OBJECTIVE
This
activity page will offer
- an
understanding of oxygen free radical formation
- an
introduction to oxidation and free radicals
- the
opportunity to observe the effect of oxidants and free radicals
- an
activity using vitamin C to retard the actions of oxidants
and free radicals
MATERIALS
- Vitamin C (1000 mg capsule)*
- Water-filled beaker
- Cotton-tipped applicator
- Apple
- Knife
- Stopwatch or clock
*TEACHER
NOTE
Vitamin C capsules are available
at most markets and drug stores.
PROCEDURE
- 1. Work in pairs. With your instructor's permission,
carefully slice off a section of apple so that the inner
"meat" is exposed to the surrounding air. Begin timing.
- Determine how long it takes for the apple to show signs
of browning. Record your results.
- Repeat steps one and two using different sections of the
same apple and record. Add the results you've obtained for
each of the two trials. Divide this number by 2 to obtain
an average time for discoloration.
- Add about 100 mL of water to a beaker. Hold a vitamin
C capsule over the beaker. Carefully twist open the capsule
allowing its contents to fall into the beaker. Use a cotton-tipped
applicator to mix the solution. This applicator will also
be used to "paint" the exposed apple tissue.
- Expose another slice of apple to the air.
- As soon as the apple is cut, use the applicator to "paint"
the exposed surface with the solution of vitamin C.
- Determine how long it takes for this apple slice to show
signs of browning. Record your results.
- Repeat steps 5, 6 and 7. Add the results you've obtained
for each of the two trials. Divide this number by two to
obtain an average time for the discoloration with vitamin
C.
QUESTIONS
- What happened to the appearance of the exposed apple
surface without Vitamin C?
- What caused this change?
- From where did this oxygen come from?
- Identify the substance that was "painted" onto the exposed
apple surface.
- Did the vitamin C covering affect oxidation? How could
you tell?
POOLING
RESULTS
Pool the class results. Then, find the average time for a
slice of apple to brown. Did your apple brown before or after
the class average time? What might account for differences
in the rate?
ON
YOUR OWN
The speed of a reaction is dependent upon the temperature
of its reactants: the lower the temperature, the slower the
molecules. At slow speeds, collisions are less frequent as
evidenced by a slower reaction rate. At high speeds, collisions
increase in frequency and reactions speed up. Design an experiment
that would illustrate the effect of temperature on the oxidation
of exposed apple.
EXTENSIONS
FREE RADICALS
The
existence of free radicals was first proposed as a way of
explaining collisions that are unlikely to occur. Consider
the formation of ozone (O3 ) from oxygen gas (O2). This reaction
is represented by the following equation:
In this reaction, three particles of oxygen must collide at
the same time to produce the ozone. As you might imagine,
a 3-way collision is highly unlikely. If we reexamine the
reaction in terms of free radicals, we can separate the reaction
into two distinct steps. During the first reaction, sunlight
splits oxygen gas into two free radicals of oxygen. No collision
between molecules is required for this first step.
In the second step, a collision
between only two particles needs to occur. As you might imagine,
a 2-way collision is much more likely event. Here, the oxygen
radical formed in the first step collides with an oxygen molecule
. The product of this 2-way collision is a molecule of ozone.
Work
in a team of three. Each team member gets a tennis ball. Position
yourselves about two meters apart from each other (at the
corners of an equilateral triangle). At a given signal roll
the three balls towards the center of the triangle. Your objective
is to have the collision of all three balls occur at the same
time. Then, try to collide only two balls at the same time.
Use this experience to explain the more plausible mechanics
of free radical reactions.
WEB
CONNECTION
The
Anti-Oxidant Vitamin
A summary of vitamin C's benefits, including its role as an
anti-oxidant.
Anti-Oxidants
Cut Free Radical Risk
An informative study on the connection between anti-oxidants
and free radicals.
What
Are Free Radicals and What do Antioxidants Do?
Discusses the relationship between free radicals, antioxidants,
and cell disorders.
The
activities in this guide were contributed by Michael DiSpezio,
a Massachusetts-based science writer and author of "Critical
Thinking Puzzles" and "Awesome Experiments in Light & Sound"
(Sterling Publishing Co., NY).
Academic
Advisors for this Guide:
Corrine Lowen, Science Department, Wayland Public Schools,
Wayland, MA
Suzanne Panico, Science Department, Fenway High School, Boston,
MA
Anne E. Jones, Science Department, Wayland Middle School,
Wayland, MA
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