Which substances are always produced in an acid-base neutralization reaction?

1 Answer
Mar 27, 2016

Typically, water (#"H"_2"O"#) is always a byproduct of an acid-base reaction.


The answer to this actually depends on how you define an acid and a base. Under the classic Arrhenius theory, an acid is defined as a substance that provides #"H"^+# ions in solution, and a base is defined as a substance that provides #"OH"^-# ions in solution.

A good example of a reaction whose outcome would be well-predicted by the Arrhenius theory would be the reaction of hydrochloric acid and sodium hydroxide.

#"HCl + NaOH"->"NaCl + H"_2"O"#

Let's take a closer look at what goes on here. Firstly, as an acid is defined as a substance that provides #"H"^+# ions, we will predict that an #"HCl"# molecule, in solution, will dissociate into an #"H"^+# ion, and a #"Cl"^-# ion.

#"HCl"->"H"^+"+ Cl"^-#

Likewise, since we have defined a base as a substance that will provide hydroxide ions, we will predict that a #"NaOH"# molecule will dissociate into a #"Na"^+# ion, and an #"OH"^-# ion.

#"NaOH"->"Na"^+"+ OH"^-#

Chloride ions will have a stronger attraction to sodium ions than to the hydrogen ions, so they will ionically bond, forming sodium chloride. Similarly, hydrogen ions and hydroxide ions will combine to form water.

#"HCl + NaOH"->"NaCl + H"_2"O"#

Interestingly, under this definition of acids/bases, water is just about always formed as a byproduct, no matter which particular acid/base we're looking at. This is because at some point, we have hydrogen ions combining with hydroxide ions, which forms water.

However, in the real world we sometimes have compounds acting as bases even though they don't dissociate into hydroxide ions. In this situation we define a base simply as a proton acceptor and an acid as a proton donor, which is the premise of Brønsted-Lowry acid-base theory.

Thus under Brønsted-Lowry theory we sometimes have reactions where water is not a byproduct. An example could be the reaction of ammonia and hydrogen chloride gas to form ammonium chloride:

#"NH"_3" + HCl"->"NH"_4"Cl"#

So, to sum it all up, under Arrhenius theory water is always a byproduct of an acid-base reaction, but this is not necessarily true if you expand your definitions a bit.