65 Strength of Bases
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Strong Bases
Strong bases either dissociate completely in solution to yield hydroxide ions, or deprotonate water to yield hydroxide ions.
LEARNING OBJECTIVES
Recognize strong bases and their chemical properties.
KEY TAKEAWAYS
Key Points
- In chemistry, a base is a substance that can either accept hydrogen ions (protons) or, more generally, donate a pair of valence electrons; it can be thought of as the chemical opposite of an acid.
- Strong bases are commonly, though not exclusively, formed from the hydroxides of alkali metals and alkaline earth metals.
- Superbases are stronger than hydroxide ions and cannot be kept in water; they provide examples of bases that do not contain a hydroxide ion (and are therefore strong Lewis and/or Bronsted-Lowry bases, but not Arrhenius bases).
Key Terms
- base: a proton acceptor, or an electron pair donor
- solvate: a complex formed from solvent molecules attaching to a solute
- dissociation: the process by which compounds split into smaller constituent molecules, usually reversibly
As discussed in the previous concepts on bases, a base is a substance that can: donate hydroxide ions in solution (Arrhenius definition); accept [latex]\text{H}^+[/latex] ions (protons) (Bronsted-Lowry definition); or donate a pair of valence electrons (Lewis definition). In water, basic solutions have a pH higher than 7.0, indicating a greater concentration of [latex]\text{OH}^-[/latex] than [latex]\text{H}^+[/latex].
Strong Arrhenius Bases
A strong Arrhenius base, like a strong acid, is a compound that ionizes completely or near-completely in solution. Therefore, the concentration of hydroxide ions in a strongly basic solution is equal to that of the undissociated base. Common examples of strong Arrhenius bases are the hydroxides of alkali metals and alkaline earth metals such as [latex]\text{NaOH}[/latex] and [latex]\text{CA(OH)}_2[/latex]. Strong bases are capable of deprotonating weak acids; very strong bases can deprotonate very weakly acidic [latex]\text{C-H}[/latex] groups in the absence of water.
Some common strong Arrhenius bases include:
- Potassium hydroxide ([latex]\text{KOH}[/latex])
- Sodium hydroxide ([latex]\text{NaOH}[/latex])
- Barium hydroxide ([latex]\text{Ba(OH)}_2[/latex])
- Caesium hydroxide ([latex]\text{CsOH}[/latex])
- Strontium hydroxide ([latex]\text{Sr(OH)}_2[/latex])
- Calcium hydroxide ([latex]\text{Ca(OH)}_2[/latex])
- Lithium hydroxide ([latex]\text{LiOH}[/latex])
- Rubidium hydroxide ([latex]\text{RbOH}[/latex])
The cations of these strong bases appear in the first and second groups of the periodic table (alkali and earth alkali metals). Generally, the alkali metal bases are stronger than the alkaline earth metal bases, which are less soluble. 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.
Superbases (Lewis bases)
Group 1 salts of carbanions (suc