Lecture 03. Chemical Equilibrium Pt. 3.
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Lecture 03. Chemical Equilibrium Pt. 3.

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3

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26

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2013

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English

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Abstract 
UCI Chem 1C General Chemistry (Spring 2013) Lec 03. General Chemistry  Chemical Equilibrium  Part 3 Instructor: Ramesh D. Arasasingham, Ph.D. Description: UCI Chem 1C is the third and final quarter of General Chemistry series and covers the following topics: equilibria, aqueous acidbase equilibria, solubility equilibria, oxidation reduction reactions, electrochemistry; kinetics; special topics. Index of Topics: 0:02:02 Aqueous Acids/Bases 0:04:42 Determining which is Acid/Base/Ca/Cb 0:07:04 Autoionization of Water 0:21:06 Introducing "pH" 0:27:09 Finding COncentration from pH 0:30:59 pH and Concentration Relationship 0:42:32 Acid Equilibrium

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OK I couldn't attention and start there is 1 of the important announcement 1 American that is that if you have checked the classes website and you check that page that talks about cost information you know about the .period distributions for example you notice that there bonus points for participating in activities in the class and so at the end of the quarter will add to bonus To your final exam and what you need to do to that to us said is that there will be 3 surveys that we want you to complete the 5 minutes of it's alright sold this week beginning April 7 the 1st survey to find it's a mail would take a long time but the first one will come out of this Sunday all right inside like a complete that the 2nd 1 will appear in the 9th week the last week of class of wheat before the last week of classes are right and in the end we would like to complete their that the teaching evaluations to the complete altered surveys then we will have a twopercent bonuses are right so I want to look out for a you know where to find you know the survey instrument is on the Tripoli website and you should be able to access their appeared art so let us pick up where we start last time and we got remember last time we started looking at the ground Lowry definition for acids and bases are right and we finished up by saying that if you
01:49
take away the socalled intake was solutions because the water and we said that the take announcing an acid is a proton donors and sell an example of an asset a generic example age 8 and water water is the base analysts said that for the reverse reaction the hydro undermines the confidence passes and Aminus is the constant Bates said take that solution the acid base pairs in that solution would be a change in minus and Anhedonia mining and each 2 will also in this case water is acting as a
02:28
base all right and we sets and this is an equilibrium we can write the quorum expression and equilibrium constants for acid in water is called the acid dissociation constant alright likewise so we can have this is unique with solutions and you have a base and that's it is a generic space that has that is electrically neutral then basin water this act as the biggest water now acts as an acid are right and you end up with BHP lost which is the conjugate asset of the and hydroxide which is the conjugate base of the acid water are right and so what you have here is in that solution you have to conjugate Aceves fares will plus is the acid form of being so this is the acid this would be corresponding conjugate base In unwanted access acid here and it's going to get basically the hydroxide since the solution behaves as it's in water now you can write this equipment expression in this form an equilibrium constant is called the base dissociation constant so we went through that last time that today I wouldn't want to looking at wanted in greater detail and 1 of the things that you would know that is when you look at water in the top reaction to see that water act as a base and the 2nd reaction water access and if so you can see that water has the ability to act as an acid or base so there and look at and we know that water can act as an acid or base so and a substance that can that depending on the situation of substance that can act as a possibility is is said to be an terror all right and that it will since water can act as a passive or a big case it turns out watermelon fields can react with themselves alright to undergo an acidbase reaction right so water molecules can react and if you have 1 water molecule it's 2 0 EU which mines where this
05:27
