Acids and bases

[chiddler]

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Which is not possible?

A lewis acid that is not a bronsted acid.
A bronsted acid that is not a lewis acid.

the second is not possible.

BF3 is an exception that helps prove this answer, but is there anything to really learn besides...

well i'm not sure. what is there to learn from this question?

 

[MedPR]

Which is not possible?

A lewis acid that is not a bronsted acid.
A bronsted acid that is not a lewis acid.

the second is not possible.

BF3 is an exception that helps prove this answer, but is there anything to really learn besides...

well i'm not sure. what is there to learn from this question?

I think the concept is that all bronsted acids are also lewis acids. Which is basically what the question/answers say.. so yea I don't know either.

 

[shoehornlettuce]

Which is not possible?

A lewis acid that is not a bronsted acid.
A bronsted acid that is not a lewis acid.

the second is not possible.

BF3 is an exception that helps prove this answer, but is there anything to really learn besides...

well i'm not sure. what is there to learn from this question?

A lewis acid is just an electron pair acceptor. A bronstead acid is a proton donor. By definition a proton must be accepting an electron pair i.e. when you put HCl in water the oxygen atom in water donates an electron pair to the proton (proton accepts the electron pair) to form hydronium ion thus bronstead acids are lewis acids.

 

[MedPR]

Ok so let me get this straight.

Arrhenius definition is the most specific definition, while Lewis is the most broad definition and Bronsted-Lowry is an intermediate? Meaning, all Arrhenius acids are Lewis acids, but not all Lewis acids are necessarily Arrhenius acids?

 

[shoehornlettuce]

Ok so let me get this straight.

Arrhenius definition is the most specific definition, while Lewis is the most broad definition and Bronsted-Lowry is an intermediate? Meaning, all Arrhenius acids are Lewis acids, but not all Lewis acids are necessarily Arrhenius acids?

My understanding is that Arrhenius and Bronstead kind of say the same thing differently. Whether you create OH- or H30+ or if you are a proton donor or proton acceptor, either way you are creating hydroxide or hydronium ions in water. Lewis acids and bases encompass all of this with regards to acids and bases in water (generation of H30+ and OH-) but often times you are using organic solvents so OH- and H30+ are not being formed.

 

[chiddler]

A lewis acid is just an electron pair acceptor. A bronstead acid is a proton donor. By definition a proton must be accepting an electron pair i.e. when you put HCl in water the oxygen atom in water donates an electron pair to the proton (proton accepts the electron pair) to form hydronium ion thus bronstead acids are lewis acids.

oh yeah you're right about that...

cool thanks.

medpr: i never thought of them as specific/non specific, rather just different ways of describing acidity.

 

[chiddler]

Another question, please:

Why is the hydride ion more basic than hydroxide ion? If two salts like NaOH or NaH, I would expect the same basicity.

 

[MedPR]

Another question, please:

Why is the hydride ion more basic than hydroxide ion? If two salts like NaOH or NaH, I would expect the same basicity.

Think about what a base does. Lewis bases donate electrons. "Hydride" implies H 1-. As in NaH, the Na is 1+ so the H is 1-. A hydride ion free in solution will more readily donate it's electron than will a hydroxide ion which is more stable due to the electronegativity of Oxygen as well as the negative charge being a bit more delocalized than in H-.

 

[mcloaf]

Another question, please:

Why is the hydride ion more basic than hydroxide ion? If two salts like NaOH or NaH, I would expect the same basicity.

Well, a lewis base is an electron pair donor, so whichever molecule more wants to get rid of its negative charge is going to be the better base. The relatively large and electronegative oxygen (with hydrogen attached) is going to be a lot happier stabilizing a negative charge than a single hydrogen ion.

 

[mcloaf]

Think about what a base does. Lewis bases donate electrons. "Hydride" implies H 1-. As in NaH, the Na is 1+ so the H is 1-. A hydride ion free in solution will more readily donate it's electron than will a hydroxide ion which is more stable due to the electronegativity of Oxygen as well as the negative charge being a bit more delocalized than in H-.

MedPR, you're dominating me in response time!

 

[chiddler]

MedPR, you're dominating me in response time!

this isn't a competition lol

 

[chiddler]

Think about what a base does. Lewis bases donate electrons. "Hydride" implies H 1-. As in NaH, the Na is 1+ so the H is 1-. A hydride ion free in solution will more readily donate it's electron than will a hydroxide ion which is more stable due to the electronegativity of Oxygen as well as the negative charge being a bit more delocalized than in H-.

and i also just realized that if you compare conjugate acids, H2O is a stronger acid than H2, therefore OH is a weaker base.

thanks very much for both responses

 

[MedPR]

MedPR, you're dominating me in response time!

Haha sorry, I try to teach what I can on here because it helps the concept stick in my head. I don't know anyone who is studying for the MCAT or who is still a premed so I'm basically on my own besides everyone here also studying 🙂

 

[mcloaf]

Haha sorry, I try to teach what I can on here because it helps the concept stick in my head. I don't know anyone who is studying for the MCAT or who is still a premed so I'm basically on my own besides everyone here also studying 🙂

Yeah, there's definitely truth to the saying that you don't understand something until you can explain it to others. Cheers.

 

[MT Headed]

My understanding is that Arrhenius and Bronstead kind of say the same thing differently. Whether you create OH- or H30+ or if you are a proton donor or proton acceptor, either way you are creating hydroxide or hydronium ions in water. Lewis acids and bases encompass all of this with regards to acids and bases in water (generation of H30+ and OH-) but often times you are using organic solvents so OH- and H30+ are not being formed.

I'm not comfortable with this analysis. EK audio osmosis describes the differences in detail (including an awful swedish accent for arrhenius) but the gist was this:

Arrhenius: acids create H+ (or H3O+) and bases create OH- in aqueous solution.

Bronsted-Lowry: acids donate a proton, bases accept a proton. This non-aqueous definition allows for things like acetic acid donating a proton to ammonia to create ammonium acetate in the absence of water.

Lewis: acids accept an electron pair, bases donate an electron pair. This non-hydrogen definition allows for things like AlCl3 being an acid and NF3 being a base, in the absence of hydrogen atoms.

I think MedPR had the right idea. Arrhenius most specific; B-L intermediate; Lewis most general.

 

[SaintJude]

May I recap, ('cause it helps me synthesize too)

Arrhenius is only applicable to aqueous solutions and defines an acid or base, of how much H+ ions or OH- ions are produced in water, respectively.

Bronsted Base is a proton acceptor (eg. NH3). Lewis Base is an electron donor (OH-, NH3).

Bronsted Acid is a proton donor (H+) (like H2SO4). Lewis Acid is an electron acceptor.

Every Arrhenius Acid is also a Bronsted Acid. Every Bronsted is a Lewis.


I hope they remove this somewhat useless topic for the 2015 MCAT.

 

[mcloaf]

It's not too bad once you get the hang of it. I've yet to see a question that focuses on Bronsted-Lowry or Arrhenius, mostly they want to see if you can figure out where the electrons are going to and coming from and then describe the reaction in Lewis acid-base terms, which is usually pretty straightforward.