Chemical Kinetics Lab: The formaldehyde clock reaction

Report
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


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