let's say this molecule is the protons donors and if this is the proton donors it acts as the acid and given that this is the protons accepted this would act as the baby's arrived and for the reverse reaction you know that this would be be the proton donors and so we call this the conjugate passing and hydroxide would be the proton except there and therefore this would be the country face so on city look at that as you can see that these 2 it would be a confident acidbase based there and he still would be a kind acid based there and this is when water reacts like this we call this reaction be so this reaction is called although ionization relations for or auto the proposed policy this is water so we call this order ionization proteolysis of water and if I rewrite this equation I can write it in this former agency now aged 2 molecules of water can give you had joined him on the course plus hydroxide Equus and this is an equilibrium right once again if this is an equilibrium than I can write equilibrium expression and the equilibrium constant for this is called the Iron Prada constant of water kW equals I join mind the products would be the concentration of hydrogen mind times the hydroxide once again this is pure water we know the conservation of pure water is a constant so we don't include the concentration of pure water right and so this would be the equilibrium expression for W which is called the I'm part constantly get pure water just water along then you would have this equilibrium established and he was riding with him expression in this way now at 25 degrees Celsius at room temperature kW it may make the constant and this value is 1 times 10 to the negative 14 malls squared per liter square so this is what memorizing and remembrance at 25 degrees Celsius the kW for water and is 1 times 10 to the negative 14 right now I remember I said that every time you look at equilibrium constant you wanna look at the magnitude of that number is that number with really big number or is it a really was a small number of it's really small it's much much smaller than 1 I right it that delays really small letters that tell you about that equilibrium human have lots of reactants and very little products so what this tells us is that an equilibrium the amount of high journeyman hydroxide items have to be really really small so the preferences for water to stay as H 2 Tuol rather than former to your loss in which miners out of millions of molecules of water very very small fraction will form her during a mine and hydroxide and it that we can calculate the concentration of the High junior mining and hydroxide science and to do that we would sit at the table our you know let's try to figure out what is the concentration of hydrogen mind hydroxide or so this is the equation looking at looking at liquid water undergoing are ionization to give you I join you mind Klaus hydroxide answer our initial concentrations would be 0 0 since this does not appear in the court expression were really not interested in the conservation of water you know what is the conservation of pure water 55 . 6 OK so we know what the Constitution pure water but because it does not appear in equilibrium expression were not interested in knowing how much is there we can just avoid that altogether now I will look at the change we know that to begin where there's 0 amounts Newman hydroxide so we know the reactions to proceed in that direction you not establish equilibrium and therefore you can say once again we're not really interested in that uniform plus 6 retained in reality minus 2 X plus 2 X plus extra 6 plus 6 on that side so therefore at equilibrium you'd end up X X and therefore kW which is 1 times 10 to the negative for the team will squared per liter squared equals the concentration of High junior mine times the concentration of hydroxide which is X times acts which is exquisite so we exploit equals that and therefore x equals 1 times 10 to the negative 14 models for leader and therefore we know that in pure water with the same pure water he hired journeyman concentration equals the hydroxide and concentration which he calls time Anthony 14 this should be 1 times 10 tonight at 7 moles of the I'm sorry this there we're OK said he'd take a square x squared with you 1 times tend to make it 7 or 8 and so on you can figure out so you can play the concentration you can see that the numbers tend to make it 7 so the amount of the High junior miner hydroxide irons in solution is really really small receded now if you're in water and that's it we change the concentration of the 100 new mining water so in N the concentration of High junior Myron is changed to let's say 2 . 1 0 OK so instead Of the and the pure water let's say I add an acid I had tried to undermine to water and I change the hydro Newman concentration water if I changed 100 my conservation
13:23
water then automatically the hydroxide and concentration would change as well all right and we can calculate the hydroxide and concentration because we know kW is a constant as long as you're provided sell we know kW equals 1 times 10 added 14 will square the display squared which on which it equal to hide joining mind concentration at times the hydroxyl concentrations so this the concentration of high joining times the concentration of hydroxide is always a constant in America it's an equilibrium constant and as long as the temperature remains the same that constant will remain a constant are right and therefore we can calculate the hydroxide and concentration and the hydroxide and concentration would be kW divided by the High junior concentration which is 1 times 10 to the negative for the most squared meter square divided by . 1 more square a Sprenger the concentration of hydrogen mind and that will give me 1 times 10 to the negative 13 holes to lead so you kill watermarking Catholic High junior mining hydroxide concentration if you take water and change the concentration of the High junior mining water you can always calculate the corresponding concentrations of hydroxide or a similarly taking water and might change hydroxide so it the concentration of hydroxide is changed to point 1 Moeller said I could do the same thing that you add hydroxide to that solution and now is dealt with pure water you at hydroxide to that solution and you change the concentration of hydroxide and to change the conservation of hydroxide than we know that the high donor my concentration which changed because we know kW equals a junior miner times hydroxide and therefore the High junior mind concentration would be kW divided by hydroxide concentration which is 1 times 10 to the negative 14 malls pretty to squared divided by . 