Chemical Kinetics Lab: The formaldehyde clock reaction
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Transcript Chemical Kinetics Lab: The formaldehyde clock reaction
Modified from P.W.W. Hunter, “Chemistry Laboratory Manual (CHEM 162)”, Michigan State University
Thermodynamics – if a reaction occurs
Kinetics – how fast the reaction occurs
and route the reaction takes
› Examples :
how fast unwanted chemical substances
break down (environmental)
how long before chemicals like drugs
metabolize in the body(biological)
how fast will this fuel ignite to run my car
(energy!)
Tells us how the reaction proceeds
Chemical reactions occur through a
series of steps (intermediate reactions)
Some steps are fast, some slow
Slowest step in mechanism determines
rate
› Can’t go any faster than slowest step!
A visible change occurs when a certain
point in the reaction is reached (typically
the end point)
Time is inversely proportional to rate
› The longer a reaction takes, the slower the
rate must be
› Something we can measure in the lab!
This is a complicated clock reaction
called the Briggs-Rauscher Reaction
Goes through 10 to 15 cycles
Chemical oscillator is due to changes in
iodine and iodide ion concentrations in
solution (several chemical equations)
B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V2, 1983, p248
A simpler example of a clock reaction
Step 1:
› HSO3- + H2O ↔ SO32- + H3O+
Step 2:
› H2O + HCHO + SO32- → CH2(OH)SO3- + OH-
Fast
Slow
Produce OH- (basic!) Can use pH indicator
phenolphthalein
› Colorless to pink at certain pH
Modified from P.W.W. Hunter, “Chemistry Laboratory Manual (CHEM 162)”, Michigan State University
B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V4, 1983, p70
Rates depend on initial concentrations
of reactants
A change in either one will affect
reaction rate
› Rate = k [formaldehyde]a [bisulfate-sulfite]b
The order of each reactant (a & b) add
to give the overall reaction order
For the general reaction X Z
Say a reaction is first order in X
› Then as the concentration of X doubles,
the rate also doubles
Say a reaction is second order in X
› Then as the concentration of X doubles,
the rate quadruples
Shows concentration dependence on rate
Rate = k [formaldehyde]a [bisulfate-sulfite]b
Hard to understand what to change and what
to hold constant… so let’s rearrange by taking
log of both sides
log(rate) = log(k) + a log[formaldehyde] + b log[bisulfate-sulfite]
If the concentration of F is held constant, then
(a log[F] ) is constant and we can plot log(rate)
vs (b log[BS]) to get order with respect to BS
The concentrations of each reactant will be varied, while holding the other
reactant constant. The time for each reaction to go to completion (in
seconds), via a color change of phenolphthalein, will be recorded in the table.
Trial
[F] (mL)
[BS-S] (mL)
1
5
2.5
Reaction
Time (s)
31.6
2
5
5
31.2
3
5
10
31.7
4
5
15
31.9
5
5
5
15.8
6
10
5
7.9
7
15
5
5.0
8
20
5
3.5
T. Cassen, J. Chem. Ed. 53(3), 1976, pg 197
Rate (1/s)
Trial
[BS-S] (mL)
log[BS-S]
1
2.5
0.399
Reaction
Rate (1/s)
0.03165
2
5
0.698
0.03205
-1.494
3
10
1.0
0.0315
-1.500
4
15
1.17
0.03135
-1.500
Trial
[F] (mL)
log[F]
5
5
0.698
Reaction
Rate (1/s)
0.0632
6
10
1.0
0.1265
-0.897
7
15
1.17
0.200
-0.699
8
20
1.30
0.285
-0.544
log(rate)
-1.499
log(rate)
-1.19
0
Log (rate)
-0.4
-0.8
-1.2
-1.6
0
0.5
1
Log[BS]
1.5
2
0
y = 1.0697x - 1.9472
Log (rate)
-0.4
-0.8
-1.2
-1.6
0
0.2
0.4
0.6
0.8
Log[F]
1
1.2
1.4
Reactant
Slope of line
Order
F
1
1
BS-S
0
Overall
reaction order:
0
1
Order of overall reaction is one.
Reaction depends directly on
concentration of formaldehyde, but does
not depend on bisulfite
B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V4, 1983, p70
Dependent vs independent variables
Calculation of slope
Using log function to “simplify” expression
Significant figures/digits
Fluctuation in measurement/results
Can perform experiment if desired and
use discovery style