Hey it’s professor Dave, let’s talk about neutralization reactions.
We know that acids will generate hydronium in aqueous solution, and bases will generate
hydroxide in aqueous solution, because of the way these substances react with water
molecules.
And it is the concentration of hydronium or hydroxide in solution that we can use to describe
the acidity or basicity of that solution, whether using pH or pOH, or some other calculation.
We also know that pure water is a neutral substance.
This doesn’t mean there is no hydronium or hydroxide in solution, but in the autoionization
of water, in the rare instances that two water molecules collide and transfer a proton from
one to the other, this will always result in the production of precisely one hydronium
and one hydroxide.
So there must be precisely equal concentrations of hydronium and hydroxide, and therefore
any possibility for acidity or basicity is cancelled out.
A solution will only be acidic or basic if one of those ions is present in greater concentration
than the other.
But pure water is not the only solution that can be neutral.
Other solutions can also be neutral even if they contain a lot of hydronium ions and hydroxide
ions, they simply need to be present in the same initial concentrations such that they
neutralize.
This is what can happen when we react strong acids and strong bases with one another.
Protons from the acid and hydroxides from the base, if present in precisely the same
amount, will react completely to form water molecules as well as an ionic salt from the
counter ions, resulting in an entirely neutral solution, since neither of these substances
is an acid or a base.
We call this kind of reaction a neutralization reaction, which will occur between a strong
acid and strong base in aqueous solution.
Of course, this will only result in a neutral solution if precisely stoichiometric amounts
of acid and base are mixed, and even then, it will only be neutral if the acid and base
are both strong.
For example, as we can see here, hydrochloric acid and sodium hydroxide, if mixed in equal
proportions, will generate water and sodium chloride.
Furthermore, because hydrochloric acid has one proton and sodium hydroxide has one hydroxide,
these can be mixed in equimolar quantities.
If, for example, the base had two hydroxides, like calcium hydroxide, we would need twice
as much acid as base to generate a neutral solution, since each calcium hydroxide would
need two protons from two acid molecules to neutralize.
But as long as this is the case, a strong acid and strong base will react to form a
neutral solution.
If instead, a strong acid like hydrochloric acid reacts with a weak base like ammonia,
the resulting solution will be relatively acidic, because the acid will completely protonate
the base, giving us the ammonium ion, and the ammonium ion is weakly acidic, so it will
generate hydronium in solution.
Similarly, a weak acid like acetic acid and a strong base like hydroxide will generate
a slightly basic solution, because hydroxide will completely deprotonate acetic acid, leaving
the acetate ion, which is just basic enough to react appreciably with water molecules
in solution, generating a small amount of hydroxide and making the solution weakly basic.
And then lastly, when a weak acid and weak base interact, it becomes complicated to predict
what kind of solution will result, and we typically have to do some math.
We might get an acidic solution, a basic solution, or even a neutral solution, depending on which
is slightly weaker, the acid or the base.
But the key is to understand that in a neutralization reaction, protons, which dictate acidity,
and hydroxides, which dictate basicity, are reacting with one another to form water molecules,
which are neutral.
And while the strength of the acid and base will determine whether the resulting solution
is acidic, basic or neutral, these will all qualify as neutralization reactions.
Let’s check comprehension.