# Question 34a11

Sep 14, 2014

These are equilibrium constants for various equilibrium reactions.

• ${K}_{\text{eq}}$

If you have an equilibrium of the type

aA + bB ⇌ cC + dD

you can write an equilibrium constant expression:

K_"eq" = (["C"]^c["D"]^d)/(["A"]^a["B"]^b), where [ ] indicates concentration

You do NOT include the concentrations of solids or liquids in the ${K}_{\text{eq}}$ expression, because their concentrations do not change.

For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

K_"eq" = ["NH"_3]^2/(["N"_2]["H"_2]^3)

For the reaction, C(s) + H₂O(g) ⇌ CO(g) + H₂(g)

K_"eq" = (["CO"]["H"_2])/(["H"_2"O"])

• ${K}_{a}$

If one of the reactants is an acid, the equation for the equilibrium is

HA + H₂O ⇌ H₃O⁺ + A⁻

The equilibrium constant expression is

${K}_{\text{eq" = (["H"_3"O"^+]["A"⁻])/"[HA]}}$

To show explicitly that this equilibrium involves an acid, we call the equilibrium constant ${K}_{a}$.

So ${K}_{a}$ is the equilibrium constant for the dissociation of an acid.

${K}_{a}$ is usually called the acid dissociation constant.

For the dissociation of HF in water, we write

HF + H₂O ⇌ H₃O⁺ + A⁻

and

K_a = (["H"_3"O"^+]["F"⁻])/"[HF]"#