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## Organic chemistry

### Course: Organic chemistry > Unit 2

Lesson 3: Organic acid-base chemistry- Acid-base definitions
- Organic acid-base mechanisms
- Ka and acid strength
- Ka and pKa review
- Using a pKa table
- Using pKa values to predict the position of equilibrium
- Stabilization of a conjugate base: electronegativity
- Acid strength, anion size, and bond energy
- Stabilization of a conjugate base: resonance
- Stabilization of a conjugate base: induction
- Stabilization of a conjugate base: hybridization
- Stabilization of a conjugate base: solvation

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# Ka and pKa review

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

## Want to join the conversation?

- Why does making the log negative mean the number is easier to work with? Thanks(1 vote)
- 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)

- I guess the "a" on Ka and pKa means "acid", but what does "K" on Ka and "pK" on pKa stand for?(1 vote)
- 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)

- what is the physical significance of Ka and pKa ... that is what each means?

what the use of pKa(1 vote)- 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)

- 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)
- 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)
- Look up “table of Ka values” on Google. If you have a chemistry textbook there’s probably a table with a bunch of them too.

AgCl will be a Ksp one...totally different from this videos topic....(1 vote)

- is Ka always given ? if not how do solve with just the equation with the product over the reactant(1 vote)
- If you have to calculate concentrations, the Kₐ will be given.

Sometimes they will give you concentrations, and you will have to calculate the Kₐ.(1 vote)

- What is the relationship between pKa and side chain charge?(1 vote)
- Unfortunately, there's no general relationship between these two.(0 votes)

- Why is the pKa concept useful if you already got pH to mesure acidity? When should you use one measure or the other?

Thanks(0 votes)- It depends on what information you have been given to solve the numerical problem.(1 vote)

## 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.