If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:8:57

Brønsted–Lowry acids and bases

AP.Chem:
TRA‑2 (EU)
,
TRA‑2.A (LO)
,
TRA‑2.A.1 (EK)
,
TRA‑2.B (LO)
,
TRA‑2.B.1 (EK)
,
TRA‑2.B.2 (EK)
,
TRA‑2.B.3 (EK)

Video transcript

you've probably heard the term acid used in your everyday life but what we want to do in this video is get a more formal definition of an acid and particularly focus on the one that is most typically used although we'll see in future videos that there's other fairly common definitions of acids used as well beyond the one that we're going to see here but the one that we're going to focus on is the bronsted-lowry definition the bronsted-lowry definition of acids and bases and this is a picture of bronsted this is a picture of Larry and they came up with this acid-base definition in the 1920s so we're going to do the bronsted-lowry bronsted-lowry definition definition of acids and bases so according to them according to them an acid an acid is a proton proton or instead of writing proton we could actually write hydrogen ion donor so why is a proton and a hydrogen ion the same thing well in the most common isotope of hydrogen we would in its nucleus we would find just a proton and no neutrons and if it's neutral you would have an electron buzzing around jumping around in its orbital so you would have its electron jumping around in its orbital but if you were to ionize it you're getting rid of its electron so if you're getting rid of its electron so if you're getting rid of this all you're going to be left with is a proton so that's why a proton and H+ is usually referring to the exact same is referring to the exact same thing so that's what an acid is so what would it base be well you could imagine by this definition a base a base would be a proton would be a proton or you could say a hydrogen ion acceptor acceptor so let's make this a little bit more tangible with some examples so one of the stronger acids we know is hydrochloric acid and let me let me draw so it's a hydrogen having a having a covalent bond having a covalent bond with chlorine with chlorine with chlorine right over there and if we want to let's draw chlorines lone pairs so outside of this this will the the electron that is contributing to this pair in the covalent bond it also has it also has three other lone pairs it also has three other lone pairs just like that so if you were to take hydrochloric acid place it in an aqueous solution so it's in an aqueous solution right over here and actually an aqueous solution you'll see this written like that that just means it's in a solution of water so you could write like this you could write a hydrochloric acid in an aqueous solution if you want to make it a little bit more explicit you could say hey look this is going to be around some water molecules in its liquid form aqueous solution is means it's it's dissolved in liquid water so some water molecules in their liquid form so this is a water molecule whoops water molecule right right over here so an oxygen bonded to two hydrogen's and sometimes you'll see it written like this that it's in its liquid it's in its liquid form well what do you think is going to happen well I already said that this is a strong acid right over here so this is going to really want to donate protons it's really one it's really going to want to donate this hydrogen but not let the hydrogen keep its electrons so what's likely to happen here well the both of these electrons in this pair are going to be grabbed by this chlorine and then this hydrogen ion because it's electron was grabbed well this could be nabbed by some some water molecule passing by remember in a real solution you know just not like they know what to do they're just all bumping past to each other and based on how badly they want to do things there these reactions happen and so you could imagine this lone pair right over here well maybe it it's able to form a covalent bond with this hydrogen and so what's going to happen what's going to happen and I'll draw it with just an arrow because the this reaction favorably goes very strongly goes to the right because this is such a strong acid well then you're going to be left with you're going to be left with the chlorine is now going to have it it's three lone pairs that it had before and then it also gate it also grabbed these two electrons right over here it also grabbed those two electrons right over there so it gained an extra electron it now has a negative charge it is now the chloride anion so it has a negative charge and what about this water molecule well this water molecule you have your oxygen you have your hydrogen's you have your hydrogen's but now you don't just have two hydrogen's you grab two this hydrogen right over here maybe I'll do this hydrogen in a slightly different color so that you could keep so that you could keep track of it you have this hydrogen right over there and this lone pair this lone pair you can view it as now forming this covalent bond you had your other two covalent bonds to the other two hydrogen's and then you still have this lone pair right over here you still have that lone pair sitting right over there and what just happened well this water molecule just gained a proton it didn't this hydrogen did not come with an electron so if you just gained a proton you are now if you were neutral before you are now going to have a positive charge so what just happened you put hydrochloric acid in a water solution in an aqueous solution this thing has donated a proton to a water molecule and so what is the acid and what is the base here well when we look at the reaction this way we see we see that this is the acid the hydrochloric acid it's literally called hydrochloric acid and here water is acting as a base water is acting as a base as you can see water can actually act as an acid or a base so water is acting as a base now you might be saying okay this reaction goes strongly to the right hey but like you know I could imagine in certain circumstances where chloride might accept might accept a proton because it has this negative charge and you would be right this reaction goes strongly to the right but once an acid has donated its proton the thing that is left over this is called a conjugate base and I'll do the same color so this is the conjugate base of hydrochloric acid the chloride anion conjugate conjugate base base of hydrogen hydrochloric acid and this right over here is the conjugate acid because you could imagine this hydronium ion this could under the right circumstances donate protons to other things donate a hydrogen without donating electron to other things and so this is actually a conjugate the conjugate acid of h2o cons you get conjugate acid of water of a water molecule and as we'll see water can act as an acid or a base but this gives you a kind of a baseline of at least the bronsted-lowry definition of acids and bases and well actually one other thing I want to add and some in some books here so over here I say you put this in an aqueous solution you're going to form some hydronium sometimes you'll see it written like this and I'll I'll just write it a little bit a little bit sometimes you will see it like this so you have your hydrochloric acid and I won't draw the details this time in an aqueous solution so it's in a solution of water and they'll just draw the reaction going like this where they say hey you're going to be left with you're going to be left with some I some hydrogen ions these protons and you're going to be left with and you're going to be left with and actually we could say it's going to be in an aqueous solution aqueous solution and you're going to be left with some chloride anions some chloride anions and it's in an aqueous aqueous solution now this isn't incorrect but it's important to realize what they're talking about when they're talking about these hydrogen ions right over here we know that if you have the hydrogen ions in an aqueous solution they don't just don't hang out by themselves they get grabbed by a water molecule and they form hydronium so it's much more I guess it's it's much more close to the actual of what's happening is if you actually talk about hydronium forming as opposed to just the protons because these protons in an aqueous solution in a water solution they're going to be grabbed by a water molecule to form hydronium and that's why I did it the way this way up here