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# Heterogeneous equilibrium

## Video transcript

let's say we wanted to figure out the equilibrium constant for the reaction boron trifluoride in the gaseous state plus three so for every mole of this we're going to have three moles of h2o in the liquid state and that's in equilibrium it's going forwards and backwards with three moles of hydrofluoric acid hydrofluoric acid so it's in the aqueous state it's been dissolved in the water if it wasn't dissolved if it was in the solid state you would call this hydrogen fluoride once it's in water you call that hydrochloric acid and we'll talk more about naming in the future hopefully plus plus one mol of boric acid also in the aqueous state it's dissolved in the water h3 b0 b o3 in the aqueous state so what would the what would be the expression of the eke for the equilibrium constant look in this situation look like in this situation so you might be tempted to say okay that's easy enough Sal you just take so the equilibrium constant you just take the the right-hand side that's just the convention I could have read it because there's symmetry here I could have rewritten it either way but let's just say you take the right-hand side say okay this is dependent on the concentration of the hydrofluoric acid the concentration of the HF the or the molarity of the HF to the third power times the concentration of the boric acid H 3 vo 3 and remember this intuition of why you're taking this to the third powers you know what's the probability because in order for the reaction to go this way you need to have three I guess you can kind of view it as three molecules of hydrofluoric acid being very close to one molecule of the boric acid so this is if you watch the last video I just made about the intuition behind the the equilibrium constant this is a this is an this is indicative of the probability of this reaction happening or the probability of finding all of these molecules in the same place and of course you can adjust it with a constant essentially what that does but that's on the product side or the reactant depending on how your what direction you're viewing this equation divided by you'll say okay the molarity of the boron trifluoride times x and i'll do this in a different color times the molarity of the h2o to the third power and that's of course h2o liquid say there you go we'll just figure this out and my rebuttal to you is figure out I want you to figure out the molarity of the water what is the concentration of the water remember the the concentration is moles per volume but in this case what's happening I'm putting some boron trifluoride gas essentially into some water and it's creating these aqueous acids these these other molecules are dissolved completely in the water so what's the solvent here the solvent is h2o the solvent is h2o and this might be how the reaction happens but pretty much there's water everywhere that water is in surplus so if you were to really figure out the the the concentration of water it's everywhere it sits there is no I mean you could say everything but the boron trifluoride but it's a it's a very high number and if you think about it from the probability point of view if you say okay in order for this reaction to happen forward I need to figure out the probability of finding a boron trifluoride atom or molecule actually molecule in a certain volume and it also needs three moles of water in that certain volume but you say hey there's water everywhere this is this is the solvent there's water everywhere so I really just need to worry about the concentration of the boron trifluoride so you could say the forward reaction you could say the forward reaction rate rate forward is going to be dependent on some forward constant times just the concentration of the boron trifluoride the water is everywhere so you don't have to multiply it times the concentration of water whatever that means because the water is everywhere so the denominator here you do not put the solvent so the correct answer for this one is you only put whatever is actually dissolved in the solution dick because frankly do the concentration doesn't actually make sense for everything I also if you think about it from the probability point of view that also makes sense because there's always water around so if you said okay what's the you know probability of finding water at any small volume of our fluid it's going to be one so you could just multiply it by a one there but that doesn't make a difference now what about the following reaction and this is called a well any equilibrium where you have different states of matter is called a heterogeneous equilibrium or heterogeneous heterogeneous genis heterogeneous equilibrium and so let's let me write another heterogeneous equilibrium so let's say I have h2o in the gaseous state and that's essentially steam so it's not going to be the solvent this time plus carbon in the solid state and let's say that that's an equilibrium with hydrogen in the gas state plus carbon dioxide in the gaseous state this is a heterogeneous equilibrium because you have things in the gaseous and the solid state and solid state by definition it can't be dissolved either into the gas or into the when we talk about solutions for m in you know we talked about colloids and suspensions and all of them mixtures before but we're talking about solutions by definition if this is in the solid state it's not dissolved if this was dissolved we would write an aq here would be the aqueous state so if you talked about the forward reaction what's the problem what's the forward reaction going to be dependent on so the rate rate forward well the solid there's a big block of carbon sitting there there's a big cube of carbon there and there's steam there's water gas all around it so if you pick any volume especially if you pick it some volume near the boundary of the carbon you're always going to have carbon around it's just what matters is is the concentration of the Hydra the water gas that's what's going to drive the forward rate so the forward rate is going to be dependent on some constant times times the concentration of the water gas and of course the the backwards rate so you need to get some h2 you know some some molecules of this let me draw it like that because it has two hydrogen molecules and then plus a carbon dioxide so maybe carbon dioxide looks like that so the the reverse reaction so rate let's say I call that reverse it's going to be equal to some constant times the probability of finding both of these molecules in the same place and of course the probability is is related to or it's on a first level approximation depending on the concentration so it's concentration of h2 times the concentration and to find both of them you multiply the probabilities you need this and that plus times the concentration of the CEO so when an equation is when a reaction is in equilibrium these two equal each other this is an R right here so this is going to be equal to the reverse rate of reaction h2 times carbon dioxide divide both sides by the k's both sides by the h2o and you get the forward coefficient or constant whatever you want to call that divided by the reverse constant I'm just dividing both sides by that is equal to this is equal to that let me just copy and paste that is equal to that divided by this Yugos take for that you divide it by that and so if we call this the equilibrium constant because it's just two arbitrary constants so we could just call this equilibrium constant you see that it actually makes a lot of sense to ignore the solid state in your equilibrium reaction so the two takeaways here is when you're trying to calculate an equilibrium constant you should ignore especially when it's in a heterogeneous equilibrium you should ignore ignore the solution or not the solution ignore the solvent in that first example where I did it with boron trifluoride with water water was a solvent so I ignored it because water is everywhere and you also ignore the solid state ignore the solid anyway when and we'll we'll probably use these in future things where we actually calculate the equilibrium constant see you in the next video where we'll learn about the chatelier's principle
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