Unit 4: Topic 8 - Introduction to Acid-Base Reactions

Question 1

Define what Brønsted-Lowry acids and bases are, and then give some examples
of reactions involving them.

Answer 1

In a chemical reaction, a Brønsted-Lowry acid will donate a proton, or in other words lose a proton as
the reaction progresses. For example, if H3O(+) will donate a proton in a chemical reaction
to form H2O (water), then H3O(+) would be the Brønsted-Lowry acid. Take the reaction
H3O(+) + OH(-) —> H2O + H2O. a Brønsted-lowry base will accept a proton, or in other words gain a
proton as the reaction progresses. In the previous example, OH- is the Brønsted-Lowry base, because
it accepts a proton to form H2O. Acids commonly have some H+ ion, or proton, to donate and bases commonly
have OH- to accept this proton. However, there are exceptions. If we examine the reaction:
NH3 + H2O –> NH4(+) + OH(-), we can see that NH3 has gained a H+ so it is a Bronsted-Lowry base.
H2O on the other hand has lost a H+ so it is a Bronsted-Lowry acid.

Question 2

Is water a Brønsted-Lowry acid or base? Depending on your answer, give an
example of a reaction where water acts as the acid or base (or both!).

Answer 2

Water can be both the Brønsted-Lowry acid and base; we determine whether or not it is the acid or base
depending on the property of the other reactants. If water is mixed with ammonia, a basic compound,
then by writing out the reaction NH3 + H2O —> OH- + NH4+, we see that H2O has donated a proton to
ammonia, and this water is the Brønsted-Lowry acid. If water is mixed with hydrofluoric acid, HF, then we
see HF + H2O —> F(-) + H3O(+), and that water now acts as a base because it accepts a proton.
Water is considered amphiprotic: depending on the circumstances, water can both donate and accept a proton.

Question 3

Can water act as both a Brønsted-Lowry acid and base in the same reaction?

Answer 3

Looking at the previous example, water could act as an acid if it donated a proton to form OH-, and it
could also act as a base if it accepts a proton to form H3O+. The reaction would be
H2O + H2O —> OH- + H3O+. This is actually a possible reaction! Although it proceeds nowhere
near completion, at 25 degrees Celsius about 6 in 10^8 water molecules proceed with this reaction.

Question 4

Define for an acid, what its conjugate base is, and for a base, what its conjugate
acid is.

Answer 4

When an acid reacts with water, the acid will donate a proton to the water to form H3O+, and
since it loses a proton, the remnant of this acid is the “conjugate base.” For example, when HCl is
dissolved in water, the reaction is HCl + H2O —> H3O(+) + Cl(-). HCl is the acid, and after donating the
proton becomes Cl-. Thus, Cl- is the “conjugate base” of HCl. In this reaction, H2O acts as the base, and
after accepting the proton becomes H3O+. H3O+ is the “conjugate acid” of H2O.
In every acid-base reaction, there is always an acid/conjugate base pair (HCl, Cl-) and a
base/conjugate acid pair (H2O, H3O+). The reactants will contain one “acid” and one “base,” while the
products will contain the “conjugate base” and the “conjugate acid.”

Question 5

HCl is a strong acid, and HF is a weak acid. Predict which acid would have the
stronger conjugate base.

Answer 5

Strong acids, such as HCl, desperately want to lose their H+ ion. In doing so, they become stable, such
as when HCl loses its H+ to form the conjugate base Cl-, a very weak base. Weak acids, such as HF, are
less likely to lose their H+ ion. For example, when HF loses its H+ to form the conjugate base F-, a weak
base is formed, but not as weak as Cl-. The same idea applies for bases: stronger bases will have weaker
conjugate acids. We can conclude that strong acids will have weaker conjugate bases than weak acids,
and strong bases will have weaker conjugate acids than weak bases.