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Lecture 09. Acids and Bases. Pt. 6.


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Title Lecture 09. Acids and Bases. Pt. 6.
Title of Series Chemistry 1C: General Chemistry
Part Number 9
Number of Parts 26
Author Arasasingham, Ramesh D.
License CC Attribution - ShareAlike 3.0 USA:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor and the work or content is shared also in adapted form only under the conditions of this license.
DOI 10.5446/18998
Publisher University of California Irvine (UCI)
Release Date 2013
Language English

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Subject Area Chemistry
Abstract UCI Chem 1C General Chemistry (Spring 2013) Lec 09. General Chemistry -- Acids and Base -- Part 6 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 acid-base equilibria, solubility equilibria, oxidation reduction reactions, electrochemistry; kinetics; special topics. Index of Topics: 0:00:00 Common Ion Effect Review 0:03:35 Buffered Solutions 0:09:20 Examples of Buffers 0:15:07 How Does a Buffer Work? 0:32:24 Calculating the pH of the Buffer
can I have a residential places other any questions before it began great so I just think of what we stopped last time and you guys remember the last time we talked about the common side effects and we said that if you take a weak acid for example In that we gasses that equilibrium and not to disturb the equilibrium by adding a common that's common to that it all agree on that and that it would be disturbed and the equivalence disturbed them we can predict how the system is going to react the summation of his principle because we know what machetes principal tells us is that if you disturb stress systematic program it's going to respond In a way in which you can counteract stress right now another way of solving common scientific problems is to have both of them to start with Sony last time took this problem where we had a we acid
and so the we gathered here was a chance so we looked at Ajab equalized in the water and because this is a weak acid it's going to establish this equilibrium where 87 7 . 2 times tend to negative for itself rather than have a system where you have people will be established and then you add a comment I and you can start with mixtures of bold and so there is an example of problem where you have to begin with you have 1 molar so this is a new show you have 1 or more of their and you have 1 more back you see them so the way to solve this problem is the same thing you can have a system that established common iron or you can start by having a weak acid and the conjugate base there was an hour to have is a mixture of all in the past we look at examples where you know how happened the pH of just a week after the it was just a wee gas along you don't have any of the conjugate base to begin with a in this example what you have is you have vote the weak gasses and the common I'm which is the conjugate base about assaulted at and based on that you can figure out what the pH about solution is a case so these are also like distinctions of the the approach that we take in solving different problems involving acids bases and figuring out the key to the solution is to see that now has a slight variations that today wouldn't want to looking at a phenomenon that is very similar to this ,comma 9 fact and we call this buffer solutions of offers that everyone to looking at buffer solutions of profits and many of you have heard the term office solutions and above all buffers before we look at solutions for we call
these buffers and if you want to describe but that is buffered solution for the past 3 years solution that maintains an approximately constant pH despite small additions of these rights so only a buffer solution is any solution that maintains its pH right despite small additions of acids and bases now we typically can take and you had asked to water was what is that solution because could you take your client is neutral and yet acid to pure water solution becomes a city and somewhat aware about pH features Google from 7 to a smaller value it might take fuel water which is neutral another strong base to it worried aloud that solution is going to be the based and so what does that tell me about Pete Peterson go up so that if you just have pure water no offers are special solutions where do you add an acid the pH does not change dramatically this just a slight change in pH maintains the pH or take a buffer which is a special solutions and that a strong base like hydroxide to it usually take fuel water and a strong basis features that go up dramatically right about offers a special solutions where you add a strong base of yet a strong acid it's pH remains relatively unchanged in the altered slightly but it maintains the peaches are so it turns out the special solutions have said very distinct characteristics so we need to define what that it's so we can say about contains 1 El-Amin acids and only weak phase that ,comma conjugated to 1 another write to me cities a special solutions to and the characteristics of the special solution is that a buffer contains bowl a weak acid and its conjugate base together are right so in other words the words admits sealed lead-acid and base what that means is that you have a weak acid rain and the salt of its conjugate the all right everyone admits a weak acid and it's kind of embarrassing the condemnation is negatively charged has to be solved to buy at CNN I take ABC in which the weak acid H