12.115

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Hyein Cha 2I
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12.115

Postby Hyein Cha 2I » Thu Dec 07, 2017 10:03 am

Question is:
Acetic acid is used as a solvent for some reactions b/w acids and bases.
a) nitrous acid and carbnonic acids are both weak acids in water. Will either of them act as a strong acid in acetic acid? explain your answer
b) will ammonia act as a strong or weak base in acetic acid? Explain your answer.

I do not understand the explanation provided in the solution manual. How do you determine whether an acid will be strong/weak acids depending on the solvent?

Curtis Tam 1J
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Re: 12.115

Postby Curtis Tam 1J » Thu Dec 07, 2017 10:59 am

You can think of it in terms of Le Chatelier's Principle.

When nitrous acid is dissolved, it produces a weaker conjugate base than that of acetic acid. This means that the conjugate base of acetic acid is better at eating all the hydronium ions (in the solutions manual, it says H+, not sure if they're referring to hydronium). For the system to go back to equilibrium, nitrous acid must dissociate even more to account for all the hydronium ions that the conjugate base of acetic acid has consumed. So in effect, there will be a higher Ka value for nitrous acid, which means it will act as a strong acid.

Carbonic acid is a weaker acid so it does not contribute as many hydronium ions in solution than acetic acid so no shift in equilibria.

Ammonia will act as a strong base based on the same logic (pun intended). The conjugate acid of ammonia is weaker than acetic acid itself. This means that acetic acid is better at consuming OH- ions. Therefore, to make up for the lost OH- ions, ammonia will have to dissociate even more, leading to a higher Kb value.

Hyein Cha 2I
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Re: 12.115

Postby Hyein Cha 2I » Thu Dec 07, 2017 12:03 pm

Curtis Tam 1C wrote:You can think of it in terms of Le Chatelier's Principle.

When nitrous acid is dissolved, it produces a weaker conjugate base than that of acetic acid. This means that the conjugate base of acetic acid is better at eating all the hydronium ions (in the solutions manual, it says H+, not sure if they're referring to hydronium). For the system to go back to equilibrium, nitrous acid must dissociate even more to account for all the hydronium ions that the conjugate base of acetic acid has consumed. So in effect, there will be a higher Ka value for nitrous acid, which means it will act as a strong acid.

Carbonic acid is a weaker acid so it does not contribute as many hydronium ions in solution than acetic acid so no shift in equilibria.

Ammonia will act as a strong base based on the same logic (pun intended). The conjugate acid of ammonia is weaker than acetic acid itself. This means that acetic acid is better at consuming OH- ions. Therefore, to make up for the lost OH- ions, ammonia will have to dissociate even more, leading to a higher Kb value.


How did you determine whether the nitrous acid produces a weaker conjugate base than that of acetic acid?

Isita Tripathi 2E
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Re: 12.115

Postby Isita Tripathi 2E » Thu Dec 07, 2017 2:16 pm

Nitrous acid has a pKa of 3.16, so the pKb of its conjugate base must be 14-3.16 = 10.84
Acetic acid has a pKa of 4.75, so the pKb of its conjugate base must be 14-4.75 = 9.25
Since 10.84 > 9.25, and a large pKb corresponds to a weaker base, the conjugate base of nitrous acid is weaker.

This can also be done conceptually: Knowing that HNO2 is a stronger acid than CH3COOH, we can conclude that it must have a weaker conjugate base than the conjugate base of CH3COOH because of the conjugate seesaw.

Hyein Cha 2I
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Re: 12.115

Postby Hyein Cha 2I » Fri Dec 08, 2017 8:59 am

Isita Tripathi 3H wrote:Nitrous acid has a pKa of 3.16, so the pKb of its conjugate base must be 14-3.16 = 10.84
Acetic acid has a pKa of 4.75, so the pKb of its conjugate base must be 14-4.75 = 9.25
Since 10.84 > 9.25, and a large pKb corresponds to a weaker base, the conjugate base of nitrous acid is weaker.

This can also be done conceptually: Knowing that HNO2 is a stronger acid than CH3COOH, we can conclude that it must have a weaker conjugate base than the conjugate base of CH3COOH because of the conjugate seesaw.


Thank you so much!


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