Enter equilibrium

The steady state approximation applies to kinetic systems, but if we think about the nature of chemical reactions, we can say that all chemical reactions are microscopically reversible.  For the simple reaction A↔B

If we assume that the forward and reverse reaction both represent elementary steps, we can write rate law expressions for each direction:

Rate_forward = k_forward[A]₀

Rate_reverse = k_reverse[B]₀

After some time has passed, the rate of the forward reaction will be equal to the rate of the reverse reaction.

Rate_forward = Rate_reverse
k_forward[A]₀ = k_reverse[B]₀     

Rearranging to group constants and concentrations, we get the expression for the equilibrium constant.

K_eq = [products]_eq / [reactants]_eq  

If [products]_eq = [reactants]_eq, then K_eq = 1 and the reaction does not favor products or reactants.  If the equilibrium is product-favored, the the value of K_eq will be larger than 1; if the equilibrium is reactant-favored, the the value of K_eq will be less than 1 but still positive.