C and what is its conjugate base it's Ian contemplates Soon that so that CMI as you know that a charge fees is not to be a lot of so needs a car so we had a .period level to take the sodium cyanide he could be potassium cyanide can call that assault of its conjugate base so about where is a weak acid page 8 HF acidic gasses that a weak acid and its based so a If it's a CNC and miners would be confident that it's a called its conjugate my it's acetic acid what is it's going to be acetate all right so all of those are negatively charged ions and therefore you have to consult the right so imbued whenever like sodium potassium or sell weak From start weak acid and the salt of its kind in it's all all right or of base like ammonia and the so it's conjugate asset "quotation mark oxygen atom types you can have a weak acid like BC and HA acidic gasses and assault of its kind to to be used or you can have a weak that's neutral like ammonia and it's ammonia What is the conjugate passive ammonia in its full plus a right to the salt about it's foreseeable so the way to identify any of the offer is to look for these characteristics so if I have for example hm In India wearing letters is on line which is the conjugate base a later you can have some and here in this year if you have a city gasses then over here you have CH 3 C O 2 in each of these this is a salt and worry about solitude with salt and water What is assault to dissociate so it's going to release the Anaheim which is the conjugate database of this would have seen minus this would release it's 3 0 to minus an interview take something like formic acid which is that then it would be a salt of folic acid and therefore this would really that again or are you were dealing with a weak then what we do in acid form would be in CEO right and always here you would have in history so once again this is an effort this which I contradict 1 another and this is also the view In each for blast and so would you end up with is resentful for plus ministry these are a combination of a weak acid and a weak base I want to look at that table that I believe that you take something like C H 3 any see how this is also just up to what we're doing as were replacing 1 of the heightened security to see them and that would be a combination of C H 3 and H 2 all right where less would release the cat and C H 3 In 3 plus and so that would be the acid form because this is the captain of the call right is there the so this would realize release the acid form and that would be it's contemplates all right so don't get thrown up by the formula because you can kind of sit down and look at it and say that seems to be its unity in HCC Allen you know that that's assault and therefore it's got a cat in the canon component would be that if you look at it it's very similar to for plus the only difference is that 1 of those languages have been replaced by the industry and therefore that would be the acid for another basic form so we said that that's the 1st criterion you won't have a buffer you have to have both of those components are right there and they have been contradict 1 another the 2nd property that is often has to have it is 2 the union from both components you have to be approximately equal amounts In other words their concentrations should be similar for the best offers you want to make sure that also In approximately equal amounts to case you have to have sold about taking a weak acid and my concentration is 1 more loss the best offer would be like on base also 1 more so you have equal concentrations of all you can they can be slightly different from each other but the ideal would be the concentrations are equal to each other can and that there is a whole should be in substantial amounts should be In substantial announced In other words
but that means that a high concentrations right so when I say higher compensation 1 Moeller is considered to be quite high concentration right so it there is the order of 1 bowler to there that the more concentrated both of them on the back of a case so whenever anyone asked to identify had offered that have to look this you have to have a mixture of economy a weak acid in its conjugate base so that their contradicted 1 another you have to have approximately equal concentrations initially substantial amounts are so these are the criteria for good after announcement see how does a bottle of water so as not looking at how does a lot of work and good buffer she had in the kitchen not change all that much right and so if we want to look at a lot of things said about 100 million that has a weak acid and its conjugate base so less likely cooler remember we're looking at Aceves equilibrium in the underlying theme for all of this is equilibrium that we establish safe I haven't and its conjugate base then let's start with the generic we gasses call that 1888 and by placing a Jaranwala I'm established the equilibrium where I have each 3 0 plus the inquest plus A-minus requests and the form in which this equilibrium is what is written is in the form of an acid associations of his equilibrium described a key right so we find start with initial concentration let's say I have 1 molar theirs and 1 more that and to begin with these initial concentrations I have initially to begin with 1 more than the weak gasoline and 1 Waller of the base of approximately equal amounts to a substantial amounts and I have bolted their case and we know that we believe is established and at equilibrium now I can jump a step busily minus