Chemical Reaction End Point Timer. - page 7
Keywords: Physics Chemistry Practical Build device end point timer
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 7 of 9 pages: 1 2 3 4 5 6 7 8 9
would expect a graph for 2.5M to look.
The values for judging by eye also show this anomaly with the value for 1M not fitting the pattern very well, at 96 seconds it is faster than you would expect given that the values for 0.5M and 1.5M are 140 and 91 seconds respectively.
I am not sure of the cause of this anomaly, as I took 3 readings it is unlikely to be a single human error although it was the first one that I did out of my experiments so that could be a contributing factor. However as I used the same HCl for all 3 experiments it is possible there was a fault with that, which may have affected its reaction time. It was the solution I used in my preliminary experiments and was out in the classroom for several weeks while I set up and built my sensor before I used it and it could have been affected in this time while all the others were only out for the one lesson in which I used them, also it had already been mixed and it could have been made some time before I used it, it had probably also been in used in various other experiments by different people and could have become contaminated, unlike the others which were made up specially, also it is possible that its mix was just slightly off.
My final sensor used a LDR whose resistance ranged from 0 to at least 46,000Ω, a 10kΩ resistor and the brightest LED I could find. Each of my final experiments used 3ml of sodium thiosulphate (0.25M) & 3ml hydrochloric acid. The sodium thiosulphate had a constant molarity of 0.25M and the hydrochloric acid had molarities of: 0.5, 1, 1.5 & 2M. For each experiment I measured the voltage every 10 seconds from 0 to 300. I used 12V power packs to power the both the circuits. I constructed the sensor myself using clamps and a stand to hold it together, I also designed a cover for apparatus, which to I made out of black paper. This cover reduced the random error that was caused by movement in the room making surrounding light levels change by shielding the LDR from all light except the LED’s.
That random error was my main difficulty in the experiment but I also had problems pouring the chemicals into the test tube and
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