Chemical Reaction End Point Timer. - page 3
Keywords: Physics Chemistry Practical Build device end point timer
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 3 of 9 pages: 1 2 3 4 5 6 7 8 9
random error - it varies significantly if there is any movement near it.
However, the error definitely wasn’t more than 50 ohms.
Difference in resistance of LDRs (Ω approx.) Minimum difference in resistance of LDRs (-50) (Ω approx.) Maximum difference in resistance of LDRs (+50) (Ω approx.)
LDR A 400 350 450
LDR B 150 100 200
LDR C 150 100 200
The minimum difference of LDR A is greater than the maximum differences of the others, so even allowing for those fluctuations LDR A is still the best.
For my experiment I will build a cover around my apparatus to reduce the affect of movement in the classroom, and so reduce the random error.
Resistance of LDR A with box around apparatus (kΩ)
With LED lit 3
With LED not lit 46
The difference between when the LED is lit and when it is not is 43 kΩ.
% resistance change = 43/46 x 100 = 93.5%
However as my LED is quite bright I discovered that the result with the LED off is not the same as the reading with the LED on and shining through an opaque fluid.
Resistance of LDR A with box around apparatus (kΩ)
With LED lit 3
With LED not lit 16
To maximise the change in resistance I need the resistor to be as close as possible to half way between the 2 values. The nearest I can get is 10 kΩ so I will be using a 10 kΩ resistor in the circuit.
I also need to decide on the molarity of the chemicals, variations in this have a large affect on my experiment as it controls how fast the reaction takes place.
I conducted another preliminary experiment with all the apparatus set up using 3ml of 0.25M Sodium Thiosulphate and 3ml of 1M Hydrochloric Acid, I used those quantities as they were the largest amount that would fit in the test tube.
I decided to measure the voltage every 5 seconds from the start of the reaction.
The reaction took 190 seconds, it reacted moderately fast leading me to decide to use some higher and some slightly lower concentrations in the other experiments, however 5 seconds was slightly to quick for me to accurately record every result so I decided to record every 10 seconds in future experiments.
I decided to keep the concentration of the Sodium Thiosulphate the same and vary that of the Hydrochloric Acid using concentrations of:
0.5M, 1M, 1.5M, 2M and 2.5M.
Next I decided that to make sure I got fair results I would take three readings at each concentration and use
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