Sunscreens
(sample OCW assignment)
You have entered
the realm of Our Chemical World. What does that mean?
When you come here you will typically read a brief introduction,
go to a web site related to a particular topic, then return
here to answer a few simple questions. Great news! This activity
earns a few points of extra credit while helping you solidify
your understanding of material from Chapters 4 and 5.
To receive extra
credit points for this OCW, you must fill out the next section
completely AND answer ALL of the questions found below. Your
answers must include explanations when they are requested.
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The web site, What’s That
Stuff, discusses and explains the chemistry of some everyday
materials whose “active ingredients” are simple
ionic and covalent compounds. This assignment
is focused on the ionic and covalent compounds used in sunscreens.
Click on the link to learn about
sunscreens, then come back and answer the questions that follow.
Remember, the answers are not necessarily all found in the article,
but you should be able to combine the information in the article
with the chemical knowledge you now have, information from your
lecture notes, and information from your textbook to answer
the questions.
Sunscreens
| Some
materials used as sunscreens are ionic compounds that are
prepared in a lotion or cream base. Let's look at the names
and formulas of a few of these ionic compounds. |
| 1. |
Zinc
is a transition metal. In C101, we don’t worry about
predicting the stable ions that are formed by metals in
the d-block. However, if you know that zinc oxide is a compound
that has 1 oxide ion for every zinc ion, what must be the
charge on the zinc ion? Explain. |
| |
|
| 2. |
Chemists use
sets of rules to name chemical compounds. However, substances
that have been known for a long time are often known by
a common name. For example, the systematic name for water
(H2O) is dihydrogen oxide.
Titanium dioxide, a white pigment that is used in paints
and sunscreens, is the common name for TiO2.
Although the common name implies that TiO2
is a covalent compound, its melting point is 1855oC.
Based on what you know about properties of ionic and covalent
compounds, which system (ionic or covalent) should be
used to name TiO2? (Hint:
Do you need a Roman numeral?) What would the correct systematic
name be? |
| |
|
| 3. |
Explain how a glob
of zinc oxide or titanium dioxide cream on your nose keeps
you from getting a sunburn. |
| |
|
| |
|
PABA,
an acronym for p-aminobenzoic
acid, is a compound that
was used for years as a sunscreen. Its structure is shown
here. It is a covalently-bonded compound. If you shop
the aisles at your local drugstore, you are not likely
to find sunscreens containing PABA, although you may find
many that highlight the fact that they are "PABA-free." |
|
PABA
p-aminobenozoic
acid
 |
|
| PABA
has been replaced by other compounds such as octyl
methoxycinnamate
(aka OMC or Parsol) shown here. Use the structural formula
shown below to answer the questions about OMC. |
| |
|
octyl
methoxycinnamate
(aka OMC or Parsol) |
| |
What's That Stuff, a web site maintained
by the American Chemical Society, has links to other fun stuff
like Cheez Whiz, new car smell, and teeth whiteners. Bookmark
it and check it out sometime for fun. (Caution: Geek alert!)
What's
That Stuff
You may change your mind as often as you wish.
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button.
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