What determines the strength of a base?

Answer 1

Compounds which have pH values greater than 7 are all bases, with the strongest bases closest to the maximum pH value of 14.

Edit:

This is not entirely true. In fact, technically speaking, it is quite false (compounds do not have pH values). Base strength depends on a number of factors. The pH scale mentioned above is strictly for applications of acid/base chemistry in aqueous solutions. For our purposes it is much more useful to discuss pKa values. A lower pKa value indicates a stronger acid, meaning that a compound with a lower pKa will be more likely to lose a proton and become its negatively-charged conjugate base. When we speak about pKas, we are discussing this term in relation to the protonated form, the acid (thus the "a" in pKa). Alkanes such as hexane have pKas. We do not think of these compounds as being at all acidic, but there are ways to generate the deprotonated form of all kinds of molecules. The pKa of an alkane like hexane is somewhere around 50 (for reference, acetic acid is 4.76, and is quite easy to deprotonate. Remember that the pKa scale is logarithmic so a single unit increase in value indicates a ten-fold increase in bulk property - in this case relative proportions of molecules in acid/base equilibria). This means that under normal conditions, hexane will never ever spontaneously deprotonate. However, if we WERE to generate the anion of hexane (it is indeed possible), we would have an EXTREMELY strong base that will immediately abstract a proton from virtually any source. Therefore you can use the pKa scale to predict the relative strength of a base, which will have a larger pKa value for increases in this strength. pKa values are delineated through empirical data, but a few factors can be used to predict relative strengths. The presence of electron-donating substituents will increase the strength of a base. Since alkyl groups are slightly donating, the anion of tert-butane is stronger than that of n-butane, and will deprotonate n-butane if the two compounds are mixed. Polarizability and resonance stabilization are also contributing factors that are outside the scope of this discussion. Suffice it to say, a strong base will have a weak conjugate acid. That is: a molecule of sufficient strength to deprotonate another species does so because the conjugate acid it thus forms is more stable. Predicting base strength is useful only in relation to another base.

Answer 2

Acids and bases are not all of equal strength in producing H+ and OH- ions in solution. The amount of H+ (acid) or -OH (base) determines their strengths. And if the acid or base conducts electricity strongly, it is a strong acid or base. We use pH values to indicate this.

Answer 3

The strength of a base is determined by its ability to accept protons. Strong bases completely dissociate in water to release hydroxide ions, while weak bases only partially dissociate. The concentration of hydroxide ions produced by a base in solution is a measure of its strength.