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Acids, bases and equilibrium.

Why is ethanoic acid a stronger acid in liquid ammonia then in aqueous solution?
A Ammonia is stronger base than water.
B Ammonium ethanoate is completely ionised in aqueous solution.
C Ammonium ethanoate is strongly acidic in aqueous solution.
D Liquid Ammonia is a more polar solvent than water.

Quick answer with explanation will be appreciated. Pre mocks starting this Monday.

(edited 4 years ago)

Reply 1

Pigster

Original post by Heidi002

Quick answer with explanation will be appreciated. Pre mocks starting this Monday.

What do you think it is and why?

Failing that, which options do you think are definitely wrong and why?

Reply 2

Heidi002

Original post by Pigster

What do you think it is and why?

Failing that, which options do you think are definitely wrong and why?

Well it would be really helpful if you can explain pretty much the whole thing to me.

Reply 3

Trumbles

Original post by Heidi002

Why is ethanoic acid a stronger acid in liquid ammonia than in aqueous solution?
A Ammonia is stronger base than water.
B Ammonium ethanoate is completely ionised in aqueous solution.
C Ammonium ethanoate is strongly acidic in aqueous solution.
D Liquid Ammonia is a more polar solvent than water.

Quick answer with explanation will be appreciated. Pre mocks starting this Monday.

Hi Heidi,

This question (acidity strength when using ammonia as a solvent) is not covered exactly on the courses I'm most familiar with, so you may want to do some follow-up checking.

First of all, do you understand what makes an acid strong or weak?
This is generally related to the tendency of a molecule to dissociate to form protonated ions (ones with an H⁺ involved) in a given solvent.
HCl is a strong acid (in water) because it ionises pretty much completely. If you add 1mol of HCl molecules to sufficient water you get 1mol of H⁺ions, or more accurately H₃O⁺ions, because it reacts with the water. So HCl + H₂O –> H₃O⁺ + Cl^-

Weak acids have more of a tendency to stay as the molecule. That is, an equilibrium is set up so that
C₂H₅OH + H₂O ^<=_> H₃O⁺ + C₂H₅O^-
The ethanoate ions have a strong tendency to (re-)react with the hydronium ions to reform the molecule and water. So a quantity of water to which 1mol of C₂H₅OH has been added might only contain 0.001mol of H₃O⁺, which is why you can eat chips with vinegar on without causing acid burns to your face.

Read that through a couple of times to make sure you've got it before going on.

An acid is a substance which tends to donate a proton; a base is a substance which tends to accept a proton
Notice that in the reaction going from left to right, the H₂O molecule is absorbing a proton, so it's acting as a base. (In the reaction going the other way the ethanoate ion is acting as the base.)

Looking back to the question...
Answers B and C look confused. We're not talking about ammonium compounds in aqueous solution. So while 'completely ionised' makes B look tempting I don't think that's it.
The polarity of NH₃on its own doesn't seem to be an explanation. (I don't think it is more polar anyway.)

So I think the answer is actually A. If we look at:
C₂H₅OH + NH₃ ^<=_> NH₄⁺ + C₂H₅O^-

If ethanoic acid is a stronger acid in ammonia than water, it must be because there are more protonated ions in solution. So eg 1 mol of ethanoic acid must produce more NH₄⁺ions in ammonia than it produces H₃O⁺ions in water. That is, the equilibrium is further to the right. Why would the that be? Well the NH₃has a greater tendency to take protons from C2H5OH to make NH₄⁺ions. The greater tendency to accept protons is to be a stronger base.

Hope that helps.