Concept Test 10

Internet Concept Test Chemistry 22
Prof. J. C. Baird
Marn-Yee Lee, Silvia Georges, Derek Cummings

Topic: Oscillating Chemical Reactions

A Question of Non-Linearity, Feedback, Periodicity.

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There are 7 questions.

Chemical Oscillations in the Laboratory and in Nature

Background material. It is not necessary to read this, but there is a wealth of material if you click here.

Oscillating chemical reactions are ubiquitous in nature. Taking a mechanistic view of life, the sleep-wake cycle, breathing rhythms, the heart beat, the rhythmic chirping of crickets are all no doubt the result of chemical reactions. Let us class all oscillations in chemical systems as circadian rhythms. The question is: when do chemical oscillations occur, how stable are they and how are they explained by our understanding of kinetics, equilibrium, acid/base reactions, free energy, electro-chemistry and complex ions? The following questions try to rely on our knowledge of chemistry for their answers, but we must use our intuitive notions as well.

Part of the biological energy cycle (the glycolytic pathway) involves nicotinamid adenine dinucleotide or NADH. This molecule fluoresces and provided the first observaton of oscillations in biochemical systems. These experiments can be done with yeast cells (A. Ghosh, and B. Chance, Biochem. Biophys. Res. Commun., 16, 174 (1964)). Below is a figure showing these oscillation of NADH and the oxygen concentration varying in time in a suspension of horseradish. Obviously, the chemical conditions have provided a stable oscillating system. What might cause this stability to be lost, or gained? This is the kind of understanding we would like to have.

Please select an answer to the following questions.

Question 1

How many chemical steps like A-->B are required to produce the necessary conditions for an oscillating reaction?:

a. One.
b. Two.
c. Three or more.

Reason for Answer:

Question 2

The buzz of the cicada's in summer in Norman, Oklahoma has a repetitive frequency of about 880 Hertz (cycles per second) at 109F. If the temperature during a cold wave drops to 106F the buzzing frequency is expected to

a. not change.
b. drop.
c. increase.

Reason for Answer:

Question 3

Feedback of a chemical sort may be necessary for oscillations (analogous to sound feedback from a microphone giving oscillations in an audio system). This can occur by:
(check all that apply)

The analogy is false.
one reaction product in a mechanism producing a reactant in another step.
one of the reactions in a mechanism producing heat (an exothermic reaction) therefore causing some of the rate constants to increase.

Reason for Answer:

Question 4

A topic of long standing interest in science is non-linear phenomena. Are non-linear phenomena unusual in chemistry?

The change in concentrations with time in most elementary chemical reactions (mechanism steps) are characterized by:
(check all that apply)

a linear relationship.
an exponential relationship.
an inverse relationship.
none of the above.

Reason for Answer:

The Model:

A solution of ferrous ion, bromide ion, malonic acid and some other components are mixed and produce an oscillating reaction that is conveniently seen by a color change. If you have time you can see the movie by clicking here.

Question 5

The BZ reaction involves oxidation steps between iron, bromine compounds and malonic acid (an organic acid) in which it is obvious that electrons flow from species to species. One of the first steps is

. Given this as a first step, if

was used as an initial reactant would the BZ reaction start?
(Note:

)

Yes.
No.
One cannot say.

Reason for Answer:

Question 6

In the Belusov-Zabotinsky oscillating reaction a color change from red to blue monitors the progress of the reaction. This color change is due to a complex of iron with phenanthroline,

, as part of an oxidation-reduction step

. The

is colored red while the

is blue. The blue color may be explained using the color wheel by

a. the energy levels of d-electrons of in the complex are closer together than those of .
b. the energy levels of d-electrons of in the complex are farther apart than those of .

Reason for Answer:

Question 7

From the BZ mechansism we find the following two steps

We can combine these in a couple of ways:

which reaction, a. or b., provides chemical feedback and is called "autocatalytic?"

Reaction a.
Reaction b.

Reason for Answer:

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