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For our investigation we will be looking at how the temperature affects the rate of reaction between sodium thiosulphate and an acid. To make sure the experiment is a fair test we must first see which other factors may effect the investigation and how we can control them. The temperature, concentration of the two solutions, the light intensity, the depth of the solution and the person judging whether they can still see the cross are all factors which may affect the rate of the reaction. Description of reaction:
When sodium thiusulphate are added together they react as shown below: HCL + sodium thiosulphate sodium chloride + sulphur dioxide + sulphur + water. HCL(aq) + Na2S2O3(aq) NaCl(aq) + SO2(g) + S(s) + H2O(l) The sulphur produced is held in suspension turning what was a colourless solution into a clouded one. This is what causes the cross to disappear. When we are measuring the time for the cross to disappear we are also measuring the time for a set amount of sulphur to be produced. The rate: The rate of reaction is the speed at which the reaction takes place.
Speed are measured as distance over time such as metres a second (m/s) and miles per hour (mph). A speed could also be the speed at which someone works out at, say a factory worker makes 5 footballs per hour so there work speed would be number of footballs over hours so in this case 5 footballs/hour. The speed (rate) at which a reaction happens at is written in the same way but in stead of a distance over time or footballs over time it is the point of the reaction you are measuring upto over time. The point you measure upto in this experiment isTheoretical background and prediction:
Since starting chemistry in year 7 we have performed and been shown experiment and demonstrations which involve reactions. The speed and how vigorously these reactions take place have been shown to vary greatly. The reaction between a strong acid and a strong alkali can happen in a few seconds and they react together extremely vigorously while the rusting (oxidising) of a metal statue can take years. Reactions occur as described in the collision theory when two molecules collide (intermolecular collision); this is shown in the diagram below:
In our experiment we will be measuring the rate of the reaction by measuring the time taken for the reaction between the hydrochloric acid and the sodium thiosulphate to reach a set point (when we could no longer see the cross). The time taken to reach this point could be changed by increasing or decreasing the number of reactions, which took place every second. This could be done in two ways: a) Increasing the number of molecules of the two reactants (increasing concentration) in a given volume of the solution b) Increasing the speed of the molecules by increasing the temperature of the reactant.
Both of these increase the chance of collisions occurring. Kinetic theory tells us that the higher the temperature then the more energy the molecules have and the faster they move around. A basic rule followed by most chemists is that for every 10 Kelvin’s the temperature is raised through the rate of reaction is doubled therefore for each 1 Kelvin raised the rate of reaction increases by about 10 percent. However experiments using the kinetic theory show however that the increase in total number of intermolecular collisions is only about 2 per cent for each 1 Kelvin rise in temperature.
Only a certain proportion of collisions actually produce a reaction. This proportion rises more rapidly with increase in temperature than the total number of collisions. It was first suggested in 1889 by a man named Arrhenius that a molecule would only react on collision if it had higher than the average energy i. e. a necessary amount of energy is required for the reaction to occur. If they do not have this minimum amount of energy they will just bounce off each other and no reaction will occur. The minimum amount of energy required for the reaction to take place is called the activation energy (Eact).
Light intensity will affect how long it takes before the person judging cannot see the cross anymore. As if there is a higher light intensity it will be a lot easier to see the cross. Also the person who is judging whether or not they can see the cross makes a difference as we all have different eyesight’s and judgement. If two different beakers are used and both filled with the same volume of water then if one has a small surface area then there will be a greater depth of solution. The greater the depth then the quicker the time taken before the cross can no longer be seen as the light would have to pass through a greater amount of sulphur.
I predict that the higher the temperature is the quicker the time will be for the reaction to take place and that the higher the temperature the higher the rate of reaction. I predict this because as I have stated above that the more energy the molecules are given by heating them the faster they move thus more collisions occur with a greater force. This results in more reactions taking place in a shorter space of time. If more reactions are taking place in a shorter space of time as the temperature increases then more sulphur will be produced per second as you raise the Temperature.
This means that the more you raise the temperature the quicker the amount of sulphur required for the person judging the experiment to be unable to see the cross. When two liquids of different temperatures are mixed together the solution produced will have a new resultant temperature. The formula used to tell us what this resultant temperature will be is: V1T 1 + V2T2 V1 + V2 T3 =Resultant Temperature This formula can be rearranged so that the temperature the sodium thiosulphate needs to be heated to can be worked out so that the desired resultant temperature is achieved when the HCL is added which is at room temperature.
