The constant substance Essay
The constant substance
So I must keep the temperature of the entire investigation constant, because otherwise the molecules will have more energy (unfairly) and will result in more collisions which are unfairly accelerated. This will tamper with my results and could make them anomalous. I must also ensure that I do not mechanically stir the solution as this will alter my results and make the investigation unfair. Variables: I will change the concentration of the sodium thiosulphate by changing the volume of water used to the volume of sodium thiosulphate solution.
These changes may affect rate of reaction, and that will affect the time taken for the precipitate to form . I will be measuring the time taken for the cross to disappear. The variables in this test are: The concentration of hydrochloric acid The concentration of sodium thiosulphate The amount of hydrochloric acid The amount of sodium thiosulphate The temperature The surface area of the reactants The size and colour of the cross The size of the conical flask Temperature will keep the temperature the same because increasing the temperature increases the rate of reaction.
I will do this by collecting all measurements on the same day and doing all tests at room temperature. If the measurements were taken on a different day, the temperature might vary. Measuring Apparatus I will keep the same type of measuring cylinder, because if I decide to use a different one with a different scale, it could change my results or confuse me when taking measurements. Dependant My independent variable is going to be the concentration of sodium thiosulphate. My dependant variable will be the rate of the reaction, or time taken for the reaction to take place.
Watching the cross disappear as precipitation occurs and sulphur is formed as a solid will be the method used to realise the rate. My controlled variables will be the temperature, the concentration of hydrochloric acid, the amount of sodium thiosulphate and the amount of hydrochloric acid. I will try and control these as far as possible, and be as accurate as I can when measuring out the chemicals. Also if I did vary the volume of hydrochloric acid, I would have different rates of reactions , and there would be no reason for it because I am varying the ratio of sodium thiosulphate to water.
Independent My dependant variable I am keeping the same is the person watching the solution and stopping the timer when the ‘X’ is not visible. I am doing this to ensure that there is no delay in time-keeping when stopping the timer. To be certain of it, I will choose a person of a quick reaction-time to do this as they will be best suited to this role. My dependent variable is the time taken. Analysis of Graphs: The results show that the rate of reaction decreases when the volume of sodium thiosulfate is less concentrated.
From the graph we can see that as the concentration decreases, the time taken increases. The gradient decreases gradually, which shows that the time taken increases between each set interval of the concentration. These results prove my prediction correct, and also prove the particle collision theory. Concentration of Na2S203 vs. Time The main graph of the concentration of sodium thiosulfate (m) vs. the time of reaction in seconds has a curved line showing a negative correlation. This graph shows as you increase the concentration of sodium thiosulfate, the rate of reaction is higher.
The curved line of best fit on the graph showing the average times shows that as the concentration of Sodium Thiosulphate decreases, the time increases by a bigger and bigger amount each time. This would continue to happen until the concentration was zero, and the time would be infinite. In addition, as the concentration increases, the time decreases, until it would be almost instant. Concentration of Na2S203 vs. Time; Rate of Reaction 1/t (x amount of sulphur/time) This graph shows that as the concentration decreases, so does the rate of reaction.
This supports the first half of my prediction shown by the collision theory, which says that as the concentration increases, there will be more collisions, and so more collisions that have the energy of activation, so the rate of reaction will increase. However, it does not support my prediction that as the concentration is halved, the rate of reaction will be halved; the graphs do not support this pattern. However, the results are accurate enough to show that as the concentration increases, so does the rate of reaction, supporting my prediction.
Rate of reaction; Error Bar On the error bar graph I have plotted the highest number and lowest number from my 3 readings and have plotted a cross for the middle reading. The distance between the two points on my error bar shows how accurate my readings were. Anomalous results and experimental improvements The only anomalous result that I have recorded (value of t when v=16cm3). The result may have turned out anomalous because of basic human error, or maybe because we measured the substances wrong. It may have even been because we did not clean the apparatus properly.
Apart from this, the accuracy of my experiment has been more or less accurate. Although there are a number of ways in which we could have made the results more reliable. For instance, we could have used better measuring equipment, because the apparatus we used was mainly basic equipment. Another thing we could have done to bring more evidence is to have tried to use the hydrochloric acid as the variable substance, and used the sodium thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.
I think that the evidence, which I have received, is enough to reach a suitable conclusion, but there are a few flaws to the experiment (which I have mentioned). Apart from them, the experiment is fine. Conclusion: The rate of reaction is manipulated by the temperature, concentration, surface area, gas pressure and catalysts. In this experiment I changed the concentration whilst keeping the temperature, surface area, gas pressure and the use of catalysts the same. I determined that if you increase the concentration of the sodium thiosulfate, the frequency of collisions increased so the rate of reaction also increased.
University/College: University of Chicago
Type of paper: Thesis/Dissertation Chapter
Date: 8 October 2017