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Iodine ClockPart I – Chemical Kinetics

Collect: 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 K2S2O8 and K2SO4, separately

(2015/11/17 revised)

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Objective & SkillsObjective

Determine: the rate law of reactionS2O8

2- + 2I- → 2SO42- + I2 Rate = k[S2O8

2-]m[I-]n

Add limiting amount of thiosulfate ion (S2O32-)

reacting with iodine(I2) as a measuring tool to determine the rate of the above reaction:

2S2O32- + I2 → 2I- + S4O6

2- (a fast reaction)

Skills Manipulate graduated pipet Initial rate law

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Introduction I - Determine the Rate of Reaction

S2O82- + 2I- → 2SO4

2- + I2

2S2O32- + I2 → 2I- + S4O6

2-

I- + I2 → I3-

rate[S O ]

t

12

[S O ]

t2 8

2 2 32

Reaction occurs extremely fast

I2 formed in the above rxn is consumed immediately

While the limiting reagent S2O3

2- used up, I2 reacts with starch indicator

Purple-blue color appears

Stop the timer, record t

Rate law to be determined

[S2O32-] ＝2[S2O8

2-]Limiting reagent

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Trial No.

0.20 MNaI

(mL)

0.20 M NaCl*(mL)

0.0050 M Na2S2O3

(mL)

2%Starch(mL)

0.10 M K2SO4*

(mL)

0.10 MK2S2O8(mL)

Reaction timet (s)

1 2.0 2.0 1.0 1.0 2.0 2.0 61

2 2.0 2.0 1.0 1.0 0 4.0 31

3 4.0 0 1.0 1.0 2.0 2.0 33

Introduction II - Initial Rate Method

S2O82- + 2I－→ 2SO4

2- + I2Table 1 Volumes of reagents for the initial rate method (total volume of 10 mL)

Volume and number of moles of limiting reagent are fixed

Note*: NaCl(aq) and K2SO4(aq) are added to maintain the ionic strength in the solutions

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Initial Rate S2O82- + 2I－→ 2SO4

2- + I2

rate ＝ k[S2O82-]m[I-]n

tt

00025.0)00050.00(21

rate[S O ]

t

12

[S O ]

t2 8

2 2 32

mnm2

82

nm282

2

1

1

2

1

2 2.0)()]([I)]Ok([S)]([I)]Ok(2[S

3161

/0025.0/0025.0

raterate

11

11

tt

tt

nnm2

82

nm282

3

1

1

3

1

3 2.0)()]([I)]Ok([S)](2[I)]Ok([S

3361

/0025.0/0025.0

raterate

11

11

tt

tt

m = 0.98

n = 0.89Substitute m and n, then obtain k.

0.0050 M x 1 mL10 mL = 0.00050 (M)S2O3

2- is limiting reagent that used up

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TrialNo.

0.20 MNaI

(mL)

0.20 M NaCl(mL)

0.0050 M Na2S2O3

(mL)

2%Starch(mL)

0.10 M K2SO4(mL)

0.10 MK2S2O8(mL)

Reaction timet (s)

1 2.0 2.0 1.0 1.0 2.0 2.0 61

Rate90 2.0 2.0 1.0 1.0 4 – x x 90

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)

2 0.98 0.890.981 90 2 8 1 1

2 0.98 0.8990 1 2 8 1

rate Δt 90 k[S O ] [I ] 2.0( )rate Δt 61 k[S O ] [I ] x

)x2.00.98log()61

90log( x = 1.34 (mL), x < 4 mL

(2.7) (1.3)

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Trial No.

0.20 MNaI

(mL)

0.20 M NaCl(mL)

0.0050 M Na2S2O3

(mL)

2%Starch(mL)

0.10 M K2SO4(mL)

0.10 MK2S2O8(mL)

Reaction Timet (s)

3 4.0 0 1.0 1.0 2.0 2.0 33

Rate29 4.0 0 1.0 1.0 4.0 – y y 29

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)

2 0.980.983 29 2 8 3

2 0.9829 3 2 8

rate Δt 29 k[S O ] 2.0( )rate Δt 33 k[S O ] y

0.89

30.89

3

[I ][I ]

)y2.0log(98.0)33

29log( y = 2.12 (mL), y < 4 mL

(1.8) (2.2)

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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, Na2S2O32% Starch solution

* Notice:Use the same set of chemicalsExpel air bubbles in dispenser before use

Measure K2SO4 with graduated pipet into Erlenmeyer flask

* NoticeWash and rinse pipet with test reagent twice before use

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Procedure 2: React and Time

Start recording time Add last reactant K2S2O8 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)

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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

Summary of Calculation

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rate = ∆

∆= .

∆

： = ∆∆

= m

： = ∆∆

= n

： , = ∆∆ ,

= . m

log(∆∆ ,

) = m • log .

m = ∆∆

n = ∆∆

log x = log 2 - • log(∆∆ ,

)

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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 K2S2O8 and K2SO4 with 100 mL beaker. Then

measure accurately with 5 mL graduated pipet Recorded time should start from the last reactants, K2S2O8, 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