required practical 7 Flashcards
(12 cards)
what reaction can you use to measure the rate of a reaction by a continuous monitoring method?
the iodination of propanone
what is the equation for the iodination of propanone?
CH3COCH3 + I2 -> CH3COCH2I + HI
what is the role of the sulfuric acid in the iodination of propanone?
it provides H+ ions which act as a catalyst
why are samples removed from the reaction mixture and poured into sodium hydrogencarbonate solution?
to quench (stop) the reaction by neutralising the acid catalyst
H^+ + CO3^2- -> CO2 + H2O
why are these samples then titrated with sodium thiosulfate solution?
to determine how much iodine is present
I2 + 2S2O3^2- -> 2I^- + S4O6^2-
what is the method for measuring rate of reaction of the iodination of propanone using a continuous monitoring method?
- use measuring cylinder to place 10cm3 of sodium hydrogencarbonate solution into six conical flasks
- fill burette with sodium thiosulfate solution
- mix 25cm3 each of propanone and sulfuric acid in a conical flask
- measure 50cm3 of iodine solution into a measuring cylinder - mix with other reactants and immediately start timer
- swirl flask well, and after 5 mins withdraw 10cm3 of reaction mixture with 10cm3 pipette and filler
- put contents of pipette into conical flask 1 of sodium hydrogencarbonate solution and record time of addition
- shake flask until bubbling stops
- repeat steps 5-7 and five minute intervals using remaining conical flasks until 30 mins have passed
- 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)
- record titres in results table
how can you calculate the order of reaction with respect to iodine using the results from the continuous monitoring method?
- calculate concentration of iodine in each comical flask (1-6)
- plot concentration-time graph of concentration of iodine in conical flask and time reaction mixture sample was added to conical flask of sodium hydrogencarbonate
- draw tangents at each time point on graph and calculate their gradient which tells you rate of reaction at that time
- 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
how could you use colorimetry to determine the order of reaction with respect to iodine from a continuous monitoring method?
- produce calibration curve by measuring absorbance of a set of standard solutions of iodine of known concentration
- each time sample of reaction mixture is removed and added to sodium hydrogencarbonate, pour some of this into cuvette and place in colorimeter
- measure absorbance and record value
- use calibration curve to determine concentration of iodine for that absorbance value, then plot concentration-time graph and determine order of reaction from this
what reaction can you use to measure the rate of a reaction by an initial rate method?
the reaction between hydrogen peroxide and iodide ions in an acidic solution
what happens in the reaction between hydrogen peroxide and iodide ions?
- hydrogen peroxide, hydrogen ions and iodide ions reacts producing iodine and water (H2O2 (aq) + 2H+ (aq) + 2I- (aq) -> I2 (aq) + 2H2O (l) )
- thiosulfate ions are also produced but immediately react with iodine (2S2O3^2- (aq) + I2 (aq) -> 2I- (aq) +S4O6^2- (aq) )
- once I2 has reacted with the limited amount of thiosulfate ions present, excess I2 remains in the solution and reacts with the starch, creating a blue-black colour
what is the method for the experiment?
- use a measuring cylinder to measure and add the following solutions to a 250cm3 beaker - 25cm3 sulfuric acid, 20cm3 distilled water, 1cm3 starch solution (measure with pipette), 5cm3 potassium iodide and 5cm3 sodium thiosulfate (ensure it is last)
- stir mixture
- add 10cm3 hydrogen peroxide solution and immediately start timer, then stir mixture
- stop timing when mixture turns blue-black and record time in seconds
- repeat this 4 more times but varying the volumes of water and potassium iodide solution each time (to change concentration of iodide ions)
how can you use the results to calculate the order with respect to iodide ions?
- for each experiment, calculate log10V with the volume of potassium iodide solution used
- for each experiment, calculate log10 (1/T) with the time in seconds for the colour change to occur
- plot a graph with log10 (1/T) on the y axis and log10V on the x axis
- draw a line of best fit (should be straight line)
- calculate the gradient of the line of best fit, which is the same as the order with respect to iodide ions
