Question

1.) 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, BaCrO4, is insoluble in basic solution but dissolves readily in acidic solution.

b.) The addition of NaOH to the iron (III)- thiocyanate equilibrium causes the red color to disappear and a rust colored precipitate to form.

c.) The solubility of lead chloride, PbCl2, in 0.10 M HCl is much lower than in pure water.

d.) Limestone, CaCO3, is an insoluble salt but will dissolve in acid.

e.) For part C, how does the rate of the reaction vary as the temperature is increased? Explain.

Answer 1

(a) BaCrO₄ + H₂SO₄ BaSO₄ + H₂CrO₄ (chromic acid)

Here a stable product chromic acid is formed therefore it is not in equilibrium condition,it is a irreversible reaction

(b) [Fe(SCN)(H₂O)₅]⁺² + NaOH   Fe⁺³(aq) + NaSCN (colourless) + H₂O

In this equation Fe⁺³(aq) slowly converted into Fe₂O₃ that is known as rust and the red colour disappear

(c) PbCl₂ + HCl(aq) PbCl₂ + Cl^- + H⁺

     PbCl₂ + H₂O 2HCl + Pb⁺²(aq)

The solubility of PbCl₂ is very low in HCl because no further chemical species formed in this reaction and only the concentration of chloride ions increases but in case of water a third chemical species HCl is formed that increase the solubility.

(d) CaCO₃ + H₂SO₄ CaSO₄ + H₂O + CO₂

It dissolve in H₂SO₄ completely to form CaSO₄ & evolve CO₂ gas.

(e) There is no effect of temperature on the reaction (c) because there is no further chemical formed in this reaction.