X plus and so on so at equilibrium we know all this would be 1 minus X is to be at this would be 1 was and therefore I can approximate this to ones this so be axed because cable it's always smaller we can approximate and say it it's a small and therefore that 1 a right-sided equilibrium we can say that this is the walls the initial concentration of and equals the initial concentration in England's because the amount of excess so smart because a weak acid haze really small because X is going to be small approximated the Bishop concentration hardly changes made to see that and what is a sort of symbolic notation that we use to describe the initial concentrations we always put a subscript 0 art and what is that subscript 0 it's at time 0 right and that's what the jury at the bottom time 0 means that all the action is taking place this is what you start with at the beginning our right before any reaction takes place and so what we say is that because of the weak gasses in the country basis case really small Italian approximated saying that initial concentration doesn't change very much right when radical imam expression we don't share the walls of that's the definition which is the loss of mass action which is 8 3 0 minus A-minus over age 18 and we can say that this approximates to the equal fitness approximates the walls H 3 0 plus the initial concentration of A-minus over the initial concentration of because we know that the weak acids initial concentration state change my much we can always approximate that in state equal to initial concentration could not remember we're looking at the effects of pH are I wanna look at how is the pH affected and what appeared to depend on what is the definition of pH depends on high journeyman concentration so if we want to look at the effect of pH wonderful wasn't Hydro undermined and so that's really what equation so that we look at the high Doria Mining concentration so we haven't had during my concentration and if I rearrange this equation you can say that people stay but so if the pH is not altered that means the High junior mind-concentrating is not alter our right to react in acid and we say the pH not change what is that really need it means that the high during the concentration is not altered by much so we don't have to during concentration depends on take which is a constant and then the ratio Of the passage to the baby's arrived so so in the ratio of Of these 2 is not altered to match its CEO glass will remain unchanged is that clear so that everyone can look at a buffer any substantial amounts of people and leave the pH is not going to change it upon heightened my concentration and therefore be that way she is not altered too much we know that the 100 among concentration will not change much and so that's how often worth it turns out that in BofA's because you have substantial amounts of and that's when you have an acid or base this ratio is not altered all that much right and as long as the the ratio is not altered the hard during my concentration does not change and therefore the pH remains unchanged all right so let's look at what happens if you add an acid were based its analysis is scenario of analysis saying that is so much the 1st case that take in H 2 your class is adding arrived in other words let's say something like . 1 molar ACL Is that it OK so all of you recognize that the is a strong acids and we know that a strong acid dissociate completely survived put . 1 molar HCl what concentration of the high Georgia mind that I'm heading by 1 moment right all and 19 . 1 Moeller I join you might not like to point 1 Moeller High junior mining pure water what is the pH . 1 will head during a but what is wanted to the what negative ones to be so they want what is at once so that means .period 1 molar you take a neutral on that point 1 bullet to neutral waters because from 7 2 To ph 1 ph 1 that's a big change member it's a factor every change in pH unit is achieved by a factor of 10 of by 1 order of magnitude so now the changes by 7 orders of magnitude so that's going strong acid now what happens if I take this and added another offer now the PA to the buffet is not change you take you articles from 7 to 1 but he put it a buffer the picture is not altered all that much in the reason is that you have this equilibrium that we've established now so we have this equilibrium it's too old plus the miners and this includes established but now I'm adding had junior mind so what am I going to that this environment that of and adding join in mind too that solution so I a lot of had remind them so what happens that equilibrium Is there commonalities had only minor there already in equilibrium so now but many more I don't mind what happened that equilibrium of distributed Chavistas would Japan and its and counteract stress so what happens is that this equilibrium so that
