Acid Base Theory

I’m just about to start teaching the acids and bases topic to my students. Acids and bases are interesting compounds because the terms ‘acid’ and ‘base’ are just labels we have given to explain different types of reactions.

The topic always starts with an introduction to acid base theory and usually covers the ‘Bronsted Lowry’ theory, as well as the ‘Lewis’ theory (of acids and bases). So what are these different theories and are there any differences between what is a ‘Bronsted Lowry’ acid and a ‘Lewis’ acid (or base)?

A Bronsted Lowry acid is considered to be a proton donor and a Bronsted Lowry base a proton acceptor. So in the following example:

HCl + H₂O –> H₃O⁺ + Cl^–

HCl is the proton donor (so it is the Bronsted Lowry acid, you would have probably imagined this to be the case) but H₂O is a Bronsted Lowry base – you may not have been expecting this. This theory was put forwards independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923.

You can also the Bronsted Lowry theory as follows:

NH₃ + H₂O –> NH₄⁺ + OH^–

In this example, NH₃ is the Bornsted Lowry base and H₂O the Bronsted Lowry acid. As with the previous example, ammonia would be expected to be a base, however, you may not have expected to see water act as an acid.

This means according to this theory, water can sometimes act as an acid and a base. It is said to be amphiprotic – it can both donate or accept a proton and act as an acid or a base.

A Lewis acid / base is subtly different to a Bronsted Lowry acid / base. Lewis acids will accept lone pairs of electrons, while Lewis bases will donate them. So in the above equation with HCl it is still acting as a Lewis acid and water is still acting as a Lewis base. 

OK, so, all seems in order. We would expect HCl to be an acid and it is an acid using both definition. The same can be applied to ammonia.

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What about this example though?

Fe²⁺ + 6H₂O –> [Fe(H₂O)₆]²⁺

In this example, no protons are donated or received however Fe2+ is a Lewis acid as it is accepting lone pairs from water (the Lewis base).

There are further IB acid base theories (that are not required at IB level) such as the Lux–Flood definition and the Usanovich definition

 And you can read more about there here:

https://en.wikipedia.org/wiki/Acid%E2%80%93base_reaction

Are there any other chemistry theories you have come across that seems to conflict with another or that changes the rules? If so, what are they? It would be really interesting to find out your thoughts – please feel free to post them below