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
Acid
Base
Neutral
sour (lemon or bitter (baking soda,
sweet, salty,
taste
vinegar)
Alka Seltzer)
bitter, no flavor
usually none
none, or a
smell
may burn nose
(except NH3)
“chemical smell”
with metals to
oils and organic
varies by
reactivity
form H2
compounds
compound
conductivity
in water
in water
some do
texture of
oily, sticky,
sticky
slippery/soapy
solution
watery, slippery
pH and pOH notes
Indicators are chemical compounds that change color when the acidity of
the solution changes.
 Historically, indicators have been naturally occurring pigments
occurring in vegetables and flowers.
 More recently, indicators are manmade. These include:
o Litmus: It turns red in acid and blue in base.
o Phenolphthalein: Clear in acid and pink in base.
pH Scale
To figure out how acidic a solution is, we use the pH scale:
Using the pH scale:
 Acids fall between pH = 0 and pH = 7.
 Bases fall between pH = 7 and pH =14
 Neutral solutions have a pH of exactly 7.00000 (though people
sometimes – erroneously – refer to solutions as “neutral” if they’re
between 6 and 8).
Calculating pH:
pH = -log[H+]
First the [ ] mean the concentration in moles per liter as we just did
last unit.
The rest is a calculator thing.
Where [H+] is the molarity of the H+ ion in the solution. For strong acids,
[H+] is the same as the molarity of the acid solution.
Sample: What’s the pH of a 0.0045 M HCl solution?
Type in .0045 on your calculator then hit log not ln, definitely log! Then
change the sign, now you have the pH.
What is water called when it can be an acid and a base?
_______________
Now to see all of this in a solution
The water is in equilibrium with hydrogen ions and hydroxide ions
H2O  H+ + OHThe Kw means the concentration of water. So I guess that the K
means concentration with a K
Kw = 1.00 x 10-7
As we know from our studies of equilibria, this means that whenever water
is present, there will be some H+ and some OH- ions present. That means
that even if there is a very high concentration of OH- ions, there will still be
a very tiny amount of H+ ions present that we can use to find the pH.
Now pOH, this means the opposite of the pH.
pOH = - log[OH-]
pH + pOH =14 so subtract instead of doing the math if you have the pH.
Sample: What’s the pH of a 3.41 x 10-4 M NaOH solution?
First find the pOH as just given to you. Then subtract it from 14 to find the
pH. Easy?