what is pKa?

I don't understand the point of pK_a.

I understand K_a is the acid dissociation constant, but what does pK_a signify?

Also in my text book, it says "the pH range in which an indicator changes colour is approximately equal to pK_a±1". What does that mean?

Original post by eggfriedrice

I don't understand the point of pK_a.

I understand K_a is the acid dissociation constant, but what does pK_a signify?

Also in my text book, it says "the pH range in which an indicator changes colour is approximately equal to pK_a±1". What does that mean?

pKa is just a way of representing acid strength without all of the very small numbers.

The 'p' system (i.e. -log₁₀) allows us to write very small numbers as simple, easy to understand numbers between 0 and 30 ish

Reply 2

eggfriedrice

OP

16

Original post by charco

pKa is just a way of representing acid strength without all of the very small numbers.

The 'p' system (i.e. -log₁₀) allows us to write very small numbers as simple, easy to understand numbers between 0 and 30 ish

So it's similar to pH and [H+]?

Reply 3

Chemmajor

In some ways yes as a small pKa value signifies a higher pH.

Posted from TSR Mobile

Original post by eggfriedrice

So it's similar to pH and [H+]?

It's exactly the same idea. That's what the 'p' stands for in pH.

You have:

pka
pkb
pkw
pH
pOH

always -log₁₀ of the concept ...

Reply 5

Big-Daddy

13

Original post by eggfriedrice

Also in my text book, it says "the pH range in which an indicator changes colour is approximately equal to pK_a±1". What does that mean?

The indicator being referred to is an acid of form HInd participating in an equilibrium in the solution, HInd <-> H⁺ + Ind^-. Both Ind^- and HInd are coloured and Ind^- is most prevalent under alkaline conditions whereas HInd dominates in acidic conditions. K_Ind=[Ind^-][H⁺]/[HInd] so [Ind^-]/[HInd]=K_Ind/[H⁺]=10^pH-pK_Ind.

Essentially, what your book is saying is that when [HInd] is 10 times greater than [Ind^-] or more, then only the colour of HInd is visible, and the same is true for Ind^-. This creates a calculable pH range (plug in the values to the equation I wrote and see) which comes to 1 unit in either direction of the exact pH at which [HInd]=[Ind^-], which is the pH in the middle, which is equivalent to the pK_Ind of your indicator. (When they say pK_a they really mean pK_Ind, but it's ok to use either here because the indicator in this case is decidedly an acid.)

Reply 6

Lelyke

5

Lol I don't see the point either, if a low pH already indicates strong ionization in water hence stong dissociation what's the point of pKa?

Original post by Lelyke

Lol I don't see the point either, if a low pH already indicates strong ionization in water hence stong dissociation what's the point of pKa?

The pH is concentration dependent, the pka is a constant.

pka gives a measure of acid strength while pH is a function of both acid strength and concentration.

Reply 8

Big-Daddy

13

Original post by Lelyke

Lol I don't see the point either, if a low pH already indicates strong ionization in water hence stong dissociation what's the point of pKa?

charco summed up the crucial point but I would just add more broadly that pH is a property of the solution as a whole, which could contain anything - any amount of your starting acid, any other acid, any bases, any buffers, any salts, etc. - whereas the pKa refers specifically to your acid to give an idea of how far its dissociation will go (or at least the equilibrium constant describing that dissociation).