1 0 miles per liter and that will give me 1 times tents and negative 13 most of support the leader had only minor right they were keeping track of saying things you can see this statistic that nominated 0 here to keep it all 3 6 thinks that you just add another 0 through all of this then a significant figures will always be consistent and we have altered the numbers that are very significant figures is that could everybody's so it turns out in water you have small amounts of during mind and small amounts of hydroxide iron and if we withdraw scale like this right and the scale on 1 side we have high junior mind concentration and on the other side let's say we have hydroxide and concentration and in the US it is 1 times tend to negative 7 Moeller and this is 1 times 10 7 Moeller Jim pure water that's what your hydroxide and concentration and high duty might concessions would be all right it on this scale :colon down here let's say this is 1 times 10 to negative 14 Moeller and this is 1 time spent 14 more than up here let's say this is 1 more then what happens is that the bureau Newman concentration is 1 more then your hydroxide and concentration will go down you raise the concentration of hydrogen lines in the conservation of hydroxide will go down so that there's times this gives you 1 time since 14 right if on the other hand you raise the conservation of hydroxide then what happens is now hydroxyl concentrations gone up so corresponding to the heard during my concentration go down so when you multiply 1 by the other you end up with 1 constant in the fall the right and so on In end between here and what happens is you end up with something like that so that's a good thing to keep in mind and so every time you know what the hydro my concentration is you can always calculate whether caused by an accident conservationists and there is no other hydroxide and concentration as you can quickly calculated with the corresponding Victor Newman conservationists are right side is not quickly show you so we looked at all the proteolysis finalization of water and what I want to to keep in mind is that only a very very small component of water actually undergoes this change all right so if you want to have a a look at what's happening at a molecular level and we want to so I just want to show you a picture of what this looks like and so what you have here is in the background you have millions and millions of water molecules are right if they were all the water molecules where there is very difficult for you to see what's going on so we're going to take all the water molecules that fade behind the background and we're just going to look at 2 1 molecules all right and you can see auto ionization or required all this is taking place so you can see that 2 water molecules come together to study began with you say it this city 2 water molecules coming together and then you can see the transfer of Proton from 1 to you hide during mine in the hydroxide I'm going away all right so this is not a molecular level this is what the event looks like OK so is getting back to there's no other way to look at Walter and now that we looked at the fact that if you look at the High junior mining concentration in water you can see that it can vary Over a very wide range because if you alter you add 100 my concentration you can change the concentration of to mine and if you take your water the concentrations tend to 97 balls leaders but you can add external or you had more hydroxide the high during my constitution goes down all right so it turns out that we look at high during month concentrations it is convenient for us to look at what we called the pH function and so all of you have heard about the pH are right in the reason that we look at pH it says that the hydrodynamic concentration can range from 10
21:23
Moeller to tend to the 5th team Moeller so it turns out we can adjust the height to undermine concentrations in this can have a very broad range starting from highly concentrated something like approximately 10 Moeller to something there were the concentration is really really small which is 10 to the negative 15 if you look at the range of this scale you can see that this is an enormous skill terrorism anywhere from 10 it's a difference of about 10 to the 17 can see that and so that's an enormous range to look at and it makes you lot mentally that's both a big numbers it's very difficult to come to comprehend the numbers of that magnitude and therefore it is convenient to compress that Steele said you have a skill that's why it is convenient to compress that scale by converting it to a logarithmic scale and so converting this to a logarithmic scale is convenience sell it is convenient to convert this to a logarithmic scale and the way we do it as we did find a quantity call pH and the pH is negative log base 10 of the High to a new high concentration right and so it now willing do as were going to convert the high concentrations too a more manageable scale which recall the pH scale and pH scale was just taking the native log of the height my concentration so if we have to take pure