What Determines the Strength of a Conjugate Acid-Base Pair?

In summary: I think it has something to do with how the protons are arranged. The protons in a strong acid are closer together than the protons in a strong base, so they can't share a electron as easily. This causes the acid to ionize more easily in aqueous solution.
  • #1
Dooh
41
1
Uggh finally did all the chem problems and studied for the upcoming final but there are 2 questions left unanswered. MAybe someone can help me out with this:

If given a lists of acids, how would one know which 1 is the weakest conjugate base? For example, HF, HNO_2, H_2CO_3, H_3BO_3, HCl, which 1 would be it? I didn't know to solve for it.

Also, a conjudate acid-base pair consists of two substances that:

-differ by 1 proton
-neutralize each other
-have equal number of protons
-satisfy oth definition
or
-none

BY eliminationg all the choices, i concluded that its none but I'm still not sure.
 
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  • #2
The stronger the acid the weaker the conjugate base. It works both sides - the stronger the base, the weaker the conujgate acid.

For every answer eliminated, explain why, so that we can point you to your error.
 
  • #3
I think it's none because none of the options with the definition of an acid-base pair. They obviously don't neutralize each other and the number of protons in this case is irrelevant. That's what i think anyway. Am i right?
 
  • #4
i dunno, i would imagine that HCl is the strongest acid and therefore Cl- would be the weakest conjugate base.
 
  • #5
Dooh said:
the number of protons in this case is irrelevant

Write down any pair of acid and conjugate base.
 
  • #6
Borek said:
The stronger the acid the weaker the conjugate base. It works both sides - the stronger the base, the weaker the conujgate acid.

For every answer eliminated, explain why, so that we can point you to your error.


Borek
--
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Why is it so? can please give me the reason that why the conjugate base of an strong acid is weak?

When we define a strong acid (or base), we say that it is an acid (or base) which ionizes to a higher extent in acquous solution. What is the reason that it does so? Is the polarity of its bonds that causes the ionization?
 
  • #7
acid dissociation:

[tex]HA \leftrightarrow H^+ + A^-[/tex]

acid dissociation constant:

[tex]Ka = \frac {[H^+][A^-]}{[HA]}[/tex]

conjugated base dissociation:

[tex]A^- + H_2O \leftrightarrow HA + OH^-[/tex]

conjugated base dissociation constant:

[tex]Kb = \frac {[HA][OH^-]}{[A^-]}[/tex]

water dissociation:

[tex]H_2O \leftrightarrow H^+ + OH^-[/tex]

water dissociation constant:

[tex]Kw = [H^+][OH^-][/tex]

and now Ka*Kb:

[tex]Ka Kb = \frac {[H^+][A^-]}{[HA]} \frac {[HA][OH^-]}{[A^-]} =
[H^+][OH^-] = Kw[/tex]

In other words (or symbols):

[tex]pKa + pKb = pKw[/tex]

The stronger the acid, the weaker the conjugated base.
 
  • #8
Thanks! this answers my first question, now what about the second one: Is it the polarity of bonds that characterizes a strong acid or base or something else? what is that?

Dooh! please forgive me if I'm HACKING your thread... :biggrin:
 

1. What is a conjugate acid?

A conjugate acid is the species that is formed when a base gains a proton. It is the protonated form of the base and has one more proton and one less electron than the base.

2. What is a conjugate base?

A conjugate base is the species that is formed when an acid loses a proton. It is the deprotonated form of the acid and has one less proton and one more electron than the acid.

3. How are conjugate acids and bases related?

Conjugate acids and bases are related because they are formed from the same original molecule, with one gaining a proton and the other losing a proton. They are also related by the fact that they are in equilibrium with each other in an acid-base reaction.

4. What is the difference between a strong and weak conjugate acid/base?

A strong conjugate acid or base is one that is able to completely dissociate in water, resulting in a high concentration of either H+ or OH- ions. A weak conjugate acid or base only partially dissociates, resulting in a lower concentration of H+ or OH- ions.

5. How do you identify a conjugate acid-base pair?

A conjugate acid-base pair can be identified as two species that differ by only one proton. The acid will have one more proton than its conjugate base, and the base will have one less proton than its conjugate acid. They are also usually written next to each other in an acid-base reaction equation.

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