Acid base strenghts

[151AND8TH]

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How can you tell which acid is stronger?
from the list below is there a way to tell them?

H2SO4>H3O+>CH3CO2H>CH3NH3+>NH3>CH3CH3

they are already in order from greatest to least acidic.. I have not been able to figure out "WHY"

 

[TheWiredNerv]

sulfuric acid has two protons to donate, H3O has 1 proton to donate and is a much stronger acid than acetic acid, CH3NH3+ has the positive charge so it is easier to donate a proton than NH3 alone. Lastly, ethane is the weakest because it will not donate any protons.

 

[MattH25]

NO! Acid strength has nothing to do with the amount of protons transferred. Some of the strongest acids known to man only transfer one proton. You have to look at the strength of the conjugate base.

H2SO4 ----> HSO4 (a very weak base since we know sulfuric is diprotic, a weak base is the conjugate of a very strong acid)

H30+ ----> H20 (water is a relatively weak base so hydronium will be a strong acid)

CH3CO2H ----> CH3CO2- (starting to get a bit more basic though still stabilized through resonance so it will come from a moderate acid)

CH3NH3+ ----> CH3NH2 (ok, no we are starting to get some stronger bases and stronger bases will always come from weaker acids)

NH3----> NH2- (I actually disagree with this. Amines with alkyl substituients are strongers bases than amines without substituients due to solvation etc... Nevertheless a strong base so it's conjugate will be a weak acid)

CH3CH3----> CH3CH2- (a primary carbanion..oh ya, you better believe that's a very strong base so it's conjugate will be an extremely weak acid)

 

[reliefe123]

NO! Acid strength has nothing to do with the amount of protons transferred. Some of the strongest acids known to man only transfer one proton. You have to look at the strength of the conjugate base.

H2SO4 ----> HSO4 (a very weak base since we know sulfuric is diprotic, a weak base is the conjugate of a very strong acid)

H30+ ----> H20 (water is a relatively weak base so hydronium will be a strong acid)

CH3CO2H ----> CH3CO2- (starting to get a bit more basic though still stabilized through resonance so it will come from a moderate acid)

CH3NH3+ ----> CH3NH2 (ok, no we are starting to get some stronger bases and stronger bases will always come from weaker acids)

NH3----> NH2- (I actually disagree with this. Amines with alkyl substituients are strongers bases than amines without substituients due to solvation etc... Nevertheless a strong base so it's conjugate will be a weak acid)

CH3CH3----> CH3CH2- (a primary carbanion..oh ya, you better believe that's a very strong base so it's conjugate will be an extremely weak acid)

ok...but then how do you know what is a strong or weak base...lol...i just memorized some of the stronger acids and stronger bases...i think there are only 56 primary strong acids and 5 primary stong bases to memorize..

 

[Corky Cacas]

well, i read through some of the responds but i deeply sad that no one mentioned about the number of Oxygens. if you look sulfuric acid has four oxygens which increase the electronegativity comparing to other acids and bases. none of the other ones has this many oxygen. this is why Sulfuric acid is the strongest. and please don't memorize the 56 acids. this won't help you on exam day. you will be under so much stress.

now i have a question for you:
which is the weakest base?

a. HClO4
b. NH3
c. NH2-
d. OH-
e. ClO4-

good luck guys.

 

[TheWiredNerv]

NO! Acid strength has nothing to do with the amount of protons transferred. Some of the strongest acids known to man only transfer one proton. You have to look at the strength of the conjugate base.

H2SO4 ----> HSO4 (a very weak base since we know sulfuric is diprotic, a weak base is the conjugate of a very strong acid)

H30+ ----> H20 (water is a relatively weak base so hydronium will be a strong acid)

CH3CO2H ----> CH3CO2- (starting to get a bit more basic though still stabilized through resonance so it will come from a moderate acid)

CH3NH3+ ----> CH3NH2 (ok, no we are starting to get some stronger bases and stronger bases will always come from weaker acids)

NH3----> NH2- (I actually disagree with this. Amines with alkyl substituients are strongers bases than amines without substituients due to solvation etc... Nevertheless a strong base so it's conjugate will be a weak acid)

CH3CH3----> CH3CH2- (a primary carbanion..oh ya, you better believe that's a very strong base so it's conjugate will be an extremely weak acid)

True

 

[fleur531]

I think the above question is probably E. because perchloric acid is normally a strong acid (http://en.wikipedia.org/wiki/Strong_acid) and therefore it's conjugate base would be weak.

In general--
If an acid is strong, conjugate base is weak.

If a base is strong, conjugate acid is weak.

Hope that's right / helps!

 

[tastybeef]

"well, i read through some of the responds but i deeply sad that no one mentioned about the number of Oxygens. if you look sulfuric acid has four oxygens which increase the electronegativity comparing to other acids and bases. none of the other ones has this many oxygen. this is why Sulfuric acid is the strongest."

This is illogical. If the number of oxygens matter, then acetic acid, which have 2 oxygens, would be more acidic than water, which only has 1 oxygen. However, this isn't true. Therefore, the number of oxygen doesn't tell you everything.

For sulfuric acid, it is the most acidic because you can draw many resonance stabilized conjugate base. A stable base means a strong conjugate acid.

 

[Flipper405]

For sulfuric acid, it is the most acidic because you can draw many resonance stabilized conjugate base. A stable base means a strong conjugate acid.

Bingo 👍