6 Water, water, Everywhere
6.2 Acid/Base Theory
6.5 pH and pOH
6.10 Weak Acids and Bases
6.11 The Water Around Us
One of the most fundamental reactions in chemistry is the acid/base neutralization reaction. Perfectly equivalent amounts of acid and base are allowed to react. If done stoichiometrically, both the acid and the base are neutralized to yield two new chemical species. In general we say "an acid plus a base makes water and a salt". The most over used reaction in academia to show this is...
HCl(aq) + NaOH(aq) → H2O(ℓ) + NaCl(aq)
Notice that this a strong acid (HCl) reacting with a strong base (NaOH). In this situation, the products are always water (H2O) and a salt, which in this case is sodium chloride (NaCl). If we actually write this in total ionic form where we show all the separated, dissolved ions, we get
H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) → H2O(ℓ) + Na+(aq) + Cl–(aq)
And now we can cancel any species that is unchanged on each side of the equation. These types of ions are called spectator ions because they aren't taking part in the reaction (a player or playa as some would say), they are "spectating". The aqueous Na+ and Cl– are both spectator ions and are removed to go the total net ionic equation here
H+(aq) + OH–(aq) → H2O(ℓ)
This net reaction is the same for pretty much every strong acid that reacts with a strong base. Why? Because every ion that is not H+ or OH– is a spectator and will be cancelled out on each side of the reaction.
Stoichiometry is very much in play here. Realize that this could very well be a limiting reactant type of problem and there could be left over acid or base. It is only a true neutralization when the two species are matched in a perfect stoichiometric ratio, which in this case is 1:1. The next section is about matching these amounts in a purposeful and analytical way - a titration.