Homework Help: Chemistry

Posted by Amy on Monday, November 12, 2012 at 9:30pm.

1.) Hydrogen fluoride (HF) is a weak acid, which when dissolved in water reacts slightly by the equation: HF(g) + H2O(l) = H3O^+(aq) + F^-(aq).
Predict the direction of shift of the equilibrium that occurs if some NaOH is added to the solution. Explain

-My guess is that equilibrium shifts to the LEFT because the concentration of reactants increases?

2.) Calcium fluoride (CaF2) is a sparingly soluble solid: CaF2(s) = Ca^+2 (aq) + 2F^-(aq). Predict whether CaF2 is more soluble in pure water of in water containing some HNO3. Explain in terms of shifts of equilibria.
(Hint: pay attention to your answer in question 1)

Chemistry - DrBob222, Monday, November 12, 2012 at 9:47pm
1. No and no. Le Chatelier's Principle says that a system in equilibrium will shift so as to undo what we do to it. (Yes, there are more esoteric ways of stating it but this is the heart of it.)
First, how could adding NaOH (a base) increase the concns of the reactants.
Here is what happens. NaOH, a base, reacts with the H3O^+ on the right side (an acid) to form H2O.
H3O^+ + OH^- ==> 2H2O.
Therefore you are decreasing the H3O^+ by adding NaOH so the reaction will try to add more H3O^+ to take the place of what has been removed. To do that it must shift to the right. Thus, the F^- will increase and HF will decrease.

2. Use the explanation from 1 to answer #2.

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