Law and Activation Energy (2020/05/08 revised) Collect: 50 mL - - PowerPoint PPT Presentation

1

Iodine Clock II – Integrated Rate Law and Activation Energy

Collect:

 50 mL Erlenmeyer flask (10): wash clean, oven dry, and cool  10 mL graduated pipet (2), pipet filler (1)  Cork stopper (6)  Thermometer (1)  Stopwatch (1) and stir bar (1) (given by GTA)  Labels (label graduated pipets, beakers, and Erlenmeyer flasks)

Prepare:

 100 mL beaker (2):  Wash clean and oven dry  Label Na2S2O3 and H2O, separately

(2020/05/08 revised)

2

Objective & Skills

Objective

 Determine the rate law of reaction

S2O8

2- + 2I- → 2SO4 2- + I2

Rate = k[S2O8

2-]m[I-]n

 Add limiting amounts of thiosulfate ion (S2O3

2-) reacting

with iodine(I2) as a measuring tool to determine the rate

• f the above reaction:

2S2O3

2- + I2 → 2I- + S4O6 2- (a fast reaction)

Skills

 Use of graduated pipet and magnetic stirrer  Graphic method - integrated rate law

3

Outline of Experiment

• I. Integrated rate law

Rate = k’ [S2O8

2-]m

• II. Ea, activation energy
• III. Effect of catalyst

lnA T 1 R E = ln(k)

a

  

4

Reaction

• rder

Rate law Integrated rate law Plot Slope Intercept

Zero [A] = -kt + [A]o

[A] vs. t

• k

[A]o

First ln[A] = -kt + ln[A]o

ln[A] vs. t

• k

ln[A]o

Second

1/[A] vs. t k 1/[A]o

rate d[A] dt k   

rate d[A] dt k[A]2   

rate d[A] dt k[A]   

1 [A] kt 1 [A]0  

* [A]ois the initial conc.; [A] is the conc. after reaction time, t.

Theorem of Integrated Rate Law

For a one reactant reaction, A  P

5

Measurement of Concentration of Reactant, [S2O8

2-]t with Time

 S2O8

2- + 2I- → 2SO4 2- + I2 (1)

2S2O3

2- + I2 → 2I- + S4O6 2- (2)

I- + I2 → I3

• + starch → dark blue, t

Limiting reagent

[S2O3

2-] ＝ 2[S2O8 2-]

 While the limiting reagent S2O3

2- used up, I2 reacts with

starch indicator, and the soln appears blue

 Stop the timer and record t  Calculate the [S2O8

2-]left in reaction mixture at time t

6

Trials of Integrated Rate Law

V (mL) Trial 2% Starch 1.0 M NaI 0.20 M Na2S2O3 H2O 0.15 M K2S2O8 1 1.0 5.0 5.5 1.5 5.0 2 1.0 5.0 4.5 2.5 5.0 3 1.0 5.0 3.5 3.5 5.0 4 1.0 5.0 2.5 4.5 5.0 5 1.0 5.0 1.5 5.5 5.0 6 1.0 5.0 0.5 6.5 5.0

 Keep the volume of solution same (18 mL) for each trial.  Keep concentration of reactants, [NaI] and [K2S2O8], same at each trial  Vary the amount of limiting reagent, Na2S2O3, added  The concentration of NaI is much higher than that of K2S2O8 and

assume constant during the reaction

 Investigate the reaction order of K2S2O8 only.

Step 1 Integrated Rate Law

 Wash clean 10 flasks, then oven

dry, and cool to r.t.

 Add reagents to each flasks

according to Table 2

 Add the last reactant K2S2O8 from

dispenser to the flask and start recording time

 Stop the flask with cork and shake

the flask to mix the reactants

 Since the solution turn to blue,

stop recording time

 You may add the last reatant

K2S2O8 to each flask every 30 s.

Note: It may take 10 min. for flask #1 to change color. Go figure.

7

* Add the last reactant K2S2O8 #1 #2 #3 #4 #5 #6

8

Step 2 Determine the Activation Energy

 Measure each reagents into

the flask according to trial no. 5

 Balance the temp. of flasks at

(1) ice-water bath and (2) warm-water bath, respectively

 Add the last reactant K2S2O8

and start timing

 Record the time interval of

color change, t, for each trial at various temp.

2% Starch 1.0 M NaI 0.20 M Na2S2O3 H2O 0.15 M K2S2O8 1.0 mL 5.0 mL 1.5 mL 5.5 mL 5.0 mL (1) Ice-water bath

• Temp. ca. 2 oC

* Note: it needs lots of ice to keep

• temp. constant

(2) Warm-water bath

• Temp. ca. 40 oC

* Note: measure the

• temp. of soln in the

flask

• No. 5

9

Step 3 The Effect of Catalyst on Reaction Rate

2% Starch 1.0 M NaI 0.20 M Na2S2O3 H2O 0.020 M CuSO4 0.15 M K2S2O8 1.0 mL 5.0 mL 1.5 mL 5.5 mL 2 drops 5.0 mL

