pka

[pistolpete007]

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rank the following in increasing acidity

pka=-60

pka=-40

pka=-22

i thought the it 3,2,1 bc the lower the pka the more acidic it is but it is actualy 1,2,3 does the rule change if its negative or somehting?

 

[Streetwolf]

the Lower The Pka The More Acidic It Is

-60 < -40 < -22

 

[pistolpete007]

why isnt -.60 the highest acidity isnt this the lowest pka value? do we not count the negative?

 

[Streetwolf]

Sorry must have misread something there. It's been a while since I've done pKas but I thought that -60 would be the most acidic.

Someone else know something I don't?

 

[DCRedskinsRule]

rank the following in increasing acidity

pka=-60

pka=-40

pka=-22

i thought the it 3,2,1 bc the lower the pka the more acidic it is but it is actualy 1,2,3 does the rule change if its negative or somehting?

My first gut reaction says that there must be a typo with the increasing/decreasing because as far as I can remember, the more negative and thus the smaller your pKa, the higher your Ka, which means you're producing more H+ ions, which means it's a stronger acid. So I would rank them 3,2,1 just like you. But I guess I must be missing something too. There's this one guy who's studying for his DAT now, OsimsDDS or something like that. He's bright and will probably set us straight. 🙂

 

[osimsDDS]

High Ka corresponds to a strong acid and high pKa corresponds to a weak acid...

Low Ka corresponds to a weak acid and Low pKa corresponds to a strong acid...

I think you have to view it as increasing or decreasing compared to a scale of 0...further from 0 means high and closest to zero means lower...but i am not sure. If this is the case having a negative or positive infront of the pKa wouldnt matter because -60 and 60 are the same distance from 0. I dunno if this rule corresponds to negative numbers for pKa, someone HELP!!!!

 

[TexasDent]

I am positive that a pKa of -60 is a much stronger acid than a pKa of -20... So in increasing acidity it would be -20<-40<-60.
I know this because I remember that fluoroantimonic acid is the strongest superacid and it had a very large negative numbered pKa; so the more negative your pKa gets, the stronger the acid is.

 

[DRHOYA]

I don't think its on a scale from zero. It's just a simple rule. The lower the pKa, the stronger the acid. The higher the Ka, the stronger the acid. I think we are overthinking it?

 

[250rsavage]

why isnt -.60 the highest acidity isnt this the lowest pka value? do we not count the negative?

Pete are the values -.60 or -60, you have two different values

all pKa is -log Ka

to generalized:
A stronger acid ( larger Ka) has a smaller pKa while a weaker acid (smaller Ka) has a larger pKa

-60 is the smallest pka and -20 is the largest pka

 

[pistolpete007]

what i wrote was correct....i got this from kaplan DAT lesson book pg 321 under hybridization it says acidity increases with the amounts of bonds a carbon has....there for acidity increases as we go from single bond>double>triple

however it showed that triple bond had pKa -22
double bond had pKa -40
and single had pKa-60

now if we arnt counting relative to 0 as AWESOME OSMIASdds said.....then it would appear that single bond with be the most acidic since it has a far greater negative pKa value....as u can see this is inconsistent with single bond>double>triple


speaking of bonds triple bond has less intramolecular attraction than double bond which has less than single bond
....what exactly does this mean is it saying that single bond is stronger than triple bond? that doesnt really make sense

 

[osimsDDS]

DUUUUUDE I am looking at the lesson book right now!!!

its not a negative!!! its a squiggly line....one of these ~...hahah

Therefore, its pKa~22, pKa~ 40, pKa~ 60 not negatives...

So yea it is correct, the more bonds a carbon has the more acidic and they show the triple bond has 22 pka and single bond has 60 pka...22 is smaller so it has a higher Ka and strong acid...

 

[osimsDDS]

oh and when its talking about intramolecular bonding, think of when you dont have triple bonds, you have hydrogens or other elements bonded to the C so there is more intramolecular bonding, when you have a triple bond there are less elements and the intramolecular attractions between those elements is less....

 

[250rsavage]

yeah - and ~ make a BIG difference.

 

[pistolpete007]

wowwwww i feel ******ed i need to get my eyes checked b4 i take the dats....im really sorry guys i've been studying for about 8hours now today so im really tired

 

[osimsDDS]

its alright you taught us all a valuable lesson, ~ is not the same as -