water so if I take pure water you know that in pure water had junior mind concentration is 1 times 10 to the negative 7 moles per liter therefore if I wanted calculate the pH of pure water it will be negative logs based 10 1 times 10 to the native 7 known as all of you recognize that concentration has units its most Khaleda all malarial Moeller but you can't take that the of the unit so when we apply the concentration today we dropped the unit because you can take a lot of the unit and and so if you put this in your calculator it will tell you it's negative negative all right and that gives me the pH is 7 no when you keep track of significant figures when you go from the lot you know from this concentration if you look at that concentration can you tell me how many sig figs as the concentration have to begin with 3 significant figures are right now whenever you take a quantity that history significant figures and you want to take a lot of that and you wanna carried over so that your your pH also conveys the appropriate number of significant figures you want to keep in mind that when you go to the pH scale what happens is this number 7 because you're going to allow concern is that are the 1st didn't actually tells you the exponent of 10 right and then these are the numbers that give you significant figures of this kind significant figures and therefore the number of decimals here is what conveys the significant figures so we need to take a lot of a number you start with a number that has 3 significant figures in a converted the lobbying this tale go number that goes to 3 decimals are right because the 1st number gives you the exponent of 10 I right and so you do not count that as significant figures so whenever you can wear a number to the logarithmic scale which the number of decimals that conveys the appropriate numbers according to get such a cheesy big number therefore you go to 2 decimals alright right and you can use the calculators but if it's 1 time something then you know that always has to be the exponents 71 times negative 7 and take a lot of that you will end up being 7 all right it's a onetime said the negative you take a lot of that with that pitch come up to me during the rights that 1 times anything then they explanation would be the corresponding pH so you don't have we should be able to do this quickly when you're doing sort of quick calculations case so yeah so it my height during my concentration is now might might take water and change the Hydra my concentration so it I join mind concentration is now . 1 0 Malaga and your ass account with a pH you know that . 1 0 Moeller is actually were at 1 time Spitzer to sink the numbers of the 1 times 10 to the negative 1 more and therefore pH is minus logs 1 times 10 to the negative 1 which gives you this is expert at oneself if he had to come out to be the case yuppie it has no unity because you know I'm taking a lot of something and sell the outcome has no unit so if he had to come out to be 1 and because the significant number given recorded 2 2 dB right likewise is you have hydroxide so let's say it your hydroxide and concentration is . 1 Morla arrived and so you know that once again this would be 1 time said the negative ones and therefore I want account the pH if I wanted pH I need to know what the high among concentration is and remains you know what the hydroxide and concentration is you can always count with the corresponding are doing my concentration so In group the hydroxide constitution is theirs and I know that my high during mind concentration would be kW divided by hydroxide and concentration which is 1 time 10 negative 14 malaria square and divided by 1 times 10 the negative 1 molar which gives me 1 times 10 to the negative 15 Muller so not counted the hydrogen survive hydroxide is given a Catholic high dramatic concentration can take negative lather of that Sino pH would be negative log about 1 times 10 to the negative 13 which is at the age of 13 alright and 2 significant figure is we carried over to decimals right now most of you have seen this pH scale in high schools and so on and you know that when we talk about the pH scale some of the things that you want to remember and so on 0 reminders 1 as if did you have a low pH appear low
29:22
pH means high concentrations having age 3 open to us and low concentrations Our people which minus sh will read that OK so where we say something has a low pH a pH music is you taking the negative log of that number of low pH means that you have large concentrations of hydrogen mine and ideological concentrations of rejoining and it means that you have low concentrations of hydroxide arrived in the pH is hiding you have a high pH highpitched means that you have low concentrations I join in my hand and high concentrations of hydroxide I'm OK so that's worth remembering too Of but there is that if you are looking at a change in pH so a change in pH of 1 unit is actually make it a logarithmic scale to the base 10 so a change in 1 pH unit means that the change in a factor of 10 sets of 10 fold change so the pH increases by 1 unit so pH increases by 1 unit that means means an increase in Hydro Newman concentration by a factor of 10 A 1 order of magnitude right it is pH increases by 2 units what is that means means that means an increase in Hydro Newmont concentration by a factor of 100 says not to orders to all right so every time the pitch limit changes it's going to be by orders of magnitude are right said that its unit changes by 