The formula is rearranged to: T3 (V1 + V2) – V2T2 V1 Throughout the experiment T1 and T3 are the only parts of the experiment, which will be changed, all the others will remain constant. V1 and T 1 are the volume and temperature of the water and sodium thiosulphate solution, the volume will be 50 ml for the whole experiment and the temperature will vary. V2 and T2 are the volume and temperature of the HCL acid, the volume will be 5 ml throughout and the temperature will be room temperature (around 20 degrees Celsius).
T3 is the temperature of the sodium thiosulphate and water solution after the HCL has been added and is the temperature, at which the reaction takes place, the temperature will either be 20, 30, 40, 50, 60 or 70 degrees Celsius. T3 ( C) T1 ( C) 20 20 30 31 40 42 50 53 60 64 70 75 The table above shows the temperatures the sodium thiosulphate and water solution will need to be at for the desired resultant temperature to be achieved when the HCL is added. Trial experiments: Before we could start the experiment we needed to run some trial experiments to see if our method was correct and to decide what concentration to use.
Previously we had performed a similar experiment to see how concentration affected the rate of reaction. This gave us a starting concentration of around 40 ml of distilled water to 10 ml of Sodium thiosulphate. This concentration proved to be slightly to fast when doing it at 70 degrees celcius. So we tried using 42 ml of distilled water to 8 ml of sodium Thiosulphate. This concentration was fine. Water (ml) Thiosulphate (ml) Concentration (g/dm) Time at 20 C (s)
Time at 70 C (s) 40 10 8 42 8 6. 4 We will use 42 ml of water and 8ml of Sodium thiosulphate which gives us a concentration of 6.4 g/dm for our experiment as if we use a concentration which is any higher it will react to quickly for us to record accurately when the experiment is done at 70 degrees celcius. If the concentration was any lower then it takes too long when the temperature is at 20 degrees Celsius. Aparatatus: Apparatus:
2 x 150ml Beakers. 1 x 100ml measuring cylinder, 1 x 25ml measuring cylinder, 1x 10 ml measuring cylinder. 1 x stopwatch 1 x alcohol thermometer (1 degree Celsius graduations) 1 x Bunsen Burner 5 x piece of paper with a Large cross computer printed on it 1 x Tripod 1 x Gauze 96 ml of Sodium Thiosulphate 60ml of Acid 504 ml of distilled water 1 x Petra dish Method:
1. Measure out 8ml of 40g/dm Sodium Thiosulphate into a beaker using a 10ml measuring cylinder. Measure out 5ml of Acid into a beaker using a 10ml measuring cylinder and Measure out 42ml of distilled water into a beaker using a 100ml measuring cylinder. 2. Pour the sodium disulphate into the same beaker as the distilled water. 3. Heat the solution to 30 degrees Celsius using a Bunsen burner Place a pertri dish on top of the beaker to stop the solution evaporating out of the beaker.
4. Place the beaker containing the Sodium Thiosulphate on top of the cross, which should be in a plastic Esselte. 5. Pour the acid into the beaker containing the Sodium Thiosulphate and start the stopwatch. 6. When you judge that you can no longer see the cross stop the stopwatch and record the time taken for this to happen. 7. The temperature is taken at the end of the experiment. 8. Clean out the beakers and repeat the experiment again varying the temperature to 20, 40, 50, 60 and 70 degrees Celsius. Do every experiment twice. Fair test:
All variables except temperature will be kept constant so that the experiment will be a fair test. The concentration will be kept the same by using acid and Sodium Thiosulphate from the same batch each time. The sodium thiosulphate and water solution will be mixed in bulk so that the concentration will remain the same through out even if an error is made on the part of the person creating the solution. The same person will be judging when the cross cannot be seen anymore as different people have different eyesights. The light intensity will be kept constant by only using natural sunlight and keeping classroom lights off.
The same cross will be used each time and will be printed by computer so if something happens to the original there is an exact duplicate of it to take its place. The same Beaker will be used every time so that the person doing the experiment will have to look through the same depth of solution each time. Each experiment will be repeated to avoid anomalous results. If the repeat experiment isn’t within a reasonable range of the first experiment then it will be repeated once more. The temperature will be taken at the beginning and end of the experiment to make sure that the temperature didn’t drop too significantly throughout the experiment.
