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Ka and pKa review

A short review of Ka and pKa for students taking organic chemistry.

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  • blobby green style avatar for user Greenwolfe
    Why does making the log negative mean the number is easier to work with? Thanks
    (1 vote)
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    • piceratops tree style avatar for user Chanceus
      Almost all of the acids you work with in organic chemistry have a Ka that takes the form a*10^-b, where a is a positive real number and b is a non-negative integer. As such, almost all of the pKa values will be positive. If you don't take the negative log and instead take the positive log, then you'll be working with negative numbers constantly. Making the value negative doesn't really distort the meaning of the final number; it just changes the direction and switches the sign. Instead of having lots of negative numbers where the lower values are stronger acids, you have lots of positive numbers where the higher values are weaker acids.
      (4 votes)
  • starky ultimate style avatar for user William Shiuk
    I guess the "a" on Ka and pKa means "acid", but what does "K" on Ka and "pK" on pKa stand for?
    (1 vote)
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    • leaf red style avatar for user Richard
      I haven't been able to find any material to definitively say exactly why, but my best guess is that the 'K' stands for constant. The scientists which popularized this idea of chemical equilibrium were Norwegian chemists Maximilian Guldberg and Peter Waage. Constant in Norwegian is spelled "konstant", starting with a 'K'. So the 'K' in equilibrium equations (and also kinetics) could just be constant in Norwegian.

      Alternatively Guldberg and Waage also published papers in Danish and German where constant is spelled "konstant" and "konstante" respectively. So the 'K' could also mean constant, but in either of those languages too.

      Again I'm not 100% sure that's the origin, but it's the most likely origin in my opinion.

      Hope that helps.
      (2 votes)
  • piceratops seedling style avatar for user shravya
    what is the physical significance of Ka and pKa ... that is what each means?

    what the use of pKa
    (1 vote)
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    • leaf green style avatar for user SLRK.Bones
      Ka is defined by the equation given, [H+][A-]/[HA]. In general, you can kind of sort of think of equilibrium constants as the ratio between the products and the reactants (you have to be careful to include any coefficients and to check the states of matter when dealing with some kinds of reactions, but this is close enough to develop loose intuition). Since equilibrium is a situation where the reaction goes both ways, looking at the equilibrium constant gives us an idea of which way the reaction goes more. When it is larger than one, more products are formed than reactants, so the reaction mostly goes forward. When it is smaller than one the opposite is true (note that for weak acids Ka is much smaller than one, which means that, while some of the acid dissociates, most of it stays together).

      pKa is simply negative one times log base 10 of Ka. This is done just to make the numbers easier to work with, the same way we tend to talk about acidity in terms of pH rather than [H+].
      (1 vote)
  • leaf grey style avatar for user Josiah Garza
    Would the pattern of weak acid strength in relation to Ka/pKa values directly correspond to the strength of weak bases and their respective Kb/Pkb values?
    (1 vote)
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  • duskpin seed style avatar for user asmaaltahan
    how did you get those numbers like HF= 3.5 x 10? how do i know for example [ag+][CI-] number would it be? is there a rule for each that would have numbers written for them? I dont understand that part from where do those number appear 😅
    (1 vote)
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  • blobby green style avatar for user Kobe
    is Ka always given ? if not how do solve with just the equation with the product over the reactant
    (1 vote)
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  • blobby green style avatar for user Sammi Browne
    What is the relationship between pKa and side chain charge?
    (1 vote)
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  • blobby green style avatar for user Red
    Why is the pKa concept useful if you already got pH to mesure acidity? When should you use one measure or the other?

    (0 votes)
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Video transcript

- We've already talked about how to write an equilibrium expression, so if we have some generic acid HA that donates a proton to H2O, H2O becomes H3O+ and HA turns into the conjugate base, which is A-. Here's our equilibrium expression and the ionization constant Ka for a weak acid. We already talked about the fact that it's going to be less than one. Here we have three weak acids: hydrofluoric acid, acetic acid, and methanol. Over here are the Ka values. You can see that hydrofluoric acid has the largest Ka value, so even though they're all considered to be weak acids, 3.5 times 10 to the negative 4 is larger than 1.8 times 10 to the negative 5. So hydrofluoric acid is stronger than acetic acid, and acetic acid is stronger than methanol. But again, they're all considered to be weak acids, relative to the stronger ones. Let's talk about pKas. The pKa is defined as the negative log of the Ka. If we wanted to find the pKa for methanol, all we have to do is take the Ka and take the negative log of it. So the pKa is equal to the negative log of 2.9 times 10 to the negative 16. Let's get out the calculator and let's do that. Negative log of 2.9 times 10 to the negative 16. This gives us 15.54 when we round that. So the pKa of methanol is equal to 15.54. We could write in a pKa column right here, and for methanol it's 15.54. If you did the same calculation for acetic acid, you would get 4.74; and once again, if you did this for hydrofluoric acid, you would get 3.46. So as we go up on our table here, we're increasing in acid strength. Out of our three weak acids, hydrofluoric acid is the strongest, so it has the largest value for Ka, but notice it has the smallest value for the pKa. The lower the value for pKa, the more acidic your acid. 3.46 is lower than 4.74, and so hydrofluoric acid is more acidic than acetic acid.