3 it's actually increasing concentration by thousands years and because we are in a logarithmic scale and it's going to be every change is all order of magnitude now also you know that if I pH is less than 7 it means that the solution is acidic and the reason it's it is because I join you my concentration is greater than hydroxide and concentration so honest PAC scale if your pH is less than substance it then you know that that solutions can be acidic because the during my concentration is greater than the hydroxyl concentrations when pH is something we don't peel water has pH equals to 7 and the reason that pure water has a pH of 7 is because the High junior my concentration is 1 times hydroxyl and times 7 and hydro hydroxide and doing my concentration equal to each other and therefore newest interning at 7 hydroxide incident its 7th and self at 384 Sabine we say that the solution is neutral because as it's neither acidic or basic because the concentration of hydroxide and had drawn young are equal to each other are right in the pH is greater than that and we say that the solution is being because now when look at the balance between high journeyman hydroxide never hydroxide and concentration is much greater than the whole tournament concentration and therefore the solution is facing and so now we compare the 2 we know that hydroxide is greater than had join and then lastly it is clearly yeah In the pH is give and then these concentration of I joined and can be calculated from now on if 1 cabinet had during a minor concentration the relationship between to hire new demand and pH is that if you want if you're you know what the pH anyone calculated that journeyman concentration then extended to the negative right and so on it's a good idea for you to use your calculators and know how to go from 1 to the other are at the concentrations given how using calculated or the teachers and to move in the reverse direction Senomyx quickly where 2 examples of applying what we just let so let's say your ass
35:19
calculated so let's say calculated the pH appearing at 25 degrees Celsius and it was solution that has hydroxide so that has hydroxide and concentration equal to 1 . 2 times tend to negative 6 right 1 counted a solution that have hydroxide iron concentration equal to that so you kind of know the general strategy it if you want out of pH regional agreement concentration rights of hydroxide is given we have to use that information to calculate adjoining date and so you know they even use kW to figure that out Sorrentino ahead tournament concentration is taken there is a lot of that case so were given hydroxide and concentration the therefore I join mind concentration would be kW divided by hydroxide kW is a constant as long as you 25 degrees Celsius divided by the hydroxide and concentration right and it's to 6 think numbers that he put in a captain work it out it comes out to be 8 . 3 times since to negative 9 more and then it I take the pH of this speech would be negative longer 8 . 3 times to the negative 9 more alert and that comes out to be 8 . 0 8 right now ask yourself if you look at the solutions you can see that the pitch comes out to be greater than 7 so you know that the solution is basic and if you compare the concentrations of hydroxide with drawing and you can see that the hydroxide and concentration is greater than the journeyman concentrations right and so when you check your solution you know that it makes sense this has to be a basic solution and therefore to PHS to be greater than 7 no likewise if you're given the pH you should be able to calculate what the corresponding high during an iron concentration in let's say the pH of great use the pH the a sample of grape juice and at 25 degrees Celsius here is 2 . 8 5 calculated tournament concentration and hydroxide iron concentration are right so once again were given pH so we know that the pH is given then we can figure out how to remind concentration it's alright and I dream concentration would be 10 to the negative 2 . 8 5 on or depending on your calculator you know it's the inverse of the 2nd function the in assisting with the block itself you just put negative 2 . 8 5 in interstate inverse logic that should give you the hydrogen concentration obtained from now take a look at this number we on the pH scale so looking at his number 1 is appropriate number 6 figs WE have the decimals alright so there only to decimals there so it should be to thinking of our rights and therefore you put that in your calculator working out this number comes out to be 1 . 4 times 10 to the on negative 3 most of the right so now that figured out what the High mind concentration is you can convert I'm sorry that agreement concentrations now you can convey that hydroxide cases that we know that hydroxide and concentration equals kW divided by journeyman concentration of W 1 times 10 14 squared divided by 1 . 4 times tend to negative the molar and that comes out to be 7 . 1 times 10 to the negative 12 Moeller right once again just to make sure that your calculations are correct you know the pH is 2 . 8 5 is a number that's much less than 7 so the solution has to be acidic and it is same the High junior my concentration should be greater than the hydroxide and concentration and you concede that that is true as well all right so what sort of I wanna make sure that this is sort of the basic stuff all right and so it's very important that you kind of review this material because the material gets more complex this stuff that we talked about today should be just intuitive or I wouldn't look at lots of massive bases acids and bases in water and which would keep in mind is as long as you're watering have this relationship that playing our right to not only look at water still are interests is is that looking at acids and bases so most of looking at passes in greater detail and so on In the last few minutes of the class let's look at Athens in general it was start by looking at acids are right now if you take asses acid and strong acids 4 we guesses are right
