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Using a pKa table

How to use a pKa table to determine relative acid strengths.

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  • aqualine sapling style avatar for user Anita Haydar Dickerson
    At approximately what pka is it a good idea to start thinking of something as a base rather than an acid?
    (9 votes)
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    • starky ultimate style avatar for user Greacus
      pKa does not show if a molecule is basic or acidic. Basic molecules gain protons and acidic molecules donate protons. If a molecule is a base or an acid, depends on their functional groups. Certain functional groups give away (carbonic acid functional group) or gain protons (amide functional group) at specific pKa's. So one molecule can have multiple pKa values.

      So to answer your question: you can not tell if a molecule is basic or acidic by looking at their pKa('s). It can only tell you how strong the base or acid is.
      Hope this helps!
      (10 votes)
  • duskpin tree style avatar for user Jared Cosby
    So, I have a question. What characteristic of our reaction would , let's say, help Oxygen "cope" with losing its "beloved" Hydrogen? Are (organic) chemistry reactions simply whoever has the lower pKa will deprotonate... and what compound it deprotonates depends on its own pKa and its stability? Apologies if I am unclear.
    (4 votes)
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  • starky sapling style avatar for user Lucifer Broke
    Yeah but why are some alcohols more acidic than the other? What factors affect the acidity of an organic compound?
    (2 votes)
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  • female robot ada style avatar for user Anastasia Nesterova
    I guess that weak acid with pka=50 is pretty stable. Under which circumstances it can loose its proton to become the base?
    (2 votes)
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  • leafers seedling style avatar for user Morgwic
    Can someone explain how he came up with the other acid with pKa of 10 being 10^6 stronger than the acid with pKa 16? Is it like relative strength of Y = pKaY^(pKaX-pKaY) (where Y = 10 and X=16 (the pKa values for those acids)) so; strenght = 10^(16-10)? Bcs I don't think he explained how he came to that conclusion but that's the only explanation I can think of, but I'm not sure if it makes sense. thx
    (1 vote)
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  • leafers sapling style avatar for user maxime.edon
    I don't understand. I used to see that fluorhydric acid (HF) is more powerful than chlorhydric acid (HCl) (for example in Breaking Bad). Yet, in the "Ka and pKa review" video, we see that HF has a pKa of 3.5, which is of course greater than the -7 of HCl. I don't get it...
    (1 vote)
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    • piceratops sapling style avatar for user Ron McMullen
      Regarding the halide acids:
      Size of the Atom Bearing the Negative Charge: Of their anions, iodide ion is the largest; its charge is delocalized over a larger volume of space and, therefore, is the most stable. HI is the strongest of the halogen acids. Conversely, flouride ion is the smallest anion; its charge is the most concentrated, and flouride ion is the least stable. HF is, therefore, the weakest acid of the halogen acids. -quoted from "Organic Chemistry" Brown, Foote, Iverson, Anslyn. Sixth Edition
      (3 votes)
  • leaf green style avatar for user Ansar
    May it be said, that the higher pKa for a base is, the stronger the base is?
    (1 vote)
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  • blobby green style avatar for user neveikliba
    I have come across a task where you have to determine whether a functional group in an organic compound will be protonated or deprotonated at ph 7, pKa values were provided. What is the reasoning on how to determine this based on pKa values? And how does the pKa values relate to pH values when it comes to protonation and deprotonation?
    (2 votes)
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    • leaf red style avatar for user Richard
      We can relate pKa values to the pH of a solution by the Henderson–Hasselbalch equation which is: pH = pKa + log([A-]/[HA]), where A- is the conjugate base and HA is the acid. The Henderson–Hasselbalch determines the pH of buffers where a buffer is essentially a solution of a weak acid and a consequential amount of its conjugate base. Pretty much all organic functional groups are considered weak acids so the Henderson–Hasselbalch equation applies to them.

      So if we're given a pKa of a functional group then the pH can have three scenarios; the pH < pKa, the pH = pKa, and the pH > pKa. If the pH is equal to the pKa then the Henderson–Hasselbalch equation yields a [A-]/[HA] ratio of 1. This means that at this pH half of the molecules are protonated (HA) and the other half are deprotonated (A-). If pH < pKa then the molecules are mostly protonated, and if pH > pKa then the molecules are mostly deprotonated. Again these last two results come from using the Henderson–Hasselbalch equation and finding the [A-]/[HA] ratio.

      Hope that helps.
      (1 vote)
  • blobby green style avatar for user Björn Sundelin
    Great videos! What is the acid with pKa ~ -3 called?
    (1 vote)
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  • stelly blue style avatar for user bhart814696
    Why does Jay say pka of "proton" , shouldn't it be pka of "starting specie"?
    (1 vote)
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    • leaf red style avatar for user Richard
      It's interchangeable with simple acid/base reaction with only one hydrogen atom in the molecule. Frist of all, saying 'pka' tells me I'm deal with an acid which will donate it's proton to a base. For compounds with only one hydrogen like HCl, we can refer to the pka of the hydrogen or the entire compound, it's the same thing since only one proton is being donated. For compounds with different types of protons, this can mean different pka values for different hydrogens. This means a single compound can have multiple values of pka with each hydrogen having its own value.

      For an inorganic example we can look at sulfuric acid which is a diprotic acid, H2SO4. Sulfuric acid has two pka values because those two protons have different levels of acidity. So in this example we would refer the pka of one proton being -2.8 and the pka of the other proton being 2.

      For an organic example we can look at phenol at . This is a compound where it has two types of protons, the alcoholic proton and the aromatic protons in the ring. The pka of 10 only refers to the alcoholic proton so its only appropriate to refer to the particular hydrogen as having a pka of 10 and not the entire compound. The aromatic protons would have pka values higher than 10 showing that they're less acidic than the alcoholic proton. Hope this helps.
      (2 votes)

