A base in chemistry is a substance that can accept hydrogen ions (protons) or more generally, a substance that can donate a pair of valence electrons. In water, by altering the autoionization equilibrium, bases give solutions with a hydrogen ion an activity lower than that of pure water, meaning the solutions will have a pH higher than 7.0 at standard conditions.
Bases can be thought of as the chemical opposite of acids. A reaction between an acid and base is called neutralization. Bases and acids are seen as opposites because the effect of an acid is to increase the hydronium ion (H3O+) concentration in water, whereas bases reduce this concentration. Bases and acids are typically found in aqueous solution forms. Aqueous solutions of bases react with aqueous solutions of acids to produce water and salts.
A strong base is a base that dissociates completely in water. This means that all the base molecules break up into ions and solvate to water molecules. Therefore, the concentration of hydroxide ions in a strong base solution is equal to the concentration of the base. Common examples of strong bases are the hydroxides of alkali metals and alkaline earth metals such as NaOH and Ca(OH)2 (Figure 2). Strong bases are capable of deprotonating even weak acids; very strong bases are able to deprotonate very weakly acidic C–H groups in the absence of water.
Some common strong bases include:
- Potassium hydroxide (KOH)
- Sodium hydroxide (NaOH)
- Barium hydroxide (Ba(OH)2)
- Caesium hydroxide (CsOH)
- Sodium hydroxide (NaOH)
- Strontium hydroxide (Sr(OH)2)
- Calcium hydroxide (Ca(OH)2)
- Lithium hydroxide (LiOH)
- Rubidium hydroxide (RbOH)
The cations of these strong bases appear in the first and second groups of the periodic table (alkali and earth alkali metals) (Figure 1). The conjugate bases of very weak acids (acids with a pKa of more than about 13) are strong bases. When writing out the dissociation equation of a strong base, assume that the reverse reaction does not occur, because the conjugate acid of a strong base is very weak.
Group 1 salts of carbanions, amides, and hydrides tend to be even stronger bases due to the extreme weakness of their conjugate acids—stable hydrocarbons, amines, and dihydrogen. Usually these bases are created by adding pure alkali metals such as sodium into the conjugate acid. They are called superbases, and it is not possible to keep them in water solution. This is due to the fact they are stronger bases than the hydroxide ion, so they will deprotonate the conjugate acid water. For example, the ethoxide ion (conjugate base of ethanol) in the presence of water will undergo this reaction:
CH3CH2O− + H2O → CH3CH2OH + OH−
Some superbases include: