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What change will be caused by addition of a small amount of HCl to a solution containing fluoride ions and hydrogen fluoride?

Answer 1

We interrogate the equilibrium:

#HF(aq)+ H_2O(l) rightleftharpoons F^(-) + H_3O^+#

For #HF#, #K_a=10^(-3.17)#

#HF# is a weak hydrohalic acid given (i) the relative strength of the #H-F# bond, and (ii) more importantly, the large, entropically disfavoured charge density of the anion.

Addition of hydrogen chloride will drive the equilibrium to the left.

Note that fluoride salts of alkali metals give SLIGHTLY basic aqueous solutions, due to the equilibrium:

#F^(-) +H_2O(l) rightleftharpoonsHF(aq) + HO^-#

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Answer 2

Wyatt Adkins

Adding a small amount of HCl to a solution containing fluoride ions (F-) and hydrogen fluoride (HF) will increase the concentration of hydrogen ions (H+) in the solution. This will shift the equilibrium of the reaction HF ⇌ H+ + F- to the left, according to Le Chatelier's principle. Consequently, more fluoride ions will react with the added hydrogen ions to form additional hydrogen fluoride, decreasing the concentration of fluoride ions in the solution. This process is an example of the common ion effect, where the addition of an ion common to an existing equilibrium causes the equilibrium to shift in a direction that reduces the concentration of that ion.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.