For MCAT purposes, a weak acid has Ka < 10^-4. For MCAT weak bases, Kb is ______

[JFK90787]

Can anyone fill in this blank?

Also, a question about Arrhenius acids. EK just told me ethanol isn't an Arrhenius acid. I was like 😡, I thought pretty much ANYTHING could be considered an Arrhenius acid since pretty much anything will dissociate a proton in water, albeit at very low concentrations. Can a dawg help me out? Thanks

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[Akarat]

Can anyone fill in this blank?

Also, a question about Arrhenius acids. EK just told me ethanol isn't an Arrhenius acid. I was like 😡, I thought pretty much ANYTHING could be considered an Arrhenius acid since pretty much anything will dissociate a proton in water, albeit at very low concentrations. Can a dawg help me out? Thanks

I think it's going to be same, in that a weak base has a Kb value of less than 10^-4.

An Arrhenius acid is a species that releases H+ in water. If ethanol is placed in water, it's going to act as a base than an acid because its pKa is greater than water's (Alcohols roughly have a pKa of 15 - 19 compared to water which has a pKa of about 7). Specifically, ethanol will act as a Bronsted-Lowry base, because it is able to accept a H+ from water.

The acid-base definitions go from more specific to more general, if you were to go from Arrhenius to Bronsted-Lowry to Lewis. For example, an Arrhenius acid can be classified as both a Bronsted-Lowry acid and Lewis acid. However, a Lewis acid can neither be classified as a Bronsted-Lowry acid nor an Arrhenius acid.

 

[JFK90787]

So anything with a conjugate base more basic than its conjugate acid is acidic, I.E something with a lower pKb than pKa will not be an Arrhenius A.?

 

[Akarat]

So anything with a conjugate base more basic than its conjugate acid is acidic, I.E something with a lower pKb than pKa will not be an Arrhenius A.?

Ethanol and water aren't conjugate acid/base pairs. H2O/-OH would be a conjugate acid/base pair and CH3CH2OH/CH3CH2O- would be a conjugate acid/base pair. In order to be a conjugate acid/base pair, they must differ only by a single H+. H3PO4/H2PO4- is a conjugate acid/base pair, but not H3PO4/HPO4 2-.

If chemical A has a pKa that is larger than chemical B's pKa, then chemical A will act as a base and chemical B will act as an acid, if the two are mixed. It's easier to just compare the pKa's of the two chemicals, or the pKb's of the two chemicals. It's harder to compare the pKa of one chemical to the pKb of another chemical, and decide which will act as the acid and which will act as the base.

As to if its Arrhenius, Bronsted-Lowry, or Lewis depends on how the two react. If, say chemical A, possesses a dissociable -OH group, then it can be classified as an Arrhenius base. If does not have a dissociable -OH group, but it can accept a H+ from water, and thus forming -OH in solution, then it is classified as a Bronsted-Lowry base. If it does neither of those, but can donate an electron pair to chemical B, and thus forming a bond, then it is a Lewis base.

 

[RogueUnicorn]

no, for the purposes of the MCAT a weak acid is anything that isn't a strong acid. this arbitrary Ka is pointless

 

[godcyning]

(Alcohols roughly have a pKa of 15 - 19 compared to water which has a pKa of about 7)

Water's pka is more like 16.

IIRC an Arrhenius acid is anything that increases [H+] in water; an Arrhenius base is anything that increases [OH-] in water. A Bronsted-Lowry acid is an H+ donor; a B-L base is an H+ acceptor. Lewis acid is an electron acceptor; Lewis base an electron donor.

So while in theory ethanol could lose an H+ in water, that alone doesn't make it an Arrhenius acid. While an H+ is perfectly able to fall off, an H+ from water is perfectly able - more able, in fact - to latch on and form EtOH2+. Because the equilibrium/pKa happens to be that way. Thus it won't increase the total [H+] in the solution.

 

[RogueUnicorn]

Water's pka is more like 16.

water's pka is exactly 14 at STP