# How to identify lewis acids and bases

What is the general methodology to identify Lewis acids and bases in an acid base reaction? Do we need to draw structures to identify which species is the electron donor and which one the acceptor, or is there any other way of doing this?

If you consider $\ce{HClO_4}$, oxygen $\ce{O}$ has lone pair(s), so there is a confusion whether it can donate the pair to other species or not. So in that case, although it is an acid, one may say it is a base. Similarly, in $\ce{H2S}$, sulfur $\ce{S}$ has got lone pairs and I am confused whether it will act as base or acid.

You need to draw the structure and identify the most acid and most basic site(s). Notice how I didn't say "the most acid molecule". Boron tribromide ($\ce{BBr3}$) contains both a super-strong acidic site (the trivalent boron atom) and potentially basic sites (the lone pairs of the three bromine atoms).
Whether a molecule (e.g. $\ce{H2S}$) will act as an acid or as a base depends on the reagents.
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