42:09
and that we start with a strong acid an example word of the hydrochloric acid so an example of a strong acid is hydrofluoric acid the fact that it is an acid tells us that if we were in water equilibrium that would be established would be HCl Equus in water Stephen Hi join mind concentration plus CEO minus Inglis all right and so the reason it acts as an acid is because you place it in water it produces high to remind our right and so you have in the background you have water in the back on the water has changed the negative 7 high junior M. and tendinitis and hydroxide but by which the fact that you put a ceiling there now you hydroxyl high during my contributions lot all right because what's happening is the new places and water you producing high junior my case of this and acts as an acid because it's producing hydrogen line and since this is an equilibrium and we're looking at an acid we know please this is a a poor women described by which is the acid dissociation constant and this is of the order of 10 to the 7 so if you look at 10 to the power 7 so there's an equilibrium remember every time you look at an equilibrium and you have an equilibrium constant you look at the magnitude of that number we get the magnitude of that number that number is almost but really small the enormous this number is so big that it's difficult to accurately calculate that number because you know you can experiment with measure numbers that huge so if you look at the became make value it's an enormous number so what does that tell you when he clearly was established in you have the mixture what is that tell you it's got lots of products Little reactants to see that and whenever you lots of High junior mind lots of products it means that it's almost going to completion requirement established the reaction is almost completely on the righthand side are right and that's why the socalled strong acids because they carry a number is enormous remember the color of his 1 year reflected in exchange 1 this number is fall greater than 1 to see that and that's why it's called a strong acid because many clues established its producing a lot of junior mine and therefore too strong acid so much so that if I were to start with you might take a and if I take my initial concentration of 8 C out and let's say I have .period 1 molar appeared to begin with the text of the point when will it see since water doesn't appear in the equilibrium expression I'm not really concerned about the constitutional water so revised upward .period 1 molar concentration of 8 C L I know that the reactions can have preceding the forward direction to produce some products are right so it by looking at the change you can say that this is going to be negative acts this will be positive that this will be positive acts but because K is so large it's 10 to the power 7 this X is going to be an enormous number to see that because almost all of this is going to consume to give you products are right itself it's so large double approximated states almost 2 . 1 molar alright and so this will be fine 1 molar excess and easily .period 1 molar so at equilibrium it .period 1 molars consumed you can see at equilibrium it's almost 0 here and there is approximately 2 . 1 molar .period 1 the remedies in equilibrium so you have to have a minuscule amount of his left over you know that it otherwise it would just it would be a reaction that the geometry but for all intents and purposes with assumed it voter completion because the amount that's left always so minuscule that would approximated state of people to 0 the needed a strong acid strong acids we say dissociate almost completely all right so almost all of the acid that you started with will dissociate to give you finetune your mind right so you want calculate what we call Percent decomposition all our when we deal with acids this reactors called association so we call it Percent dissociation so the presented Association is the amount of the percentage of the reactant that dissociated member in this case it would be that this is the amount that associated which is this all right so the
47:48
present association with the Act's divided by what we started with witches .period once so this comes from here this number comes from there all right it's the initial amount and acts the mound that associated so you can see that almost all of its its roughly . 1 Walid Associates divided by 21 Morley's what we started with times 100 And so that gives me roughly 100 % so the tickle
48:24
messages that you take strong acids they associate to 94 99 . 9 9 9 9 which runs up to 100 you see that because it it's an equilibrium and requiring established a clear by the time of the product site it's almost all of the strong acid associates completely and so strong acids have dissociation of dissent associations that a lot it's almost of the order of 99 . 9 9 9 9 % is an excessively high right next I will take a look at what we can do