Bronsted Acids/Bases

[MedPR]

Found this question on a link from wikipremed. The link to the answer is dead, but it's not a hard question.

Just want to make sure that I'm on the right track (kind of late, since MCAT is on April 5, but still 😀)

Which of the following compounds can be Brnsted acids? Which can be Brnsted bases?

(a) H2O

(b) NH3

(c) HSO4-

(d) OH-

A, B, and C could be acids
All could be bases

Correct?

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[gettheleadout]

Looks right to me. Though oxide anion (O2-) can theoretically be formed via OH- <---> O2- + H+ the fact that O2- is so much stronger a base than OH- means the Ka for this dissociation would be probably be almost zero even under conditions allowing the reaction to take place at all.[FONT=verdana,geneva,arial,sans-serif][SIZE=+1]
[/SIZE].

 

[hellocubed]

In this sense, Would a Potassium Ion (K+)
be a Bronsted Acid?

(K+) + H2O -----> KOH + H+



although impossibly unfavorable that reaction may be?

 

[chiddler]

In this sense, Would a Potassium Ion (K+)
be a Bronsted Acid?

(K+) + H2O -----> KOH + H+



although impossibly unfavorable that reaction may be?

Bronsted acid describes something that donates protons. K+ has no protons so cannot be a bronsted acid.

I use: You're BAAD!

base accepts, acid donates. and then reverse it for lewis acid/base.

not sure if very efficient, but i started using it in high school so i'm proficient with it now

 

[hellocubed]

Bronsted acid describes something that donates protons. K+ has no protons so cannot be a bronsted acid.

I use: You're BAAD!

base accepts, acid donates. and then reverse it for lewis acid/base.

not sure if very efficient, but i started using it in high school so i'm proficient with it now

oh nvm. K+ is an Arrhenius Acid.


Thanks!~~

 

[gettheleadout]

In this sense, Would a Potassium Ion (K+)
be a Bronsted Acid?

(K+) + H2O -----> KOH + H+



although impossibly unfavorable that reaction may be?

So in this reaction we have K+, which has no protons to release (cannot be a Bronsted acid) but can accept electrons due to its positive charge (can be a Lewis acid.)

We also have H2O which can lose a proton (can be a Bronsted acid) or gain a proton (can be a Bronsted base). Since losing a proton results in gain of electrons from the bond that was broken, and gaining one results in loss of a lone pair, H2O can simultaneously act as either a Lewis acid or Lewis base respectively.

In this reaction H2O is releasing a proton, so it is acting as a Bronsted acid. K+ is gaining electrons from the oxygen in the OH- remaining after the proton is removed from H2O, so K+ is acting as a Lewis acid.

In this particular scenario, Lewis acid and Bronsted acid are not synonymous, though they often are in standard acid-base chemistry.

 

[SaintJude]

I don't think NH3 is Bronsted acid.

 

[gettheleadout]

I don't think NH3 is Bronsted acid.

Oh wow, yeah ammonia is a weak base, I totally missed that. NH3 is unlikely to lose a proton and form NH2-.

So the answer is just H2O and HSO4- then.

 

[MedPR]

Bronsted acid describes something that donates protons. K+ has no protons so cannot be a bronsted acid.

I use: You're BAAD!

base accepts, acid donates. and then reverse it for lewis acid/base.

not sure if very efficient, but i started using it in high school so i'm proficient with it now

+1 for proper use of you're!

 

[411309]

Found this question on a link from wikipremed. The link to the answer is dead, but it's not a hard question.

Just want to make sure that I'm on the right track (kind of late, since MCAT is on April 5, but still 😀)

Which of the following compounds can be Brnsted acids? Which can be Brnsted bases?

(a) H2O

(b) NH3

(c) HSO4-

(d) OH-

A, B, and C could be acids
All could be bases

Correct?

break it down; H2O could gain a hydrogen to become hydronium H3O or lose one to become OH hydroxide. NH3 could become NH4 or lose an H to become NH2. HSO4 could gain a H to become sulfuric acid (H2SO4) or lose one to become SO4. OH couldnt lose that H because elements that are by themselves typically exist in diatomic form anyways. Logically it's a base because it could accept a hydrogen to become water.


sooo yes. all of them are right :O

 

[MedPR]

I don't think NH3 is Bronsted acid.

Oh wow, yeah ammonia is a weak base, I totally missed that. NH3 is unlikely to lose a proton and form NH2-.

So the answer is just H2O and HSO4- then.

Can't NH3 lose a proton to become NH2 1-?

 

[hellocubed]

Can't NH3 lose a proton to become NH2 1-?

Yeah I would think so.

It should still be an acid, albeit a very weak one.

 

[gettheleadout]

break it down; H2O could gain a hydrogen to become hydronium H3O or lose one to become OH hydroxide. NH3 could become NH4 or lose an H to become NH2. HSO4 could gain a H to become sulfuric acid (H2SO4) or lose one to become SO4. OH couldnt lose that H because elements that are by themselves typically exist in diatomic form anyways. Logically it's a base because it could accept a hydrogen to become water.


sooo yes. all of them are right :O

If you admit OH- won't donate its proton then how are all of them right?

Can't NH3 lose a proton to become NH2 1-?

Yeah I would think so.

It should still be an acid, albeit a very weak one.

I'm seeing this as the same as the OH- situation. Yeah, it can technically lose a proton, but it's not going to. Ammonia is known to be a weak base and I can't find a Ka for NH3 <---> NH2- + H+

 

[411309]

Can't NH3 lose a proton to become NH2 1-?

i would think so also. im pretty sure that list is right.

 

[gettheleadout]

So apparently pure ammonia auto-ionizes with a K = 10^-30, and can also lose the proton in forming amides. So ammonia can in fact function as an extremely weak acid.

Still not totally sure about OH-.

 

[MedPR]

So apparently pure ammonia auto-ionizes with a K = 10^-30, and can also lose the proton in forming amides. So ammonia can in fact function as an extremely weak acid.

Still not totally sure about OH-.

I think OH- is different from NH3 because OH- becomes a monatomic Oxygen when it donates a proton. I don't know if O2- can even really exist.

 

[gettheleadout]

I think OH- is different from NH3 because OH- becomes a monatomic Oxygen when it donates a proton. I don't know if O2- can even really exist.

Just because oxide ion is only ever an intermediate or part of a transition state (and as far as I can find is never stable on its own) doesn't mean OH- can't be acting as an acid. Take the oxidation-reduction reaction for electrolysis of water.

2H2O ---> 2H2 + O2

It's more easy to explicitly see when the half reactions are written out, but given that only one proton can dissociate from H2O at a time, at some point the OH- remaining (and present in the half reaction) must lose its proton and either form an extremely short-lived intermediate of O2- or simply lose the proton while forming an O=O bond. Either way, the oxygen in the OH- molecule is no doubt losing a proton (acting as a Bronsted acid) and simultaneously gaining electrons from the dissociating bond (acting as a Lewis acid). It seems more likely to me that it loses the proton while forming the O=O bond, but that doesn't change the fact that the oxygen atom is still breaking its O-H bond, by definition acting as a Bronsted acid, even if it's giving electrons away to another oxygen (acting as a Lewis base) at the same time as it gains those from the breaking O-H bond back (acting as a Lewis acid.)

 

[gettheleadout]

So does anyone know the actual answer? I feel like there are too many technicalities with this question for it to actually be posed.