Question

6. The dissociation reaction for hydrofluoric acid is shown below. HF (aq) = H (aq) + F (aq) a. Write an equilibrium expression for the dissociation reaction shown above. b. An HF solution at equilibrium is found to contain 0.0445 M HF, 0.00549 MH', and 0.00549 MF. Based on this information, what is the value of Key for the dissociation of HF? Show calculations to support your answer. A small amount of CaCl2 is added to the HF solution described in part (b). The calcium ions react with the F ions in the solution to form Cafz(s), decreasing (F). How will (H') and (HF) change in response to this perturbation? Briefly explain your answer. d. A small amount of Naf is added to the HF solution described in part (b). How will [H') and (HF] change in response to this perturbation? Briefly explain your answer. e. A solution of HF at equilibrium is found to contain equal concentrations of HF and F. What is the concentration of H' in this solution? Assume this solution is at the same temperature as the solution from part (b). Show calculations to support your answer. f. The dissociation reaction of HF(aq) solution is endothermic. How will [H") and (HF) change in response to an increase in the temperature of the solution? Briefly explain your answer.
question 6 a-f

Answer 1

6.

a.

Keq= H+] x [F- HF

b.

[H+] [F-] Ke = [HF) 0.00549 x 0.00549 0.0445 Keq = 0.000677 Keq =

c.

HF = H+ + F

Since fluoride ions are removed (by reaction with calcium ions to form calcium fluoride), the equilibrium for dissociation of HF will shift to right. More and more HF will dissociate to produce more and more protons and fluoride ions. This will compensate for removal of fluoride ions (through reaction with calcium ions). Due to this, the concentration of HF will decrease and the concentration of protons will increases.

d.

NaF → Na+ + F-

Added NaF is strong electrolyte and completely dissociates to provide fluoride ions (common ions). This will increase fluoride ion concentration. Due to this, the equilibrium for dissociation of HF will shift to left. More and more protons will combine with fluride ions to provide HF. Due to this fluoride ion concentration will decrease. This will compensate for added fluoride ions (in the form of NaF). Due to this, concentration of HF will increase and that of protons will decrease. This illustrates common ion effect.

e.

Since the temperature is same, the value of the equilibrium constant is also same.

Keq = ? [H+] x [F] (HF] 0.000677 = "T*[] [HF] (: [F-] = [HF]) [F-] H+1 = 0.000677 M F-1 0.000677 – [HT]

f.

Endothermic reaction means heat is supplied during reaction.

HF + heat = H+ +F-

With increase in temperature, heat is supplied to the reaction mixture. Here heat appears on reactant side. Due to this, the equilibrium for dissociation of HF will shift to right. More and more HF will dissociate to produce more and more protons and fluoride ions. The concentration of HF will decrease and the concentration of protons will increases.