counteract the addition sold to counter attack the added High junior mind and what happens is that the high during mine plus A-minus will react to give you HPA it's too early it's a question was inquest and that question so that's what happens is that you've added to what had to on the system respond by this reacting with this year are in this way what it does is it absorbs the extra to mind that you added and the system will go back to equilibrium when the system was about to call area for happens is that this ratio want to go back to equilibrium this ratio is altered only slightly so the winners buffer acts is that it it absorbs the added an arson and reacts with it and forms a weak acid and when the system goes back to equilibrium that because case a constant to see that the ratio of products to reactors have been maintained because that's OK is a constant and so what happens galactic William this ratio is only altered very slightly and therefore because the ratios altered only slightly in the pH remains relatively unchanged great Sony as a result of this reaction would be the ratio h a hallway A-minus is relatively unchanged and so it that ratio is unchanged the high during my concentration is not changed all that much right now the 2nd scenario is that we added This is the end of this so I know we understand how about works and we understand that when you have an added acid because disturbs at equilibrium in the added extra I don't mind is absorbed and consumers this reaction to you until then and the ratio is not altered that much we know the during my concentration has not changed all that much know what happens if you add a base so the 2nd scenario appears to be there's always minus is added to the right in another so on and a strong basis and that they were added .period 1 molar annual wage so we know awaits KOH airline which lithium hydroxide is all good 1 metal hydroxide is also strong basis because the salsa and want to put them in water don't associate completely into the plane .period 1 will any awaits the result this year and . 1 while the hydroxide if you take hydroxide what is the pH of . 1 will hydroxide memory hydroxide .period 1 means tend 81 so appeal which is 1 EPO which is 1 word is pH 30 so you can see that if I add this amount of sodium hydroxide into pure water or fuel worries creates Saban at 5 1 0 7 hydroxide goes from Pete 70 feet teens that's a big change its 7 orders of magnitude on and therefore a 6 orders to and so there a big change but you at the same amount of hydroxide into a buffer the pH is altered only very slightly and that's about why the pH alternately slightly because of you take this equilibrium once again the buffer establishes an equilibrium and this is the equilibrium that the buffet establishes now we're adding order to minus right so added will which minus reacts with the weak acid so what happens is that it turns out and acid and a base in large amounts cannot coexist because the neutralized each other so we have weak acids in large amounts and we had a strong base in large amounts and even solution you have a week you have any acid we got strong and you have a base in large amounts they will neutralize each other they can exist in small amounts like water you have Victoria I'm attend to the needs 7 hydroxide tend to the needs of user with small amounts are I believe you have a really substantial amounts like 1 molar and . 1 more than what happened is large amounts of acid and base cannot coexist will neutralize each other and so will neutralize each other which you end up with is warranted and the minus His so what happens is that we have a strong base now you have lots of with lots of the acid which is the weak acid and you have lots of bases and we know they have substantial amounts of an accident base it neutralize each other it will neutralize each other the from water it and so what happens there's this reaction takes place and so you can see he added for which minus is consumed to produce an A-minus and with the system establishes equilibrium so when equilibrium is established again and again the ratio um acid to Vegas it is relatively unchanged are sold out so you see that you have and after that there are some reacts to the conjugate base so far added Garcia you need substantial amounts of the conjugate base you have had to undermine members during reacted the miners deviated so bad acids are absorbed by the conjugate base you had a strong wave like hydroxide in reacting what the acid part series of tension of the acid to absorb the base after we need substantial amounts of gold is the buffer has substantial amounts of the weak acid and substantial amounts of the conjugate base which is a sop any vote that those because 1 deals with any extra acid that added 1 will absorb added stresses the company's will absorb and system galactic would you have had a strong base the weak gasoline react with and absorb them and so that and on the back to equilibrium and the net result is regardless you had a strong acid a strong base the ratio always goes back to when it was because she is always a constant and we know that when he close established the ratio of products 2 reactors have to be maintained 50 is going to remain a constant every issue has to be the constant that could arise and so that's what happens and that's how coffers of the year so now that what I want you to be able to do so it whenever you given up problems yet to figure out is this a buffer or not right and which is known as about 4 is very similar to the common that problem that we dealt with before because in a common items that problem you had a weak acid and yet we base which is a common for that equilibrium in the buffer is the same thing soliciting example of a possible problems and what we do is we calculate the pH of the buffer and only had an accident and calculated how the future changes with added passes or base now once again there is nothing new here it's a kind of conforms to all the different types of equal the problem that we've been working out right so let's start looking at part so we say suppose 1 wall of formic acid and . 5 of sodium formate added to water and included a a 2 1 lead are so begins .period by saying that we have formic acid an initial concentration of folic acid is 1 of the mold over 1 leader Kate at concentrations molds divided by the volume and that gives me 1 more