 Prepare the reaction solution according to trial no. 5 and

add 2 drops of Cu2+ as catalyst

 Compare the reaction rate with that without Cu2+ added

* Note: add the Cu2+ just before the last reagent, K2S2O8

10

Notice

 Erlenmeyer flasks should be oven-dried and cooled down to

r.t.; do not wipe dry with paper towel

 Use the same set of reagents for whole series of trials  Start recording time since the last reactant, K2S2O8, is

added to the solution (remove pipet filler to drain all liquid); the way of recording time should keep constant throughout experiment

 Use enough ice in the ice-water bath to keep the temp.

constant during the reaction

 Take the temp. of reaction solution; not the temp. of the

water bath

 After class, wash clean the Erlenmeyer flasks and put them

into oven to dry

 Hand the stir bar and timer to TA

Full version report

11

Data Analysis of [S2O8

2-]t vs.

Time

No. n of Na2S2O3 added / mmol n of S2O8

2-

used up / mmol [S2O8

2- ]left

i.e. [S2O8

2-]t / M

ln[S2O8

2-]

1/[S2O8

2-]

t #1 5.5*0.2=1.1 1.1/2 = 0.55 (0.15*5-0.55)/18 = 0.0111

• 4.50

90.0 t1 #2 4.5*0.2=0.9 0.9/2 = 0.45 (0.15*5-0.45)/18 = 0.0166

• 4.09

60.0 t2 #3 3.5*0.2=0.7 0.7/2 = 0.35 (0.15*5-0.35)/18 = 0.0222

• 3.81

45.0 t3 #4 2.5*0.2=0.5 0.5/2 = 0.25 (0.15*5-0.25)/18 = 0.0277

• 3.58

36.0 t4 #5 1.5*0.2=0.3 0.3/2 = 0.15 (0.15*5-0.15)/18 = 0.0333

• 3.40

30.0 t5 #6 0.5*0.2=0.1 0.1/2 = 0.05 (0.15*5-0.05)/18 = 0.0388

• 3.25

25.7 t6 (S2O3

2-)added

(S2O8

2-)used [S2O8 2-]left

• Tabulate your calculation in details

y = 0.1039x + 16.575 R2 = 0.9698 20 40 60 80 100 200 400 600 800 Time(s) 1/[A]

12

Example of Integrated Rate Law

y = -4E-05x + 0.0385 R2 = 0.965

0.000 0.010 0.020 0.030 0.040 0.050 100 200 300 400 500 600 700

Time（s） [A]

y = -0.002x - 3.1581 R2 = 0.9983

• 5
• 4.5
• 4
• 3.5
• 3

200 400 600 800 Time（s） ln[A]

[A]: [S2O8

2-]

According to the plots  ln[A] vs. time  It’s a first order reaction, m = 1  According the slope, k = 2.0 x 10-3 [S2O8

2-] vs. time

ln [S2O8

2-] vs. time

1/[S2O8

2-] vs. time

13

Data Analysis of the Activation Energy

 Determine the rate constants, k, of reaction at various

temperatures

 Use Arrhenius eqn. to calculate the activation energy

Trial

• Temp. (oC)

t (s) 1/T (K-1) ln(1/t) 1) Room temp. T1 t1 1/T1 ln(1/t1) 2) Ice-water bath T2 t2 1/T2 ln(1/t2) 3) Warm-water bath (ca. 40 oC) T3 t3 1/T3 ln(1/t3)

lnA T 1 R E = ln(k)

a

  

14

y = -5824.7x + 14.597 R2 = 0.9994

• 8.00
• 6.00
• 4.00
• 2.00

0.00 0.00320 0.00330 0.00340 0.00350 0.00360 0.00370 1/T ln(1/ t)

lnc lnA T 1 R E = ) Δt 1 ln(

a

   

Ea = 5824.7 × 8.314 = 48 kJ/mol

Plot ln(1/t) vs. 1/T to Obtain Ea

Derived from Arrhenius eqn.:

Notice of Report

• No. 5

△t (s) T (℃) T (K) Without Cu2+ 270 14.0 287.2 With CuSO4 92 14.5 287.7 * Conclusion: add Cu2+ could increase reaction rate

15

 List the calculation in details  Print and attach the tables and figures with report  Determine the reaction order, rate constant, and

activation energy with appropriate significant figures and units

 The effect of CuSO4

Full version report

Pipet 16

Ex.1 Deliver 5.00 mL solution:



Wash a 10 mL pipet thoroughly.



Rinse twice with small portion of sample solution.



Press valve A of pipet filler and squeeze bulb to expel the air inside and create a vacuum.



Insert the top of pipet into pipet filler, press valve S to draw liquid to equal to the mark

• f 0.00 mL.



Hold pipet vertically and transfer liquid into

• container. (One hand hold pipet and the
• ther hand hold container to operate.)



Press valve E to drain liquid to the mark of 5.00 mL.



Wash thoroughly after use.

1 2 3 4 5

Partially deliver

Transfer pipet

T12 - Graduated Pipet

10 ml Ex

25

ml

安全吸球 排氣閥 吸液閥

A S E

排液閥

安全吸球

A S E

Bulb Suction valve Empty valve Pipette filler Aspirate valve