Chemistry Reference
In-Depth Information
OH
-
(aq)
+ NH
4
+
(aq)
water + ammonia
hydroxide ion + ammonium ion
As you can see, the ammonia is acting as a Brønsted-Lowry base, be-
cause it is accepting a proton from another species, the water. What is wa-
ter acting like in this equation? If you said a Brønsted -Lowry acid, you are
correct. Water is donating a proton to the ammonia, to become the hy-
droxide ion.
What do you think will happen to the hydroxide ion (OH
-
) on the prod-
uct side of this equation? This is a reversible reaction, so, as more and
more hydroxide and ammonium ions are produced, some of them will have
effective collisions with each other and start the reverse reaction. The re-
verse reaction can be represented by the following equation:
OH
-
(aq)
+ NH
4
+
(aq)
H
2
O
(l)
+ NH
3(aq)
Notice that we have an acid and a base in the reverse reaction as well.
In this reaction, the hydroxide ion is accepting a proton from the ammo-
nium ion to form water, so it is acting as a Brønsted -Lowry base. The am-
monium ion, which is donating a proton to the hydroxide ion, is acting like
a Brønsted -Lowry acid. When we write both sides of the reaction together,
the equation looks like the following:
H
2
O
(l)
+ NH
3(aq)
H
2
O
(l)
+ NH
3(aq)
OH
-
(aq)
+ NH
4
+
(aq)
So, a reversible reaction of this type shows two acids and two bases. We
will label the acids and bases, but first we must introduce two more defini-
tions. A conjugate acid is the substance that is left after a base accepts a
proton. A conjugate base is the substance that is left after an acid donates a
proton. Using these definitions, let's label the equation.
H
2
O
(l)
+ NH
3(aq)
OH
-
(aq)
+ NH
4
+
(aq)
acid base conjugate conjugate
base acid
Typically, the acid and base will be labeled on the left side of the equa-
tion. In our example, H
2
O is the acid, because it is donating a proton on the
left side of the equation. OH
-
is the conjugate base, because it is the species
that is left over when the acid donates a proton. The NH
3
is acting as a base
on the left side of the equation, because it is accepting a proton. The NH
4
+
is the conjugate acid of the base, because it is the species that is formed
when the base accepts a proton.
