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Conjugate Acids and Bases - 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.

Conjugate acids and bases are acids and bases that differ only in the presence or absence of a hydrogen ion. In acid base equilibrium both the forward and backward reaction involve proton transfer. After donating a hydrogen ion, the acid in the forward reaction becomes the base in the reverse reaction. A strong acid or base has a weak conjugate and visa versa.

Hi guys. So let's jump into some more information about acids and bases and let's specifically talk about conjugate acid base pairs which is kind of a prelude to talking about acid base equilibrium and kind of a mixture of talking about equilibrium systems which we should have already kind of encountered at some point, so remember again that an acid is a proton donor and a base is a proton acceptor when we're talking about this system.

So in an acid base equilibrium, both the forward reaction, so the reaction moving to the right, and the backwards reaction, the reaction moving to the left, involve proton transfer. So here I have a basic example of kind of an acid base equilibrium problem so we have Hx here representing our proton donor and water representing our proton acceptor, the double headed arrows like indicate that this is an equilibrium so the black arrow indicates the forward reaction, the blue arrow represents the backwards reaction and so the products of this would be x- is a proton acceptor and H3o+ as our proton donor so what happened in the forward reaction is here water accepted this proton giving us x- and H3O+ going in the backwards reaction x- abstracted the proton from the H3O+ to make H2O and Hx here so you might need to take some time to look at this severally to convince yourself of that. But here I have it written again that for the forward reaction the Hx acted as the proton donor or the acid and water acted as a proton acceptor or the base and then for the backward reaction x- acted as the acceptor or the base and the hydronium ion H3O+ acts as the donor or the acid.

So when you have in a system an acid and a base that differ only in the presence or the absence of a proton, we call those a conjugate acid base pair so the sample I just showed you in that particular instance depending on which direction you were looking at the equation, the acid is Hx and then its conjugate base is x- so every acid has a conjugate base that's formed by removal of the proton so again in the previous example x- is the conjugate base of the acid Hx and then similarly every base has a conjugate acid that's formed by adding a proton so again in this in the previous example Hx would be the conjugate acid of the base x- so here is another simple equation so we can just kind of actually put in some molecules and see how it works. So we have NH3 ammonia acting as a base and it has these lone electrons this lone electron pair and we have water acting as an acid and forming an ammonium ion NH4+ and OH- and so in this case ammonia is the base, water acts as an acid ammonium is the conjugate acid of the base ammonia right so this lone pair removes a proton from water so we have the addition of a proton to make the ammonium ion so to make the conjugate acid of ammonia and then water is the acid in this case and so it when the proton is removed becomes OH- the conjugate base so the stronger the acid the weaker the conjugate base and the stronger the base the weaker the conjugate acid and if you kind of think about that that makes sense because you can't have something being just as good at donating a proton as it is at receiving it otherwise your system will kind of fall apart and that the bases of acid base conjugate pairs.