Section Name 25 pts Exercises 18 Explain the following observations in terms of

Question

Section Name 25 pts Exercises 18 Explain the following observations in terms of shifting equilibria by Le Chatelier's Principle Write the equation for the equilibrium involved in each case. a. Barium chromate, Bacro is insoluble in basic solution but dissolves readily in acidic solution. 5 pts thiocyanate equilibrium causes the color to disappear and a rust colored precipitate to from. 5 pts c. The solubility of lead chloride, PbCl2, in 0.10 M HCi is much lower than in pure water. 5 pts d. Limestone, CaCO, is an insoluble salt but will dissolve in acid. 5 pts

Explanation / Answer

Q1.

a)

BaCrO4 (s) <--> Ba+2 + CrO4-2

note that

BaCrO4 + OH- --> no reaction

BaCrO4 + H+ --> H2CrO4 + Ba+2 (aq)

since CrO4-2 ions are being used in the second reaction ( acid production) then, the shift goes toward MORE BaCrO4 sent to soution, i.e. to the right of the equilibrium

b

NaOH = Na+ + OH-

Fe(SCN)- comes from

Fe+3 + SCN- --> Fe(SCN)+2

note that excess OH- ions will interact with Fe+3 and will produce

Fe+3 + 3OH- <--> Fe(OH)3(s)

so Fe+3 ions from the original equilibrium decreases, so it shift toward more Fe+3 (reactants)

c.

PbCl2 <--> Pb+2+ 2Cl-

addition of HCl --> H+ + Cl- will clearly favour more PbCl2 solid production, since it is pretty saturated

d.

CaCO3 + Acid:

CaCO3 <-> Ca+2 + CO3-2

note that H+ wil interact with CO3-2 so

CO3-2+ + 2H+ <--< H2CO3

so the shift goest to more CaCO3 in solution

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