Video transcript

- [Voiceover] Let's look at how to use a pKa table. Remember from general chemistry that pKa is equal to the negative log of the Ka. And the lower the pKa value the stronger the acid. pka values are used a lot in organic chemistry, so it's really important to become familiar with them. For example, if our acid is H-Cl the pKa of this proton is approximately negative seven. So if a base comes along, some generic base, and takes this proton these electrons are left behind on the chlorine. So let's use a different color here. So the electrons in magenta come off onto the chlorine to form the chloride anion. So the chloride anion is the conjugate base to H-Cl. The lower the pKa value the stronger the acid, and out of all the acids I have on this pKa table H-Cl has the lowest pKa value. So this is the strongest acid out of the ones on this pKa table. And from general chemistry, the stronger the acid the weaker the conjugate base. Because for an acid to be strong the conjugate base must be weak to resist reprotonation. If it's easy to lose a proton it should be really hard to regain it, otherwise you wouldn't have a strong acid. So H-Cl has the lowest pKa value on this table, so it is the strongest acid. And the chloride anion must therefore be the weakest base out of all the conjugate bases on the right side of our table. Next, let's look at this acid. So this is the acidic proton, so if a base came along, some generic base, and took this proton and left these electrons behind on the oxygen. We will use this color again. So the electrons in magenta come off onto the oxygen to give us acetone as our conjugate base. The approximate pKa for this proton is negative three. And I've seen lots of different pKa values for this proton. I've seen negative six, I've seen negative seven, so use whatever pKa values you are given in your course. So pKa tables don't always match exactly and this is one that I've seen lots of different values for. Next we have hydronium, H3O+. So if a base comes along and takes a proton from hydronium the pKa of that proton is approximately negative two. So these electrons would come off onto the oxygen, so the electrons in magenta come off onto the oxygen, and those would be these electrons right here, to form water as our conjugate base. So H3O+ has a higher pKa value than H-Cl, therefore H3O+ is not as strong of an acid, or we could say it the other way around. H-Cl has a lower value for the pKa, therefore H-Cl is a stronger acid than H3O+. Alright, let's look at some more examples for acids and pKa values. So next let's look at acetic acid. So this compound right here. Here's the acidic proton on acetic acid and that proton has a pKa value of approximately five. So if a base takes that proton these electrons are left behind on the oxygen, which give that oxygen a negative one formal charge, so the acetate anion is the conjugate base to acetic acid. Next, our next compound, we have these two protons right here and let's say this proton, doesn't really matter which one you chose, has a pKa value of approximately nine. So if a base takes that proton then the electrons in magenta here are left behind on a carbon. Let me go ahead and circle the carbon, so this carbon would get a negative one formal charge. Our next compound is phenol and the acidic proton on phenol is this one right here. So if a base takes that proton these electrons in magenta are left behind on the oxygen, which give the oxygen a negative one formal charge. So this is our conjugate base. So the pKa for that proton is approximately 10. Alright, let's get some more space down here. Let's look at some more. Alright, next we have water. Alright, so what's the pKa for this proton on water? It's approximately 15.7. So the conjugate base to water would be hydroxide. Next we have ethanol. So this is the acidic proton on ethanol, the pKa value's approximately 16, so the electrons in magenta here would end up on the oxygen, giving the oxygen a negative one formal charge. So this is the ethoxide anion. So how much more acidic is phenol than ethanol? Remember the lower the pKa value, the stronger the acid. Phenol has a pKa value of 10 for this proton, whereas ethanol has a pKa value of 16, approximately, for this proton. So how much more acidic is this compound compared to this compound? Well, remember the definition for pKa. It's the negative log of the Ka. And each pKa unit is in order of magnitude. So how many units difference do we have between these pKa values? This one's 10 and this one's 16. So that's a difference of six. But really each unit is in order of magnitude, so this proton on phenol is 10 to the sixth times more acidic, so this compound is one million times more acidic than ethanol. So that's really how to think about acid strength when you're looking at a pKa table. And it's so much easier to work with pKa values, which is why they're used so often in organic chemistry. Alright, next let's look at another alcohol. So this proton has a pKa value of approximately 17 and this would be the conjugate base. This proton on this next alcohol the pKa is approximately 18, so that we get this as our conjugate base. And we'll talk about the reasons why, for example, we have these alcohols here. Later we'll talk about the reasons why they have different pKa values. Alright, let's look like, let's look at some more. So the pKa value for this proton is approximately 19. So if the base comes along and takes that proton that I just circled in red, these electrons in magenta would be these electrons and they're on this carbon now, which gives that carbon a negative one formal charge. Alright, next we have acetylene. Alright, so if we deprotonate acetylene this pKa has a value of approximately 25, so the electrons in magenta here are left behind on this carbon to give us the conjugate base. Alright, for ammonia. For ammonia if a base took that proton, these electrons would come off onto the nitrogen, so that would be these electrons here, giving us our conjugate base with a pKa value of approximately 36. And this is another compound you see a lot of different pKa values for. I've seen 33, I've seen 38. So again, use whatever pKa values you are given in your class. Alright, let's finish this off here, so our last few compounds. So what's the pKa for this proton? It's approximately 44. So the electrons in magenta come off onto your carbon and this would be your conjugate base. And our last example, ethane is the weakest acid out of all the acids I have listed on this pKa table. So the pKa for this proton, approximately 50. And the electrons in magenta here would be left behind on this carbon. So if this is the weakest acid, if ethane is the weakest acid, then this must be, this must be, on the right here, this must be our strongest base. So you can also use a pKa table to think about the strengths of bases. So if you had, if you had some acid here, let me go ahead and draw one, so H-A right here. So the electrons in magenta would take this proton and these electrons would be left behind, and so you would reprotonate, you would form, you would form your ethane. So if this functioned as a base and took a proton then you would make your ethane here. So the stronger the base, the weaker the conjugate acid. So pKa tables are important for thinking about the strengths of acids, and you can also use them to think about the strengths of bases.