When the temperature reaches as near as possible to where I want it, I will move the conical flask on to the piece of paper with a cross on it and add in the 5ml of acid, simultaneously starting the stopclock, and taking the temperature.
When the yellow colour of the sulphur in the flask completely obscures the cross from sight, I will stop the stopclock, and record the time to the nearest second and the temperature to the nearest °C at the end.
I will repeat the experiment for each temperature three times. This should be most effective for eliminating anomalous points and experimental error in the given time.
Prediction/ Background Knowledge
Matter is made up of particles. Substances are made up of molecules bonded together, and molecules are made up of atoms bonded together.
When two reactant substances are mixed (acid, alkali or acid, base etc.), if the molecules collide in a certain way and with enough force the reaction will occur. This is the collision theory. The amount of energy needed to give the atoms or molecules the strength to collide so that they react is called the activation energy. Here is a graph of how this will appear during the experiment:
As everything we know has thermodynamic energy (heat and work), everything is moving. Atoms vibrate around a central point, and can move in liquid, gas or aqueous form. Thus, when two reactants are mixed, sooner or later a reaction will occur. If the reactants receive more heat energy they will have more kinetic energy – heat and work are closely related. If the two reactants have more kinetic energy, then their molecules will vibrate with greater force, and in a larger area. Therefore the probability of a reaction occurring between two atoms is much greater, so the reaction will occur more quickly.
Using Key Science – Chemistry, by Eileen Ramsden, I predict that the effect of temperature on reaction speed is not linear – for every 10°C rise in temperature, the rate of the reaction will double. To show this, I will draw graphs of the rate of reaction (1/time) against temperature (in °C). I am recording from 20° to 70°C because, to get below 20°C would be inconvenient, and, if I did it up to 80°C, it is possible that the acid would start boiling (HCl boils at 84.8°C).
Here is the equation for the reaction:
Na2S2O3(aq)+ 2HCl(aq) → 2NaCl(aq) + SO2(g)+ S(s) + H2O(l)
Sodium thiosulphate + hydrochloric acid → Sodium chloride + sulphur dioxide + sulphur + water.
The SO2 dissolves in the water.
Results/Obtaining Evidence
Analysing Evidence
I have rounded all the temperatures in my results to the nearest degree, because that is as far as the thermometers measured. Where there is a .5 after a temperature, it means that the reading was between two points, but not exactly at .5.
My results show that as you increase the thermal energy applied to a reaction, the reaction occurs faster. This supports my prediction.
The reaction occurred fastest between about 30° and 40°C.
I predicted that the rise in temperature would not be linear to time of reaction, and that the time taken would halve for every 10°C rise in temperature.
20 – 230 The results that are the nearest to this prediction are 20°, 30° and 40°.
30 – 122
40 – 70
50 – 38
60 – 29
70 – 19
The calculations on my graphs show the correspondence of my results. The values gained should be as near as possible to each other. Only the first value seems to be anomalous, however it fits in the points on my graph. As I increased the temperature, the molecules in the solutions had more energy for the reaction to take place, as they vibrated faster and further the probability of a collision of two particles colliding in the right way and with enough power to react increased exponentially (as shown by the temperature/time graph)
Evaluation
- My experiment was successful, as I proved my prediction.
- My results were quite accurate. This is because I took three readings for each temperature. I had no extreme anomalous points.
- The results could have been made more accurate by taking further readings.
- The method of telling when the cross had been obscured was vulnerable to mistakes.
- This could have been solved by using a colourimeter to gauge the exact amount of light passing through the solution.
- It was difficult to start the stopclock, add the acid, and take the temperature at the same time. This may have caused some minor inaccuracies.
- The stopclocks did not seem entirely reliable. This could have again caused minor inaccuracies.
- This could have been solved by bringing a watch.
- When the acid was added, it was a different temperature. This will have had a cooling effect on the sodium thiosulphate, before the reaction’s heat counterbalanced it. This is not a major problem, but shows that the temperature changes cannot be used to find how much heat energy was exchanged in the reaction. There was not a large cooling effect due to the acid or surrounding air temperature as the sodium thiosulphate and HCl solution was mainly water, which has a high specific heat capacity.
A different way of doing the experiment would been to have heated the two solutions separately in an oven set to the temperature of the experiment, then to have added the solutions together, into a container on a balance and calculated the weight lost for every 10 seconds of reaction.
