Neutralization (chemistry) - Acid-alkali


An acid–alkali reaction is an neutralization reaction which is considered a special case of an acid–base reaction, where the base used is also an alkali. When an acid reacts with an alkali it forms a metal salt and water.

In general, acid–alkali reactions can be simplified to

OH−(aq) + H+(aq) → H2O

by omitting spectator ions.

Acids are in general substances that contain hydrogen ions (H+) or cause them to be produced in solutions. Hydrochloric acid (HCl) and sulfuric acid (H2SO4) are common examples. In water, these break apart into ions:

HCl → H+(aq) + Cl−(aq)
H2SO4 → H+(aq) + HSO−

An alkali is a base that contains a metal from column 1 or 2 of the periodic table (the alkali metals or the alkaline earth metals). Alkalis may be defined as soluble bases, which means they must be able to dissolve in water. In general, bases are defined as substances that contain hydroxide ion (OH−) or produce it in solution. Therefore, one may also speak of hydroxide bases that dissolve in water, and thus these would also be alkalis. Some examples, then, of alkalis would be sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)2), and calcium hydroxide (Ca(OH)2). Note that only hydroxides with an alkali metal — column 1 — are very soluble in water; hydroxides with an alkaline earth metal — column 2 — are not as soluble. Some sources will even say the alkaline earth metal hydroxides are insoluble.

To produce hydroxide ions in water, the alkali breaks apart into ions as below:

NaOH → Na+(aq) + OH−(aq)

However, alkalis may also have a broader definition that includes carbonates (CO2−
3) bonded to a column 1 metal, an ammonium ion (NH+
4), or an amine (NHx radical) as the positive ion. Examples of alkalis would then also include Li2CO3, Na2CO3, and (NH4)2CO3.

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