Acids and Bases
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An Acid is a substance that
produces H3O+ (H+) when it is dissolved
in water. It is a proton donor and an electron pair acceptor or a
species that donates protons. For example: HCl, NH4,
A Base is a substance that
produces an OH- when it is dissolved in water (Arrhenius). A
proton acceptor (Brønsted), or a electron donor. For example: NaOH,
Acids and bases were first identified as specific types of compounds because of their behavior in aqueous solutions.
Acids and bases relate to each other in Conjugate Pairs, somewhat
like husbands and wives. For every acid there is a conjugate base; and
for every base there is a conjugate acid. Just like every husband has a
wife and vice versa. The two members of the conjugate pair are related
by the donating and accepting of a single proton.
The equation below, Equation (1),
demonstrates a power struggle going on between the two couples and within them. There is a competition for
which base, H2O, (keep in mind that H2O can act
as an acid or a base because it auto-ionizes itself, meaning it gives
protons back and forth within itself, thus acting as both an acid and a
base;) See Equation (2).
Then A- will get the proton. The winner is the stronger base
which has a greater affinity for H+ and everything will go its way. This
base will determine whether the equation goes to the right or the left at
HA(aq) + H2O(l) <==>
H3O+(aq) + A-(aq)
To determine the strength of an acid or base can be difficult within
conjugate pairs. Strong acids have weak conjugate bases so the
equilibrium lies far to the right. A weak base has a lower affinity for
protons that water. So water wins the H+ ion as in the reaction in
Equation (1) above.
Of course, a weak acid has a strong conjugate base so the equilibrium will
go to the left, and the acid will not dissociate that much.
In order to determine how much the H+ will get, or the degree to which a
weak monoprotic acid will dissociate, we use:
Ka, the acid dissociation constant: Ka =
[H3O+][A-] / [HA]
The weaker the acid, the smaller its Ka and the less it will
dissociate. Strong acids completely dissociate into their component
ions in aqueous solution.
Note that [H2O is omitted from the Ka expression because the
concentration of H2O is so high in an aqueous solution and
changes so little, it is basically treated as a constant.
Below are examples of strong and weak acids:
Strong acids: HCl
Weak acids: NH4+
We can also take a look at this from the base's point of view using the
base dissociation constant, Kb. KB is a measure of the degree to
which a base will dissociate:
B(aq) + H2O(l) <==> BH+ +
The weaker the base, the smaller its Kb.
Kb always refers to the reaction of a base with water to from the
conjugate acid and the hydroxide ion.