Write the equilibrium constant expression for the following reactions?

Write the equilibrium constant expression for the following reactions? enter image source here

1 Answer
Jun 30, 2018

A few principles to keep in mind:

  • Products over reactants for the forward reaction, and square brackets for concentration of aqueous species.
  • Coefficients go into the exponents of the substance concentration.
  • Pure liquids and solids are written as #1# in the #K# mass action expression.

As a result,

#A)#

#color(blue)(K_c = (["AlCl"_3]^2)/(["HCl"]^6))#

Fairly straightforward; the solid and liquid are implied to be #1#, and hence are invisible. I didn't write a net ionic reaction for this one, but did for #(B)#, because #(A)# has #"AlCl"_3#, which is not quite ionic and not quite covalent.

#B)#

#K_c = (["HNO"_3]^2)/(["Ba"("NO"_3)_2]["H"_2"SO"_4])#

This second one looks rather clunky though, given that nitrates are generally quite soluble, and nitric acid is a strong acid... I would have rewritten this as its net ionic reaction:

#"Ba"^(2+)(aq) + "SO"_4^(2-)(aq) rightleftharpoons "BaSO"_4(s)#

which is the precipitation equilibrium to form barium sulfate solid.

This has

#color(blue)(K_(sp)^(-1)) = 1/(["Ba"^(2+)]["SO"_4^(2-)]) = color(blue)(["Ba"^(2+)]^(-1)["SO"_4^(2-)]^(-1))#