Why isn't acetic acid a strong electrolyte/acid

  • Thread starter gsingh2011
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  • #1
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Since HC2H3O2 is made up of acetate which is a polyatomic ion, shouldn't it ionically bond with H+? If thats the case, why doesn't it completely dissociate in water and become a strong electrolyte?
 

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  • #2
Borek
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Since HC2H3O2 is made up of acetate which is a polyatomic ion, shouldn't it ionically bond with H+?
Just because something is polyatomic doesn't mean it is ionic.

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  • #3
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The degree of covalent vs. ionic nature of a bond is determined by the difference in electronegativity of the bonding atoms. In the case of O and H, you get a polar covalent bond, not an ionic one.
 
  • #4
Borek
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The degree of covalent vs. ionic nature of a bond is determined by the difference in electronegativity of the bonding atoms. In the case of O and H, you get a polar covalent bond, not an ionic one.
This is actually a little bit more complicated in the case of carboxylic acids - this is not just a common O-H bond like in alcohol, as there is a carbonyl group close by, which stabilizes anion after dissociation. Still, you are right that it is not an ionic bond.

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  • #5
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Yes, that's true. The nearby carbonyl group does contribute to the ionic nature of the O-H bond in a carboxyl group. Resonance stabilization is the reason they dissociate to the degree they do. The point is that the degree of ionic character of the bond is not the sole factor determining solubility. For example, compare the dissociation constants of ethanol and sulfuric acid.
 
  • #6
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Acetic acid is a weak acid because acetate and a free proton are not as thermodynamically stable as acetic acid.

Consider the acid dissociation constant (K_a): an acid with a low K_a, has an equilibrium concentration of the acid that is much greater than the product of the equilibrium concentration of the conjugate base and the equilibrium concentration of protons.

Strong acids completely dissociate; therefore, they have very large K_a's.
 
  • #7
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You might say it is thermodynamic instability. I would say it is due to the difference in energy. Maybe it's the same thing?

Is it possible to use the Botzmann distribution to compute the molar energy difference, e.g.
[tex]\Delta E= E_{Ac^-}-E_{AcH}=-RT\ln\frac{[Ac^-]}{[AcH]}[/tex]
in a solution of acetic acid?
 

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