Acids and bases Notes
Acids and bases are defined by several different models. Overall,
solutions are said to be “acidic” if they have more acid molecules
than base molecules, “basic” if they have more base molecules than
acid, and “neutral” if they have equal quantities of both.
1. Arrhenius definition of acids and bases: Acids are compounds
that give off H+ ions (also called hydronium ions, H3O+ ions, or simply
“protons”) when you dissolve them in water. Bases are compounds
that give off OH- (hydroxide) ions when you dissolve them in water.
Arrhenius acids almost always start with the letter “H” in their
formulas – this is the source of the H+ ion that comes off when you
dissolve the compound. Common examples include the following:
HNO3(l)  H+(aq) + NO3(aq)Arrhenius bases always have “OH” in their formulas,
indicating the presence of the hydroxide ion. Common
examples include:
NaOH(s)  Na+1(aq) + OH-1(aq)
2. Brønsted-Lowry definition of acids and bases: Brønsted-Lowry
acids are compounds that give H+ ions to other compounds.
Brønsted-Lowry bases are the compounds that accept these H+ ions.
Unlike with the Arrhenius definition of acids and bases, you can’t
have a Brønsted-Lowry acid by itself – it needs a base to give its
H+ ion to. As a result, we have come up with the idea of
conjugate acid and base pairs.
Let’s take a look at the following acid-base process::
HNO3 + H2O  H3O+ + NO3-1
Taking a look at this process, you can see that HNO3 has given a
H+ ion to H2O. Thus, we can define HNO3 as the acid and H2O as
the base. Likewise, if you look at the right side of the equation, H3O+ can give a H+ ion to NO3-1 – this allows us to define H3O+ as an
acid and NO3-1 as a base.
Looking a little deeper, you can see that HNO3 is an acid and that
NO3-1, which is based on it, is a base. As a result, we say that
“HNO3 is an acid, and NO3-1 is its conjugate base.” Likewise, we
can say that “H2O is a base and H3O+1 is its conjugate acid.”
Together, these are called “conjugate acid-base pairs.”
More examples (have them identify the conjugate acid-base pairs).
HSO4-1 + H2O  H2SO4 + OH-1
H2SO4 + H2O  HSO4-1 + H3O+
As you’ve seen in the example above, H2O can act as both an acid
and a base. As a result, we say that it’s amphoteric (which
means simply that it can be either an acid or a base, depending on
what it’s reacting with at the time).
3. Lewis acids and bases: Compounds that accept electron pairs are
Lewis acids and compounds that donate electron pairs are Lewis
bases.
The reason this definition was invented is because the definition of
acids and bases was formerly not very well served when speaking of
a class of compounds called “amines” (of which ammonia is the one
we’re currently most interested in). You see, when ammonia is
dissolved in water, it behaves as a base despite the fact that it
doesn’t have OH-1 ions to give away to anything.
 This was explained by Gilbert Lewis in the following way:
The electron pair on
ammonia is donated to
water and pulls of an H+
ion. Since ammonia
donates an electron pair
and water accepts it,
ammonia is a Lewis
base and water is a
Lewis acid
 An important feature of Lewis acid-base theory: An Arrhenius or
Brønsted-Lowry acid will always be a Lewis acid as well – all
Lewis did was to increase the definition of “acid” and “base” to
include amines while keeping the other acids and bases in the
same categories.
Properties of acids and bases:
It’s fairly easy to tell whether something is an acid or a base based on
observable properties. Here are just a few properties put into a simple
table that you can easily remember. Please, however, always remember
that these properties are guidelines and aren’t always going to be true for
all acids and bases!
Property
taste
smell
reactivity
conductivity
texture of
solution
Acid
sour (lemon or
vinegar)
may burn nose
with metals to
form H2
in water
sticky
Base
Neutral
bitter (baking soda,
sweet, salty,
Alka Seltzer)
bitter, no flavor
usually none
none, or a
(except NH3)
“chemical smell”
oils and organic
varies by
compounds
compound
in water
some do
oily, sticky,
slippery/soapy
watery, slippery