# Write the equilibrium constant expression for the following reactions?

## Write the equilibrium constant expression for the following reactions? 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)

$\textcolor{b l u e}{{K}_{c} = \left({\left[\text{AlCl"_3]^2)/(["HCl}\right]}^{6}\right)}$

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 $\left(B\right)$, because $\left(A\right)$ has ${\text{AlCl}}_{3}$, which is not quite ionic and not quite covalent.

B)

${K}_{c} = \left(\left[{\text{HNO"_3]^2)/(["Ba"("NO"_3)_2]["H"_2"SO}}_{4}\right]\right)$

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:

${\text{Ba"^(2+)(aq) + "SO"_4^(2-)(aq) rightleftharpoons "BaSO}}_{4} \left(s\right)$

which is the precipitation equilibrium to form barium sulfate solid.

This has

$\textcolor{b l u e}{{K}_{s p}^{- 1}} = \frac{1}{{\left[{\text{Ba"^(2+)]["SO"_4^(2-)]) = color(blue)(["Ba"^(2+)]^(-1)["SO}}_{4}^{2 -}\right]}^{- 1}}$