- Weaker acids/bases are more thermodynamically stable than stronger acids/bases
We can also use the relative stabilities of charged molecules to predict the relative strengths of acids and bases. Let’s consider a generic reaction between an acid, HA, and a base, B-.
Notice that the left side of the equilibrium has an acid (HA) and a base (B-), as does the right side (the conjugate acid of B-, BH, and the conjugate base of HA, A-). Let’s imagine that HA is a stronger acid than BH. That is, it is more thermodynamically favourable for HA to give up H+ than for BH to give up H+. The result will be that the right side of the equilibrium, where HA has released H+, will be favoured. The stronger of an acid HA is, the more thermodynamically favourable it is for HA to release H+ and become A-, and thus the more thermodynamically unfavourable it is for A- to regain H+ (i.e. act as a base) to become HA. That is, the stronger an acid is, the weaker its conjugate base. We can also conclude that weaker bases are more thermodynamically stable. Notice then that this means if HA is the stronger acid, that both the weaker base (A-) and the weaker acid (BH) are on the same side of the equilibrium, and that this side of the equilibrium is favoured. This is generally true: in an acid base equilibrium, the side of the equilibrium with the weaker acid and the weaker base is favoured. Conversely, if BH was a stronger acid than HA, then B- would be the more stable/weaker base and the left side of the equilibrium would be favoured.
If you were to carry out the same thought experiment with a generic reaction between an acid, HA+ and a base, B, then you would see that we can equivalently conclude that weaker acids are more thermodynamically stable.
This knowledge enables us to predict the relative strengths of acids and bases. If we have two positively charged acids, the more stable molecule is the weaker acid and the less stable molecule is the stronger (more reactive) acid. If we have two negatively charged bases, the more stable molecule is the weaker base and the less stable molecule is the stronger (more reactive) base.
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