concentration had to be calculated using the results; here are their formulas:
Average Time – all three results added together and divided by 3. NB, if an anomalous result is present only the non-anomalous results are added and divided by how many results were added.
Rate of reaction – rate of reaction is worked out by dividing the average time by one.
Concentration – the concentration is determined by dividing the volume of sodium thiosulphate by 50. Then it is multiplied by 0.02.
The shaded boxes are anomalous results and are also on the following pages in line graphs, as circled crosses.
As the temperature of the solution affects the results the temperature of the room was taken down,
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Day one – 21oC
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Day two – 20oC
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Day three - 20oC
Conclusion
From the first graph we can see a number of clear patterns. Firstly it can be seen that as the concentration is increased the time for the precipitate to block the cross from view is decreased. Also, at high concentration the perpetuate appears in shorter time giving a faster reaction, so at low concentration the precipitate took longer time giving a low reaction. To be more precise, when the concentration of sodium thiosulphate in increased more thiosulphate ions are put into the solution, this making all the ions closer together do there is a better chance that they will collide with one another at the right angle and with enough energy to overcome the activation energy and break the ion’s bonds. So the relationship between the number of effective collisions and the rate of reaction is ‘more effective collisions per second, the faster the rate of reaction’ as the graph results showed by showing that when the concentration is halved and is a 0.006 moles per litre, the time taken to finished the reaction was 78.3 seconds.
In the second graph a mathematical relationship between rate and concentration is found. It shows that if the concentration is doubled the rate also doubles and that the concentration is proportional. By showing at double the concentration of 0.012 moles per litre, the time taken to finish the reaction was 34.3 seconds (almost half) and the rate directly proportional to concentration as seen by the fact the graph is a straight-line graph.
So as predicted in the plan section, the higher the concentration the higher the rate, and that of the concentration is doubled then the time taken is halved.
Although some results did not fit the pattern and were not in line.
Some results did not turn out as expected and for the reasons that the reaction time for the STOP/START clock varied, the conical flask may have been cloudy which could of effected judgement, the measuring volume could have been inaccurate as could of the swirling, the times and the height and angle that the reaction was viewed from. Also the room temperature changed on different days.
Evaluation
The method that was used when the experiment was carried out was good because it was not that difficult in obtaining exact results. But more accurate results could have been reordered because:
- The reaction time for the STOP/START clock varied,
- The conical flask may have been cloudy which could of effected judgement,
- The measuring volume could have been inaccurate,
- The amount of swirling may have changed,
- The times and the height and angle that the reaction was viewed from,
- Also the room temperature changed on different days.
So if the experiment were to be repeated the above details would be improved by:
- A computer clock program could be used to take the time to make the results more accurate by using a light and a LDR sensor to record when the cross is obscured from view,
- The apparatus could be washed out using distilled and water,
- A burette could be used for all of the solutions to make the measurements more accurate,
- A magnetic stirrer would keep the solution swirling at a constant speed,
- A heating mantle may be used to keep the temperature of the solution the same.
These more precise pieces of equipment would overcome a persons subjective judgement and to give consistent conditions. Repeating the results for a 4th time could help to improve the reliability and also eliminate anomalous results.
There are some other experiments and techniques that could be used to improve of extend the coursework:
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To make your prediction more accurate the results could be taken over a wider range by increasing the concentration by smaller variations e.g. every 2cm2,
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A different experiment could be carried out to extend the work where a different acid would be used e.g. HNO3 – nitric acid,
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Also, to extend the coursework, instead of changing the concentration of the sodium thiosulphate the acids concentration would be changed.