The stopwatch will be started as soon as the first drop of acid touches the sodium thiosulphate every experiment. Safety: Goggles will be worn at all times during the experiment, as safety is paramount. As acids are being handled there is the off chance that some could well be splashed into the experimenters eyes which is goggles are warn will reduce the risk are any making contact with the eye itself. As the acid is very weak and watered down lab coats are not compulsory but if any if spilt on ones hand or other areas of bare skin it should be washed immediately.
Once we have completed our experiment we will wash our hands as we will have been handling acids which are corrosive so we will need to wash any which has managed to get onto our skin off. Treatment of results: When the results have been collected they will be put in a table of results showing both the recorded times taken for each experiment and the average of these two. Anomalous results will be highlighted in the table and will not have been taken into account when the graphs are drawn.
The results will be used to produce two graphs, the first of which will have temperature plotted against time and the Second will have temperature plotted against one over time taken, which is the same as the rate of reaction. A line of best fit will be drawn for both graphs. Method: The apparatus was set-up as shown in the diagram. 8 ml of 8g/dm sodium thiosulphate was measured into a 10 ml measuring cylinder and 42 ml of distilled water was measured out into a 100 ml measuring cylinder. Both the 8 ml of sodium thiosulphate and the 42 ml of distilled water were poured into a 250 ml beaker together.
A thermometer was then put in the beaker and a petri was placed over the top of the beaker to stop the solution evaporating when it was heated up. The beaker was then placed on top of a tripod and gauze and heated using a Bunsen burner. Heating was stopped just before the temperature reached 30 degrees Celsius, as the temperature would keep on rising for a short time after the Bunsen burner was taken away. The Beaker was placed on top of a piece of paper with a black cross-drawn on it. When the temperature cooled down to 31 degrees Celsius the HCL was added as the fact that the HCL would lower the temperature had to be taken into account.
As soon as the HCL was added the stopwatch was started. The stopwatch was stopped as soon as the person judging could no longer see the cross. The time taken was recorded and the apparatus was clean thoroughly. The same experiment was then repeated again a second time and at 40 C, 50 C, 60 C and 70 C, it was repeated for these temperatures a second time as well. The experiment was also done at room temperature so no heating was therefore involved. Results: Temperature ( C ) Time 1 (secs) Time 2 (secs) Average Time (secs) Rate (1/time)
Room temperature had changed by 1 C when the experiment was repeated for room temperature so both results have been shown and will both be plotted on the graph at there respective time and temperatures. Calculating the rates: 17 C: Time taken for cross to disappear = 307 seconds Rate of 30 C: Average Time taken for cross to disappear.
Anomalous results: The graphs showed that there were no anomalous results as it produced a smooth curve. As I used room temperature which subsequently went up a degree from 19 degrees Celsius to 20 degrees Celsius is was unable to repeat it at 19 degrees as I had no way of cooling the solution down. As these results were not done a second time we cannot be sure they are not anomalous but they appeared to be fine and were included in the graph. All the results were within a reasonable range of each other. The only results, which appeared dubious, were the pair for 70 degrees Celsius as there was such a big difference between the two.
We would have done the experiment for a third time except we had run out of time. If we were to do the experiment again I would obtain a third set of result for every temperature to make certain none of the results were anomalous. Though on the graph it appeared that none were anomalous as the results produced such a smooth curve. Conclusion: As I predicted the graph of temperature against time showed that the higher the temperature is the lower the time taken for the cross to disappear. Unlike I predicted the time does not double with an increase of 10 degrees Celsius.
My results show that as the temperature increases the percentage difference between that temperatures time and the time taken for 10 degrees Celsius less than that temperature decreases. At 20 degrees Celsius it took 238 seconds for the cross to disappear while at 30 degrees it took 123 seconds. 123 is 51. 68% of 238, which equates to a percentage decrease of 48. 32%. At 30 degrees Celsius it took 123 seconds for the cross to disappear while at 40 degrees it took 70 seconds. 70 is 56. 91% of 123, which equates to a percentage decrease of 43%. This decreasing trend continues as the temperature increases as shown in the table below.
Temperature 1 ( C) Time 1 (secs) Temperature 2 ( C) Time 2 (secs) Calculation Percentage Decrease The amount the percentage decreases does not decrease evenly. As the percentage differences are so marginal they cannot be clearly seen on the graph. The graph showing time over temperature showed that the higher the temperature the quicker it took for the cross to disappear and the lower the temperature the longer it took.
