In the movie 'Alien', they try to remove an organism from
the face of a crewmember, by cutting off one of its digits (fingers). Its blood falls to the floor, and then corrodes through the metal. They panic, realizing that if the blood continues
to on, through floor after floor, it will eat through the hull of the spaceship. They run down to the next level, and find
the acid has corroded through. On the next level down, they
look up and it appears that the acid has stopped corroding
the metal. Someone carefully touches the acid residue with
the eraser end of a pencil. It "burns" just a little then
stops.
We tend to think of acids as "tough", because they can corrode,
or even erode metal. Acids can be ranked according to strength.
There are at least two methods for defining acids and bases.
We will start with the Arrhenius definitions.
Arrhenius acids and bases-
Acid- A substance that donates a proton to water to form H3O+ is an acid.
Base- A substance that accepts a proton from water, leaving a hydroxyl OH- is a base.
Recall from the discussion of formal
charge, that oxygen prefers to have two bonds and two
lone pairs of electrons. Above we see H3O (three
bonds and one lone pair), with a
+1 charge on the oxygen, and we see OH (one bond and three lone pairs) with
a -1 charge on the oxygen. This is your first opportunity
to apply formal charge to
a real application.
Memorize H3O+ and
OH-. On a test, you can write draw these, and
then use them to remember the following two facts:
Replacing a lone pair with a bond increases the formal charge
of an atom by one.
Replacing a bond with a lone pair decreases the formal charge
of an atom by one.
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