Chemistry for Every Man: Acid - Base Reactions

Acids and bases react together to create a salt and water.  Thus, for hydrochloric acid and sodium hydroxide,

HCl + NaOH => NaCl + H2O

Let's consider the equation above in detail,

Hydrochloric acid, which is a water solution of hydrogen chloride gas, is represented by one hydrogen atom, H, attached to one chlorine atom, Cl.  Sodium hydroxide, a base, is represented by one sodium atom, Na (from the Latin, Natrium), attached to one oxygen atom, O.  The oxygen atom, in turn, has a hydrogen atom attached to it.  The portion of sodium hydroxide -OH is considered the part of the molecule that identifies NaOH as a base.

The sequence of the reaction becomes more obvious if we draw it,

H-Cl + Na-OH => Na-Cl + H-OH  with H-OH being written H2O for simplicity.

The above example is excellent in its simplicity, but as you know, life isn't generally simple in anything, and that applies to chemistry.  For instance, consider this reaction,

H2CO3 + NaOH => NaHCO3 + H2O

What has happened here?  Well, there is an acid reacting with a base here, and water and something else is produced, apparently a salt.  So what's more complicated about that? 

H2CO3 is also called  carbonic acid, and it is found in soda water.  It is really carbon dioxide, the gas you breathe out, combined with water. This acid has two hydrogens.  In our reaction, above, we chose to react only one of those two hydrogens to form our salt.  The second hydrogen could still be reacted.  Thus NaHCO3 could be viewed, in one sense, as an acid.  But carbonic acid is a very weak acid, whereas sodium hydroxide is a very strong base.  Hence, NaHCO3 is also a basic salt.  See what I meant when I said it wasn't so simple?  NaHCO3 is known by almost everyone as either sodium bicarbonate or baking soda.  The “bi” means it contains a hydrogen.

Sodium bicarbonate can be further reacted with another sodium hydroxide,

NaHCO3 + NaOH => Na2CO3 + H2O

You can see this, perhaps a little more clearly if I write it,

H-Na-CO3 + Na-OH => H-OH + Na-(CO3)-Na

We have taken carbonic acid, a very weak acid, and reacted it with two molecules of very strong base, thus the resultant Na2CO3, or sodium carbonate, is a very strongly basic salt.  It's common name is “washing soda,” as it makes a very good cleansing agent in clothes washers, and it is derived from sodium, thus “soda.”

One more item for this article.  Acids plus bases result in salts and water, yes.  But what if the base is a basic salt, or a salt made by reacting a strong base with a weak acid, and you were to pour a strong acid in with the basic salt?  The strong acid will “kick out” the weaker acid portion, replacing it and regenerating the weak acid as an acid.  Thus,

HCl + NaHCO3 => NaCl + H2CO3

In this case, the H2CO3 will go further, generating water and carbon dioxide bubbles.  Thus, if, for safety reasons we substitute hydrochloric acid with acetic acid or vinegar (which is also stronger than carbonic acid), we will get a frothy bubbling up of our solution.  We could write this as,

CH3COO-H + Na-HCO3 => CH3COO-Na + CO2 + H2O

Try, in a glass, putting in a half-teaspoon of baking soda, and adding a little vinegar, and you will have carried out the above reaction.

I might add that a similar thing happens if strong acids are involved with weak bases.  The resultant acidic salt, if reacted with a stronger base, will kick out the weaker base.