An acid-base reaction is called a “neutralization reaction.” It consists of the transfer of a hydroxide ion (H+) from the acid to the base. They are therefore usually “displacement reactions,” but can also be combination reactions. The products are a salt and usually water. Therefore, they are also called “water-forming reactions.” An example is when you take an antacid to neutralize stomach acid from an upset stomach.
In a neutralization reaction, an equivalent amount of acid combines with a base to form equivalent amounts of salt and water. The acid and base neutralize each other. We say this even if they are not FULLY neutralized, i.e. the pH does not end up 7.0. In such cases, there is not enough acid or base to neutralize the other.
Some chemical equations for neutralization reactions are:
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NaOH+HCl?NaCl+H2O 2HCl+Ba(OH)2?BaCl2+2H2O HCl+NH3?NH4Cl
In the first two equations, elements are swapped between molecules. They are called displacement reactions. Note that neutralization reactions need not be displacement reactions or produce water, as the last equation shows. The last equation is instead a combination reaction.
Note that the salt product of a neutralizing reaction has a broader meaning than just table salt. It refers to a compound with two parts, adhering to each other through an ionic bond. One part is positively charged -- the other negatively -- and so they adhere. This adhesion comes from the loss of the positive charge of the H+ by the acid and of the negative charge of the OH- by the base.
Neutralization Reactions as First Aid
Warning labels on household cleaners may give instructions, in case of ingestion, to swallow a neutralizing agent until you can get medical help. For example, a toilet cleanser with hydrochloric acid may advise to counter ingestion with bases such as chalk, soap, egg whites or milk.
Conversely, the treatment for lye on skin is vinegar, an acid.
Another example is chemistry labs that are stocked with emergency bottles of baking soda (NaHCO3), a base, for acid burns. In the case of hydrochloric acid, the neutralization equation is HCl+NaHCO3?H2CO3+NaCl. Water isn’t formed yet. The carbonic acid (H2CO3) breaks down further, into CO2 and H2O.
Neutralization can be used to determine the concentration of a solution. It involves gradually adding an acidic or basic solution of known concentration very gradually, until the pH reaches neutralization. The match between acid and base therefore indicates how much was in the sample of unknown concentration. This procedure is called “titration,” or “volumetric analysis,” because the volume of known concentration determines the answer.