Lewis vs. Brønsted vs. Precipitation reaction

Question

Is Ba2+(aq) + SO42-(aq) --> BaSO4(s) a Brønsted acid-base reaction and/or a Lewis acid-base reaction and why? I thought it was Lewis and not Brønsted but apparently that's wrong. Thanks!

Answer 1

By definition, Brønsted-Lowry acid-base reactions  require that a proton (H⁺) be exchanged in the reaction thus forming a conjugate acid and a conjugate base:

 

 HA  +  B -->      A^–      +    HB⁺

acid  base    conj. base    conj. acid

 

Clearly, this is not the case here, so your reaction does not fall under the Brønsted-Lowry definition of A/B reaction.

 

By definition, a Lewis acid base reaction will require a base to provide electrons to an acid in order to form a bond between the two compounds, thus creating a new compound called a Lewis adduct:

 

 A   +  :B   -- >   A–H

acid  base          adduct

 

In many ways, your reaction does look like a Lewis acid base reaction, where the acid is the barium ion (short on electrons), the base is the sulfate ion (electron-rich), and the adduct is the barium sulfate compound.

 

What looks "strange" to most students is the fact that most Lewis A/B reactions are presented as forming an adduct  containing a coordination bond that it significantly covalent. However, the bonding in barium sulfate is also presented in most textbooks as being essentially ionic. This is however not entirely the case, as most bonding does not purely categorize as either ionic or covalent, but is, in reality, a mix of both.

 

When you look at examples for Lewis acids and bases past most textbooks, you will find that:

• complex (an)ions, such as the sulfate ion, are indeed considered Lewis bases
• alkaline earth metals, as well as hydrogen ions and alkali metals, are considered hard Lewis acids.

 

So, to sum it up, your analysis is logical. The reaction you have does represent a Lewis acid-base reaction, although not  necessarily among the most typical ones encountered in textbooks.