concentration we're told that we have sodium formate so you need to be able to recognize that now we have to solve the conjugate base and this is a point 5 holes In 1 liter which gives me . 5 0 0 smaller so To begin with you can see that I have a mixture of a weak acid and assault of its kind could face so what does that mean that view buffeted by you just have a weak acid it just kept gasses now of a mixture of both now I dearly In the concentrations of both with the same that the best offer you see that's the reason I'm using different numbers this is because we resolve the matter .period providers his wonderful both of those it's hard for for you to see the changes that are taking place and using 2 different numbers so that you can track the changes that take place for each 1 of those that I will offer would be where you had bolted them in approximately equal concentration camp so we're taking example where you can see the salt is half as much as the acid now when we look at the Salt we know that the moment you put the salt in Wantagh them as and when it is all that you know that it's going to be associated to give me a conjugate base class the cattle yeah and we know the concentration in the initial concentration of this is . 5 we know that the concentration of the conjugate base would be . 5 and we know that the concentration of the sodium content should be . 5 because for every unit of the combined entity Associates will give you 1 unit of the Anaheim and 1 unit of the cat rights should be stopped .period 5 more of the combined unit we know that the parts that come from that combined unit will have . 5 Will concentration each the case so now like an equilibrium problems we have to start with the equilibrium that we're looking at we have a weak acid folic acid and they've given us the so we can write down equilibrium that and so on we start by saying you have folic acid In lottery giving me can you tell me what the products of this will turn out to be his acting as an asset is is that proton donors this is the proton accepted so what would end up thanks to your last C -minus and Mike a it is 1 . 7 7 times 10 to the negative thoughts a weak acid now my initial concentrations would be we just calculated that I have
formic acid which is 1 molar not concerned about whatever I don't mind going to 0 to begin with but now I also have a conjugate base so this is what I would start with ah I have combined with a weak acid with it's all so before that equals 0 my initial concentrations would be that know about the change this to be negative ads plus process carried their own want documentary therefore at equilibrium but I would have won minus X analysts and . 5 lost their right to know and I know how to solve the rest of the car right now and I have to figure out what taxes and exercise during my concentration to uncover the pH there was like finding out access that is enacted in concentration differed on the future so we can go pretty fast on this part because we were so often and so we know that 8 equals 1 point 7 7 times 10th inning for which gives me hi to eliminate concentration times the following 9 concentration divided by by folic acid which is at times . 5 0 0 plus that's divided by 1 minus effects no to that approximated because case tend to make before it's small and therefore I can make my approximation and my approximation is going to be there .period 5 plus His . 5 Texas or small we can collect them and if X is small compared to 21 obviously it's got to be small compared to what I said this is visible compared and that no 1 is a bigger numbers so excess violence more than that as well so now I can say is that 1 . 7 7 times 10 negative for equals at times .period 5 divided by 1 and so I can solve the address and if I software that's the number comes out to be 3 . 5 4 times 10 to the need for most of now I have to check validity is my approximation balance and can only see that is the pick 1 of these all right I don't need to be counted believable because it might take the top 25 C X is small compared 25 it has obvious is small compared to 1 side to take a 3 . 5 4 states need for divided by 2 . 5 molar times 100 And you worked that out becomes out to 0 . 0 7 0 8 % senator words 0 . 0 7 that's well below threshold so we know approximation is valid survival that approximation is valid but now I know that my head during my concentration is some 3 . 5 4 4 times 10 1084 rollers and if that is the case I concur with that to pH and pH comes out to be 3 . 4 5 1 established 3 significant figure number 7 and that 3 decibels and that would be the pH of this all right so that was the 1st part of the problem how to calculate the pH of a buffer and you can see by setting up equations in solving in the manner in which you solve every equilibrium problem you can figure out what the Peter that solution and knowledge of the 2nd part supposed . 1 wall of a strong acid such as ACL is added to the mixture captain the pH of the resulting solution right so now part of the now we have to figure out what happens if the you had a strong acid so once again don't get thrown off by this problem because it just looks complicated it's actually just follow the same principles that we've followed before we should be able to come to a rational conclusion of Celeste out with the high during my concentration so we have to wear adding the initial
