Question

Reaction (i) Pb2+(aq) + 2 Cl-(aq) ⇌ PbCl2(s)

If NaOH is added to reaction (i), a second precipitation reaction will occur: Reaction (ii) Pb2+(aq) + 2 OH-(aq) ⇌ Pb(OH)2(s) The important aspect of this is that the two reactions take place within the same container (just because we draw them separately on paper does not mean they are separate inside in the container). Thus, reaction (i) and (ii) are coupled at the molecular level by a common chemical species that is shared – the Pb2+ ions.

1.When NaOH is initially added to a container with reaction (i), what happens to the concentration of Pb2+ as soon as some OH‐ mixes into the solution and begins to form the Pb(OH)2 solid?

2.To reestablish equilibrium in reaction (i), what will happen to the amount of PbCl2(s)?

Answer 1

1) By that addition of OH- ions into Pb+2 solution, the concentration of Pb+2 is decreased due to formation of Pb(OH)2 solid material.

2) By the addition of OH- a second reaction takes place between Pb+2 and OH- ions and produce Pb(OH)2. So the concentration of Pb+2 in the solution decreased. Due to this reason, the equilibrium in first reaction is moved into left side by dissociation of PbCl2 into Pb+2 and Cl- ions. According to lechatliers principle, when thatthe concentration of Pb+2 is decreased, then equlibrequi is moved in a direction to increase the concentration of Pb+2 ions.