Strength of Acid and Solvent  [ENDORSED]

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ZoeHahn1J
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Strength of Acid and Solvent

Postby ZoeHahn1J » Wed Dec 06, 2017 11:08 pm

Would anyone be willing to explain how the strength of an acid varies depending on the solvent? Also, how much does the solvent impact the strength of the acid relative to its polarity/electronegativity/number of oxygen atoms?

Chem_Mod
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Re: Strength of Acid and Solvent  [ENDORSED]

Postby Chem_Mod » Thu Dec 07, 2017 8:54 am

The role of solvent in acid/base chemistry can be complicated. Not only can certain solvents already contain strong acid or base, but the dielectric constant (polarity) of the solvent can stabilize or destabilize species in the acid/base reaction.

For example, acetic acid will be "more acidic" in water than in the significantly less polar solvent dimethylsulfoxide (DMSO). This is because in order to stabilize the species on the product side of the acid base reaction (the negatively charged acetate anion and the hydronium cation) the solvent must be polar. Since less of the product can remain stable in the DMSO solvent environment, the acetic acid will not dissociate as much, equilibrium will shift to the left, the pH will not be reduced as much, and acetic acid will appear to be a much weaker acid than it already is.

The so-called "common ion effect" and the pKa of the acid will determine how much will dissociate in a strongly basic or acidic solvent environment. The pKa of acetic acid is around 4.76 at 25 degrees Celsius. Thus, if the solvent has a lot of strong acid (for example, HCl) already dissolved in it, and the pH of the aqueous environment is therefore something like pH = 2.0, then only an entirely negligible amount of acetic acid will actually deprotonate in such an environment and it will seem to behave as a much weaker acid than it already is.


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