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CHM 1301 Study Guide - Quiz Guide: Transmittance, Ethylenediaminetetraacetic Acid, Absorbance


Department
Chemistry
Course Code
CHM 1301
Professor
Natalie Goto
Study Guide
Quiz

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LAB REPORT 4 – Determining Rate of Reactions by Use of Spectrophotometry
INTRODUCTION
Fast and slow reactions are studied, for the most part, to give an understanding of the world around us;
thus, emerges a branch of chemistry known as chemical kinetics, which is the study of the factors that control
rate of reactions and the mechanisms by which the reactions occur. Factors include temperature, surface area,
catalysts, as well as concentration of solution. In this experiment, that factor that is investigated is the varying
solution concentrations of EDTA, ethylene(dinitrilo) tetraacetate, which influences a characteristic known as the
order of the reaction. The order of a reaction is based on the index, or exponent, to which its concentration term
in the rate equation is raised. For example, in a reaction such as
A + B P [1]
The rate can be expressed as
Rate = k [A]n[B]m [2]
Thus given a rate expression, rate order can be determined by variables “n” and “m” with respect to [A] and
[B]. In a first order reaction, the sum of “n” and “m” would be equal to 1. To determine order of a reaction, in
this experiment, it involves the measurement of the appearance of a coloured species as time progresses. A
solution of chromium (III) salt is added to three different solutions of EDTA (differing in pH) as to observe the
reaction progress, which slowly changes in colour from a clear/yellowish solution to a purple solution. The
change in the colour of solution indicates the formation of a complex known as the chromium (III)-EDTA
complex. The results obtained will be used to determine the partial order with respect to the Cr(III) ion by use
of spectrophotometry, which is the relative measurement of radiant energy as a function of wavelength
(Gibson).
One can quantifiable method of determining rate is by first obtaining measurements of transmittance on
a spectrophotometer. Transmittance is the ratio of the intensity of light after it passes through the medium being
studied to the intensity of light before it passes through the medium, and so, absorbance is related inversely to
the transmittance as follows:
A = – log T [3]
This is possible again, due to the formation of chromium (III)-EDTA complex, and the complex emits a purple
color that can be measured by means of spectrophotometry. The relationship between absorbance and
concentration can be recalled by using the Beer-Lambert law:
A = b c [4]ε
In this relation, A represents the absorbance, represents the molar absorptivity coefficient, b represents the ε
path length and c represents the concentration of the absorbing species in the solution. This equation shows that
absorbance is directly proportional to the concentration, and absorbance value is used to represent the
concentration of the absorbing species. Thus, this value can be plotted as a function of time in order to
determine the instantaneous rate by taking the slope of the tangent at any point on the curve plotted, which
demonstrates pseudo first-order kinetics.
PROCEDURE
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As described in the lab manual (What in the World ISN’T Chemistry, Dr. Rashmi Venkateswaran, 2000,
Exp. 4. p. 52).
OBSERVATIONS
Table 1: Qualitative Raw Data
pH 4.0 pH 4.5 pH 5.0
Before Mixing EDTA – clear transparent
solution
Chromate Ion Solution –
dark purple colour
EDTA – clear transparent
solution
Chromate Ion Solution –
dark purple colour
EDTA – clear transparent
solution
Chromate Ion Solution – dark
purple colour
After Mixing Clear transparent solution
No odour Light pink solution
No odour Pink solution
No odour
After 60
Minutes Light pink solution
No odour Lavender solution
No odour Magenta solution
No odour
After 110
Minutes Lavender solution
No odour Deep purple colour
No odour Deepest purple solution
No odour
After Heating Deep purple solution
No odour Deep purple solution
No odour Deep purple solution
No odour
Table 2: Quantitative Raw and Processed Data of EDTA solution pH of 4.0
% T T A A(CrIII) LogA(CrIII) Rate (Ms-1) Log(R)
0 Time of Mixing
5 84.0 0.840 0.076 1.02 0.009 0.340 -0.468
10 81.5 0.815 0.089 1.01 0.003 0.336 -0.474
15 81.0 0.810 0.092 1.01 0.002 0.335 -0.475
20 80.0 0.800 0.097 1.00 0.000 0.333 -0.477
25 78.0 0.780 0.108 0.989 -0.005 0.330 -0.482
30 77.0 0.770 0.114 0.983 -0.007 0.328 -0.484
35 74.0 0.740 0.131 0.966 -0.015 0.322 -0.492
40 72.0 0.720 0.143 0.954 -0.020 0.318 -0.497
45 70.0 0.700 0.155 0.942 -0.026 0.314 -0.503
50 68.5 0.685 0.164 0.933 -0.030 0.311 -0.507
55 66.5 0.665 0.177 0.920 -0.036 0.307 -0.513
60 64.5 0.645 0.190 0.906 -0.043 0.302 -0.520
65 63.5 0.635 0.198 0.899 -0.046 0.300 -0.523
70 62.0 0.620 0.208 0.889 -0.051 0.296 -0.528
75 60.0 0.600 0.222 0.875 -0.058 0.292 -0.535
80 59.5 0.595 0.225 0.871 -0.060 0.290 -0.537
85 58.0 0.580 0.237 0.860 -0.065 0.287 -0.542
90 56.5 0.565 0.248 0.849 -0.071 0.283 -0.548
95 56.0 0.560 0.252 0.845 -0.073 0.282 -0.550
100 56.0 0.560 0.252 0.845 -0.073 0.282 -0.550
105 56.0 0.560 0.252 0.845 -0.073 0.282 -0.550
123 8.0 0.08 1.10 1.10 0.041 0.367 -0.436
Table 3: Quantitative Raw and Processed Data of EDTA solution pH of 4.5
Time (min) % T T A A(CrIII) LogA(CrIII) Rate (Ms-1) Log(R)
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