This was shown by the graph having a monotonically decreasing smooth curve. The graph plotting rate of reaction (1/time) against temperature showed that the lower the temperature the lower the rate and the higher the temperature the higher the rate. This was shown by a monotonically increasing smooth curve. The reason the rate of reaction is increased as the temperature increases is due to the sodium thiosulphate and HCL molecules being given more energy, which they convert into kinetic energy. This causes them to move faster which enables more collisions to take place and reactions occur when two molecules collide.
Not only does the higher temperature increase the number of collisions but it also causes the collisions to happen with more energy. This increases the number of collisions, which actually produce a reaction. This is because for a collision to actually trigger a reaction it must happen with a certain amount of energy (activation energy). The higher temperature increases the average amount of energy each molecule has so more reactions will occur with enough energy. Errors: Measurement errors: The water, sodium thiosulphate and HCL were all measure out using measuring cylinders.
A 100ml measuring cylinder was used for measuring out the water which could only be read to the nearest ml. It is reasonable to estimate we could read it to +/- 0. 5 ml. The HCL and Sodium thiosulphate were measured out using a 10 ml measuring cylinder which could be read to the nearest half a ml. So it is fair to say we could read it to +/- 0. 25 ml. These errors can be used to decide the percentage error in our final results. Error in reading the HCL measuring cylinder: +/- 0. 25 ml.
Error in reading the Sodium thiosulpate measuring cylinder: +/- 0.25 ml Error in reading the water-measuring cylinder: +/- 0. 5 ml % error = +/- estimated error Reading So for the HCL % error = +/- 0. 25/5 x 100% = 5 % So for the Sodium thiosulphate % error = +/- 0. 25/8 x 100% = 3. 125 % So for the water % error = +/- 0. 5/42 x 100% = 1. 19 % Total percentage error = 9. 135 % The percent deciding when to stop the stopwatch may have not stopped it at the same time every experiment the 70 degrees Celsius experiment was extremely hard to judge as the experiment took place so quickly.
Method errors: The temperatures did not remain constant while the reaction was taking place, as the experiment could not be heated once it had started to react. The concentration of the HCL and the Sodium thiosulphate was supposed to be 1 mole and 40 g/dm respectively but there must have been errors involved when these solutions were originally made so there will be errors in there concentrations. Improvements: If I had another chance to redo the experiment there would be several things I’d change. Firstly rather than allowing a human being to judge when they cannot see a cross I would use a data-logger and light sensor.
The beaker would be encapsulated in a container blocking out external light and a lamp would be placed shining up from underneath it and a light sensor would be placed above it. The light sensor and data logger would be able to record how long it took for the reaction to only allow say 20% of light through to the light sensor. I would also use a water bath to heat the solution up with to allow more even heating. As the temperature would carry on increasing after the Bunsen was taken away. I would allow more time to carry out the experiment so I could repeat each experiment around 4 times so make sure I had no anomalous results.
I would also the same experiment varying the temperature with a couple of different concentrations to observe how greatly the temperature affected the rate at a higher temperature. I would try the experiment over a greater temperature. Range using intervals of five degrees Celsius rather than ten, which we were unable to do due to time. I would not use room temperature as it went up by a degree during the experiment and I had no way of cooling down the solution to allow me to do the experiment again at 19 degrees Celsius. I would investigate how exactly concentration affects the reaction.
A method for an improved version is shown below: Diagram: Method: The apparatus was set-up as shown in the diagram. 8 ml of 8g/dm sodium thiosulphate was measured into a 10 ml measuring cylinder and 42 ml of distilled water was measured out into a 100 ml measuring cylinder. Both the 8 ml of sodium thiosulphate and the 42 ml of distilled water were poured into a 250 ml beaker together. A thermometer was then put in the beaker and a petri was placed over the top of the beaker to stop the solution evaporating when it was heated up. The beaker was then placed in a water bath and heated.
Heating was stopped when the temperature reached 30 degrees Celsius. The Beaker was placed in the sealed container and the data logger was started as soon as the HCL was added. The lid was place on top of the container as soon the HCL was added. The light sensor measured the percentage of light passing through the solution and the data logger logs how long it takes for the solution to only allow 20% of the light pass through. The time the Logger showed would be recorded. The time taken was recorded and the apparatus was clean thoroughly.
The same experiment was then repeated again a second time and at 40 C, 50 C, 60 C and 70 C, it was repeated for these temperatures a second time as well.
Bibliography: Diagram and information on collision theory from Chemistry explained by j. r. palmer, B. A. J Shaw pg 219 www. gcsechemistry. com/rc1. html G. I Brown Physical Cemistry Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.