BrØnsted – Lowry concept of bases

 Yesterday, we looked at the BrØnsted – Lowry concept of bases and we were able to establish in that study that the whole idea was hinged on the concept of proton transfer. This implies that a base must be able to ACCEPT  a proton from an acid. We saw in that study that water was acting as an acid in that sense. This further buttresses the point that acid – base behavior could be circumstantial. That is, a substance may act an acid one moment and as a base the next moment.


In the formation of H3O^+, we see that water acts a base in that particular instance. Look at the reaction;

HA(aq) + H2O(l) --> H3O^+(aq) + A^-(aq)

We encountered this reaction countless times in the last few weeks. We can see that in these cases water was acting as a base, accepting a proton from the hypothetical acid HA.


Now recall the reaction that we saw yesterday;

NH3(aq) + H2O(l) --> NH4^+(aq) + OH^-(aq)

We can see that here water is acting as an acid, releasing a proton to ammonia. This further lends credence to the idea that I am trying to buttress in today’s class; ACID - BASE BEHAVIOR CAN BE MERELY CIRCUMSTANTIAL.


We would have to look at the reaction and what is playing out in it to know what substance acted as acid or base as the case may be. Also, every substance could give rise to a chemical specie that could act as an acid or a base. We would be developing this concept more fully in tomorrow’s lesson. See you there!


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