Chemical Thinking P7S1 Pre-Lab Worksheet v4.2 | 1 Project 7, Session 1
Pre-Lab Worksheet
Your TA’s name: ___Kexin Li____________ Your name: Stef Antonopoulos Lab section: _________1b________________ Your email: stavroulaa@email.arizona.edu GRADE ____ /10 p
All work must be well-written and organized. If your penmanship is poor, you must type all responses. If you need to organize your thoughts,
please use a separate sheet of paper.
Studying a reaction of the type Y(blue) Z(colorless) two students collect the following absorbance data at a max= 640 nm for Y(blue) as a function of
time from a sample of Y(blue) with a starting (initial) concentration of 0.00373 M. The molar absorptivity ε of Y (blue) is 217 cm-1M-1 at 640 nm and
the optical path length can be taken as l = 1.00 cm. Time (s)
Absorbance 0
0.810 5
0.569 10
0.431 15
0.349 20
0.294 25
0.252 30
0.223 Question 1 (6 p) Transform (convert) the above absorbance values into concentration (mol/L or M). Using graphical analysis determine the rate
order “m” and the rate constant “kobs” for the reaction. Paste-in ALL the graphs used to determine the correct values of m and kobs. Title the plots
and label the axes correctly. Clearly explain how you derived the values of m and kobs. Convert Absorbance values into concentration
Beer's Law: A = elc
A = Absorbance
E = molar absorptivity
L = path length
c = concentration Time (s)
Absorbance
Beer's Law Concentration
(M) Conversion of Absorbance Values into Concentration Results Table
0
5
10
15
20
25
0.810
0.569
0.431
0.349
0.294
0.252
0.810 = (217 0.569 = (217 0.43 1= (217 0.349 = (217 0.294 = (217 0.252 = (217
cm-1M-1)
cm1M1)
cm1M1)
cm1M1)
cm1M1)
cm1M1)
(1.00cm) c
(1.00cm) c
(1.00cm) c
(1.00cm) c
(1.00cm) c
(1.00cm) c
3.73 x 10-3 M 2.62 x 10-3 M 1.99 x 10-3 M 1.61 x 10-3 M 1.35 x 10-3 M 1.16 x 10-3 M The rate order is second order.
kobs = 23.624 P7S1 Pre-Lab v4.2 VT, MY © 2017 6-13-17 Chem Think Technical Press CBC UofA 30
0.223
0.223 = (217
cm1M1)
(1.00cm) c
1.03 x 10-3 M Chemical Thinking P7S1 Pre-Lab v4.2 VT, MY © 2017 6-13-17 Chem Think Technical Press CBC UofA P7S1 Pre-Lab Worksheet v4.2 | 2 Chemical Thinking P7S1 Pre-Lab Worksheet v4.2 | 3 Question 2 (4 p) Use the results from Question 1 to calculate the shelf-life (t90) of Y(blue). You will have to first derive an expression for t90 based
on the order you have determined for the reaction (hint: start by substituting [Y(blue)] = 0.9[Y(blue)]0 when t = t90 into the appropriate integrated rate
law. You can assume k in the integrated rate law and kobs from Question 1 are the same for this exercise). To receive credit, you must show all
work and justify the mathematical relationships that you use. 2nd Order Integrated Rate Law
[C] = concentration
k = rate constant
t = time (s)
[Y(blue)] = 0.9[Y(blue)]0
[Y(blue)] = 3.73 x 10-3 M
kobs = 23.624 1
1
=
+ kt
[C] [C ]0 1
1
=
+23.624 t
−3
−3
0.9[3.73 × 10 ] [3.73× 10 ]
Shelf −life ( t 90 ) of Y (¿ ) =1.261 P7S1 Pre-Lab v4.2 VT, MY © 2017 6-13-17 Chem Think Technical Press CBC UofA

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