concentration of ABC out would be . 1 malls and 1 leader of the because we know that the buffers 1 occasion so we start a quick 1 mole and what we end up with is . 1 0 4 molar concentration of so we calculated the initial concentration of the seal that we had we known that that is equal to 1 because of the strong acid boundary wall the matter had to remind you that we had take now I want to think about how buffers behave you add a strong acid to the buffer How is the strong acid consumed it reacts with word that the country base arrived right in this case the old mines are right so what's going to happen is that we have a limited range problem to begin with so what we have is that the added acid finally we reckon that the the ACO miners will
react with the added passes to give you a last speech to all this evidence that's how he added is consumed so that the pH is not altered very much so what happens that the incident you add a strong acid it's the system is no longer equilibrium it's been disturb our right and ways to stop announced going to responded by counteracting that's disturbance by just reacting to high-demand react the kind base to give you formic acid and water so what if you want to look at this initial at the beginning of what I have this initially before any reaction occurs where was but to begin with how much folic acid concentration we have performing at concentration is 1 this is .period 5 types so this is . 5 we added I don't mind how much time do you mind if we had played 1 more alert right and always here we had 1 molar of that member
offered to begin with I had . 5 candidates and 1 more week yes No we added this strong acid and . 1 wall of her so that is asked to words already and the end that is after any reaction occurs you would have significant is this becomes a living wage and problems I right between these 2 which is yeah .period
5 lots of this they one-to-one ratio can you may see that this is actually limiting if this is a limiting its all consumed that means it reacts with this how much of this will be left over you see them so if this is all consumed that means in 1 mall is consumed in 1 or more of this consumed and 1 wall of this consumer from 1 more of that here what I am what's happening is that because essentially what happens is you have a point 5 miners .period once arrived but I never went there to see the number you can so this is 2 significant figures to decimals 3 decibels was attracting so you end up with a number that is to decimals you have it now since this is a limiting his all-consuming so the project site you have In every student at 1 . 1 0 because on this side you'll have 1 plants . 1 0 so sensible doing and we're looking at the reaction and not be added acid been consumed so because the added after visiting consumed about concentrations of four-minute formic acid have changed right so now we go back to the equilibrium so that the US now wants the equilibrium is established what will happen Class H 2 0 giving you new H 3 0 plus quest was 8 CEO of minus the question we know is 1 . 7 7 times 10 to the negative for but now once this reaction is complete you can see my initial concentration of folic acid is 1 . 1 0 0 1 .period are concerned about water 109 0 to begin with and when I had is point 5 . 4 0 for me today so the change here would be she stated that its plus a plus and therefore it equilibrium I have one-liners when 21 miners that's not concerned about that and this is the point 4 plus a right and so now I can say equals 1 point 7 7 times tend the negative for which is the high during my concentration times deformity and concentration divided by the concentration of folic acid which gives me exercise . 4 plus sex divided by 1 . 1 0 minus X now the approximation which is administered .period 4 plus approximately 2 . 4 1 . 1 0 minus X approximates to 1 . 1 0 and I'm thinking that I can go back and say that 1 . 1 4 1 . 7 7 times 1084 for equals exercise .period was 0 divided by 1 . 1 0 I can move fast here because you know this is the general form that would solve every acid-base equipment probably offer acts and it is sulfur acts that number comes out to be full . 8 7 so the 4 . 19 because I'm keeping this to 2 significant figures because when we did that traction right we ended up with a number that was to decimals and so that comes to at 4 . 9 times 10 the negative for smaller around and then dividing check will let it will let it would be that's divided by . 4 times 100 which is 4 . 9 times since the native for smaller divided by . 4 0 Moeller times 100 and that comes out to be 0 . 1 2 per cent so we know approximation is valid and Savino axes Texas 4 . 9 times tend to native form therefore pH SO X equals the height journeyman concentration and so pH is the negative log of that and it comes out to be 3 . 3 1 right so let's look at Article lessons do you remember when we had buffer it wasn't a . 4 5 all right without adding any acid the about was 3 . 4 5 I added .period 1 Waller of acid and that it is now 3 . 3 1 seconds
in the pit was not altered all that much if I take fuel water and at the same amount of the pH changes by several orders of magnitude could take pure water in it .period 1 bowler the changes will hit 70 years 1 which is a big change in favor of offer conceded that he is the alter very slightly right and that's how about at the same
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