Question

Write the equilibrium constant expression for the following reactions?

Write the equilibrium constant expression for the following reactions?

Answer 1

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))`