Iodine Clock Part I Chemical Kinetics

Transcription

1 Collect: Iodine Clock Part I Chemical Kinetics (2015/11/17 revised) 50 ml Erlenmeyer flask (10): wash clean, dry, and cool 5 ml graduated pipet (2), pipet filler (1) Cork stopper (6) Stopwatch (1) (given out and collected by GTA) Labels (label beakers, and Erlenmeyer flasks) Prepare: Scientific calculator 100 ml beaker (2) wash clean and dry label K 2 O and K 2 SO 4, separately 1

2 Objective & Skills Objective Determine: the rate law of reaction O + 2I - 2SO 4 + I 2 Rate = k[ O ] m [I - ] n Add limiting amount of thiosulfate ion ( O 3 ) reacting with iodine(i 2 ) as a measuring tool to determine the rate of the above reaction: 2 O 3 + I 2 2I - + S 4 O 6 (a fast reaction) Skills Manipulate graduated pipet Initial rate law 2

3 Introduction I - Determine the Rate of Reaction O +2I - 2SO 4 + I 2 Rate law to be determined 2 O 3 + I 2 2I - +S 4 O 6 I - + I 2 I - 3 Limiting reagent [ O 3 ] =2[ O ] rate [S2O ] t [S O 2 ] t Reaction occurs extremely fast I 2 formed in the above rxn is consumed immediately While the limiting reagent O 3 used up, I 2 reacts with starch indicator Purple-blue color appears Stop the timer, record t 3

4 Introduction II - Initial Rate Method O + 2I - 2SO 4 + I 2 Table 1 Volumes of reagents for the initial rate method (total volume of 10 ml) Trial No M NaI 0.20 M NaCl* M Na 2 O 3 2% Starch 0.10 M K 2 SO 4 * 0.10 M K 2 O Reaction time t (s) Volume and number of moles of limiting reagent are fixed Note*: NaCl (aq) and K 2 SO 4(aq) are added to maintain the ionic strength in the solutions 4

5 Initial Rate O + 2I - 2SO 4 + I 2 O 3 is limiting reagent that used up rate [S2O ] t rate = k[ O ] m [I - ] n rate rate rate rate 1 2 [S O 2 ] t 1 2 ( ) t t 2 m n / t2 t1 61 k(2[s 2O ] 1) ([I ] 1) ( 2.0) 2 m n / t1 t2 31 k([s 2O ] 1) ([I ] 1) 2 m n / t 3 t1 61 k([s 2O ] 1) (2[I ] 1) ( 2.0) 2 m n / t1 t 3 33 k([s 2O ] 1) ([I ] 1) Substitute m and n, then obtain k M x 1 ml 10 ml = (M) m m = 0.9 n n = 0.9 5

6 Design a Set of Reaction That Will Change Color in Given Time: 90 s The longer the time for it to change color (t), the slower the reaction, the concentration of the reactant should be lower Change the formula according to trial 1 (i.e. 2.0 ml NaI) Trial No M NaI 0.20 M NaCl M Na 2 O 3 2% Starch 0.10 M K 2 SO M K 2 O Reaction time t (s) Rate x x (2.7) (1.3) 90 rate Δt 90 k[s O ] [I ] 2.0 ( ) rate Δt 61 k[s O ] [I ] x log( 90 ) 0.9log( 2.0 ) 61 x x = 1.34, x < 4 ml 0.9 6

7 Design a Set of Reaction That Will Change Color in Given Time: 29 s The shorter the time for it to change color (t), the faster the reaction, the concentration of the reactant should be higher Change the formula according to trial 3, (i.e. 4.0 ml NaI) Trial No M NaI 0.20 M NaCl M Na 2 O 3 2% Starch 0.10 M K 2 SO M K 2 O Reaction Time t (s) Rate y y (1.) (2.2) rate Δt 29 k[s O ] [I ] ( ) 0.9 rate Δt 33 k[s O ] [I ] y log( 29 ) 0.9log( 2.0 ) y = 2.12, y < 4 ml 33 y

8 Procedure 1: Prepare Sample Solution Wash 10 of 50 ml Erlenmeyer flasks Oven dry and let cool Label and carry out the three trials listed in Table 1 Transfer reagents from dispenser to Erlenmeyer flask: NaI, NaCl, Na 2 O 3 2% Starch solution * Notice: Use the same set of chemicals Expel air bubbles in dispenser before use Measure K 2 SO 4 with graduated pipet into Erlenmeyer flask * Notice Wash and rinse pipet with test reagent twice before use

9 Procedure 2: React and Time Start recording time Add last reactant K 2 O Stopper the flask Swirl the flask for 20 s Leave the flask on tabletop * Remove the pipet filler directly to expel the solution inside completely * Do not let solution splatter Observe the solution changing color Stop timing Record reaction time (t) 9

10 Procedure 3: Calculate and Play the Iodine-Clock Symphony Duplicate the three trials Take average value as t * If the reaction time of two trials are different from more than 3 s then redo trial Calculate values of m, n, and k Design a set of reagents that will change color at given time interval (t) Carry out test Compare the time with the given time to check its accuracy Iodine-clock symphony: Prepare a set of reagents Test with other students to observe whether the soln will change color on the correct beats 10

11 Summary of Calculation rate = : = = : = = =. m n m = n = :, =, =. m log(, )= m log. log x = log 2 - log(, ) 11

12 Notice Erlenmeyer flask should be oven-dried and cooled to room temp., do not wipe dry with paper towel Wash and dry the flasks after use, 10 might not be enough Do not take other people s cleaned flasks Obtain about 30 ml of K 2 O and K 2 SO 4 with 100 ml beaker. Then measure accurately with 5 ml graduated pipet Recorded time should start from the last reactants, K 2 O, is added to the solution (remove pipet filler to drain all liquid); the way of recording time should be the same throughout experiment Take 2 significant figures of reaction order, m and n, for example: m = 0.96 When designing your own set of reagents, remember to make sure that the total volume is fixed to 10 ml and mind the ionic strength. Write down the calculated volume of each components; check it with TA 12