CHM120H5 Study Guide - Midterm Guide: Reaction Rate, Isopropyl Alcohol, Sarvepalli Radhakrishnan

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9 Feb 2016
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Experiment 5
Oxidation of Ethanol by Chromium(VI): A Kinetics Study
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Purpose
In this work, we studied the oxidation of the CH3CH2OH by HCrO4- (Eq.1).
3CH3CH2OH + 4HCrO4- + 16H3O+ → 3CH3COOH + 4Cr3+ + 29 H2O (1)
Rate = kobs[HCrO4-]x(2)
By monitoring [HCrO4-] during the reaction, we explored the reduced rate law (Eq. 2), including
the reaction order (x), and the observed rate constant (kobs), as well as the reaction half life.
Experimental Methods
The reduced rate law of the CH3CH2OH - was investigated according to the procedure given
in the lab manual (pages 58-60)1 and were followed correctly and thoroughly. The calculations to
complete the reaction equation also followed the same procedure1 and are presented in Appendix
I.
Results
The HCrO4- solution was diluted with excess HCl solution, which produced a red-orange
colour, this was as described in the lab manual1. For every 10mL of diluted HCrO4- solution, KI
ions were added, the solution had produced a red-brown colour and after being titrated with
Na2S2O3 solution, it had produced a dark yellow colour. Moreover, with the addition of the
aqueous Thyodene, the solution was noticed to be a dark blue-purple colour. Finally, in the final
titration with Na2S2O3 solution, the solution was a aqua green colour. These steps were
performed as instructed in the lab manual1 to maintain accuracy. Also, these observations are
consistent with what was recorded in the data sheet, under Appendix II.
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The HCrO4- concentration reduced from the initial 0.011M to 0.003M after 48 min of the
reaction (Table 1; see Appendix I for the calculations). We explored the reaction order by the
plots of [HCrO4], ln[HCrO4] and 1/[HCrO4] against time (Figures 1-3, respectively; see Table 1
for data). The R2 values from these plots are 0.940, 0.963 vs. 0.910, respectively. This indicates
that the reaction order is 1. Similarly, Finlayson et. al. also found that the reaction is a first-order
reaction.2 The following table and figures further explain this.
Table 1. Calculated Concentrations of HCrO4- and time respectively
Reaction Time
(minutes)
[HCrO4-] (mol/L) ln[HCrO4-] (mol/L) 1/[HCrO4-] (L/mol)
0 0.011 -4.509860006 90.90909091
8 0.0077 -4.86653495 129.8701299
16 0.0082 -4.803621125 121.9512195
24 0.0062 -5.083205987 161.2903226
32 0.0049 -5.318520074 204.0816327
40 0.0041 -5.496768305 243.902439
48 0.003 -5.80914299 333.3333333
0 10 20 30 40 50 60
0
0
0
0.01
0.01
0.01
0.01
R² = 0.94
Time (minutes)
[HCrO4-] (mol/L)
Figure 1. The graph represents the results of the reaction and its trend line. X is time (t) with the unit minutes, and Y
is the [HCrO4-] with the unit M. There is a decreasing trend, as time increases the [HCrO4-] decreases. Thus the
equation, y=-0.000154x+0.01 and an R2 value of 0.94011.
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