required practical 7

Question 1

what reaction can you use to measure the rate of a reaction by a continuous monitoring method?

Answer 1

the iodination of propanone

Question 2

what is the equation for the iodination of propanone?

Answer 2

CH3COCH3 + I2 -> CH3COCH2I + HI

Question 3

what is the role of the sulfuric acid in the iodination of propanone?

Answer 3

it provides H+ ions which act as a catalyst

Question 4

why are samples removed from the reaction mixture and poured into sodium hydrogencarbonate solution?

Answer 4

to quench (stop) the reaction by neutralising the acid catalyst
H^+ + CO3^2- -> CO2 + H2O

Question 5

why are these samples then titrated with sodium thiosulfate solution?

Answer 5

to determine how much iodine is present
I2 + 2S2O3^2- -> 2I^- + S4O6^2-

Question 6

what is the method for measuring rate of reaction of the iodination of propanone using a continuous monitoring method?

Answer 6

1. use measuring cylinder to place 10cm3 of sodium hydrogencarbonate solution into six conical flasks
2. fill burette with sodium thiosulfate solution
3. mix 25cm3 each of propanone and sulfuric acid in a conical flask
4. measure 50cm3 of iodine solution into a measuring cylinder - mix with other reactants and immediately start timer
5. swirl flask well, and after 5 mins withdraw 10cm3 of reaction mixture with 10cm3 pipette and filler
6. put contents of pipette into conical flask 1 of sodium hydrogencarbonate solution and record time of addition
7. shake flask until bubbling stops
8. repeat steps 5-7 and five minute intervals using remaining conical flasks until 30 mins have passed
9. add two drops of starch solution to each conical flask and titrate contents of each one with sodium thiosulfate solution until colour change occurs for black to colourless (refill burette when needed)
10. record titres in results table

Question 7

how can you calculate the order of reaction with respect to iodine using the results from the continuous monitoring method?

Answer 7

1. calculate concentration of iodine in each comical flask (1-6)
2. plot concentration-time graph of concentration of iodine in conical flask and time reaction mixture sample was added to conical flask of sodium hydrogencarbonate
3. draw tangents at each time point on graph and calculate their gradient which tells you rate of reaction at that time
4. if concentration has no effect of rate, the order is 0
   if concentration halves and rate halves, the order is 1
   if concentration halves and rate quarters, the order is 2

Question 8

how could you use colorimetry to determine the order of reaction with respect to iodine from a continuous monitoring method?

Answer 8

1. produce calibration curve by measuring absorbance of a set of standard solutions of iodine of known concentration
2. each time sample of reaction mixture is removed and added to sodium hydrogencarbonate, pour some of this into cuvette and place in colorimeter
3. measure absorbance and record value
4. use calibration curve to determine concentration of iodine for that absorbance value, then plot concentration-time graph and determine order of reaction from this