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Bronsted-Lowry Model - Concept

Teacher/Instructor Jacqueline Spivey
Jacqueline Spivey

Ph.D.,U.C.Santa Cruz
Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

Bronsted-Lowry definitions of acids and bases are useful in non-aqueous solutions. An acid is defined as a substance that can donate an hydrogen ion while a base can accept a hydrogen ion. Water is an example of an amphiprotic substance which can act either as an acid or a base.

So let's talk about Bronsted-Lowry acids and bases, so previously we've discussed that there's arrhenius model of acids and bases and then there is the Bronsted-Lowry type of acid and base and that Bronsted-Lowry is more commonly now the definition that is used because it's more applicable as the situation doesn't have to occur in aqueous solution, so basically Bronsted-Lowry model defines acids and bases with respect to their ability to transfer protons and we saw protons here as H+. Okay so that will mean a Bronsted-Lowry acid is a substance either a molecule or an ion that can donate a proton to another substance okay so an acid with respect to Bronsted-Lowry is a proton donor.

A Bronsted-Lowry base then is a substance that can accept the proton that was donated by Bronsted-Lowry acid. Remember acid base pairs they're pair because you cannot have one without the other so here is the simple example. We have HCl which is a strong acid and water which will go to form a hydronium ion and a chloride ion okay so here HCl because it's a strong acid is a proton donor and so here water is going to work as the base and so remember it's working as a base because it has this nonbinding electron pair that's going to come and remove this proton from the chloride form the HCl so that makes H3O+ and then we have Cl– in solution as well.

Again remember acids and bases work simultaneously to transfer protons between them okay so the Bronsted-Lowry acid must have a hydrogen atom that it can actually loose as a proton like HCl did and the Bronsted-Lowry base be it a molecule or an ion must be capable of binding that proton via the nonbinding electron pair, like I just showed with water. Okay so within that there's is other type of definition that's important and that's that substances can be amphiprotic okay and that means they can act as an acid or a base depending on the situation so in this example I just showed you with water and HCl water acted as a base why because water has the nonbinding electron pair and it removed the proton from HCl here so HCl acted as the proton donor, water acted as the proton acceptor okay proton acceptor means that it acts as the base.

In another example though with water and ammonia NH3 water is going to act as an acid meaning that water is going to be the one that actually donate the proton. Let's look at that a little bit closer here so we have ammonia is going to act as the base because if you go back to your orbital diagrams you'll know that ammonia has this nonbinding electron pair on top so it is going to go and remove a hydrogen from water to produce the ammonium ion NH4+ and a hydroxide ion OH- okay so remember that water in specific is one of the ones that can act as an amphiprotic substance and that is some more on acids and bases.