The iodine was analysed by titrating the acid/propanone/iodine solution against 0.0050 mol l-1 sodium thiosulphate. The volume and concentration of the sodium thiosulphate can be used to calculate the number of moles, and therefore by proportion, can be used to calculate the number of moles of iodine and therefore the concentration of iodine.
I2(aq) + 2S2032-(aq) ==> 2I-(aq) + S4062-
1 mole <== 2 moles
Procedure:
A 50cm3 pipette was rinsed with 0.010 mol l-1 iodine solution and 50cm3 of this solution was transferred into a 250cm3 conical flask and was stoppered. Then a 25cm3 pipette was rinsed with 1.0 mol l-1 sulphuric acid and 25cm3 of this solution was transferred into a second 250cm3 conical flask. Another 25cm3 pipette was rinsed with 1.0 mol l-1 propanone solution, and then 25cm3 of this solution was transferred into the conical flask containing the acid and the flask was stoppered. A measuring cylinder was then used to add 10cm3 of sodium hydrogencarbonate solution to each of the seven 100cm3 conical flasks.
A burette was then rinsed, including the tip with 0.0050 mol l-1 sodium thiosulphate and filled with the same solution. The sulphuric acid/propanone mixture was then added to the iodine solution and the timer was immediately started. The flask was stoppered and swirled to ensure thorough mixing.
After about two minutes, 10cm3 of the reaction mixture was pipetted into one of the conical flasks containing the sodium hydrogencarbonate solution, and the time was noted when the pipette was half empty. The contents of the flask was swirled. This solution was then titrated against 0.0050 mol l-1 sodium thiosulphate, and when the solution turned straw coloured, a few (three) drops of starch solution was added. The titration was then continued to the end-point carefully to ensure the end-point was not overshot. This was then repeated after about 6, 10, 14, 18, 22 and 26 minutes. In each case the time was recored when the pipette was half empty.
The concentration of iodine was calculated in the reaction mixture each time a sample was removed from it. This was done using an accurate concentration of sodium thiosulphate which was 0.0050 mol l-1.
A graph of iodine concentration against time was drawn, and from it, the order of reaction was calculated with respect to iodine. The rate of the reaction was then calculated and so was the initial concentration of propanone and hydrogen ions in the reaction mixture. The sulphuric acid is diprotic so therefore contains two moles of hydrogen ions per mole of acid. These initial concentrations along with the rate of reaction were used to calculate the rate constant for the reaction. The reaction is first order with respect to both propanone and hydrogen ions.
The colour change is from black/blue ==> colourless.
The samples taken were added to sodium hydrogencarbonate because it is a base and was used to neutralise the acid, so that the reaction slows right down.
Results:
See sheet for table:
K= 2.26x10-6 = 1.8x10-5 mol-1 l/s
0.25x0.5
Conclusion:
The order of reaction with respect to iodine is zero. This is because on the graph the gradient was constant with respect to iodine concentration. The rate of the reaction was 2.26x10-6 and the rate constant was calculated to be 1.8x10-5 mol-1 l/s.
If a lower temperature was used the reaction rate would decrease because temperature is a measure of the average kinetic energy particles have, so therefore there would be fewer sucessful collisions.
If a lower initial concentration of propanone was used, the rate of reaction would decrease, as there would be a fewer number of particles able to collide. Lowering the concentration of iodine would not effect the reaction rate as it is zero order.
The rate constant would not be affected by a higher temperature or a higher concentration of iodine.
Iodine is not in the rate determining step because it is zero order and therefore not in the rate equation.The rate determining step is determined by the slowest step in a reaction mechanism, shown by the rate equation.
by
jimmybob126 (student)
