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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.

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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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  • $\begingroup$ @above: >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 (BBr3) contains both a super-strong acidic site (the trivalent boron atom) and potentially basic sites (the lone pairs of the three bromine atoms). How do I identify the most acidic and basic site(s)? $\endgroup$ – Dhrubajyoti Ghosh Feb 4 at 10:26
  • $\begingroup$ @DhrubajyotiGhosh Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. I have converted your answer to a comment here. With a bit more rep, you will be able to post comments yourself. Please don't add answers that do not actually answer the question, they will be deleted and no-one will be any wiser. $\endgroup$ – Martin - マーチン Feb 4 at 11:00

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