Lewis acids and pH

[Anushi Patel 1J]

How do Lewis bases/acids affect the pH of a solution? If they are bases, they should cause the OH- concentration to increase or the H+ concentration to decrease. If they are acids, the opposite. However, I don't see how they can do that if they are simply electron acceptors/donors. Can someone explain the mechanism?

[Akhil Paladugu 3G]

Many Lewis acids are water reactive and will hydrolyze in water. Lewis bases will usually act as Bronsted bases as well and protonate in water thus raising the pH

[Iona Pay]

Lewis acids and bases are generally defined as electron pair acceptors and donators respectively. When a Lewis acid and base are combined, they form a Lewis adduct (for our purposes here, a single product of two distinct molecules).

So, to understand how Lewis acid/bases affect pH:

Think of a proton (H+) as a Lewis acid - it has a positive charge, and could accept a pair of electrons. When adding H+ to a solution, we either raise or lower a pH. For example, protonating ammonia (H+ + NH3 → NH4+) lowers the pH of the solution because it produces a basic Lewis adduct. If we protonate hydroxide (H+ + OH− → H2O), we produce water, which is neutral.