Reactions at Equilibrium and the Equilibrium Constant

The equilibrium constant ‘K’ is defined as the ratio of the concentration of products to the concentration of reactants, each raised to the power of their coefficients in the chemical reaction.
The reaction quotient Q has the same form as K but applies to the system at any point—not necessarily at equilibrium. It helps assess the system’s current state. 
Δ_rG is the Gibbs free energy change of a reaction under current conditions. The standard Gibbs free energy change,  Δ_rG°, refers to standard conditions: 1 molar concentration and 1 atmospheric pressure.
They are related by the equation:
  Δ_rG = Δ_rG° + RT ln Q 
At equilibrium, Δ_rG is zero and Q = K, giving:
  Δ_rG°  = –RT ln K 
If ‘K’ is much greater than one and Δ_rG° is negative, the reaction favors the products. 
On the other hand, if ‘K’ is much less than one and Δ_rG° is positive, it favors reactants. 
When ‘K’ equals one, both reactants and products are present in significant amounts.