Every reversible reaction eventually settles into a mix of reactants and products that stops changing β not because the reaction stops, but because the forward and reverse rates become equal. The equilibrium constant Kc is the single number that tells you where that balance point sits, and the reaction quotient Q tells you how far any given mixture is from that balance point.
How Kc is calculated
For a balanced equation aA + bB β cC + dD, the law of mass action defines Kc = ([C]^c Γ [D]^d) Γ· ([A]^a Γ [B]^b), using equilibrium concentrations in mol/L. Each concentration is raised to the power of its coefficient in the balanced equation β not the number of molecules in a single reaction event, but the stoichiometric coefficient after balancing. Pure solids and pure liquids are left out of the expression entirely, since their activity is defined as 1 and stays constant regardless of how much solid or liquid is present.
Reading the size of Kc
Kc has no fixed "good" or "bad" value β it simply describes where the balance sits at a given temperature. A large Kc (this calculator flags Kc β₯ 10 as "products favored") means that at equilibrium, the numerator (products) is much larger than the denominator (reactants), so the reaction has largely converted reactants into products. A small Kc (Kc β€ 0.1, "reactants favored") means the reverse is true: at equilibrium, mostly reactants remain and very little product has formed. Values in between mean the equilibrium mixture contains comparable, non-trivial amounts of both sides.
Using Q to predict which way a reaction will shift
The reaction quotient Q has the exact same formula as Kc, but you can plug in concentrations from any moment β the instant you mix reactants, mid-reaction, or after a disturbance like adding more of one species. Comparing Q to Kc tells you the reaction's next move: Q < Kc means there aren't yet enough products relative to reactants, so the forward reaction proceeds; Q > Kc means there's a temporary excess of products, so the reverse reaction proceeds; and Q = Kc means the system has already settled at equilibrium and concentrations will hold steady (until something disturbs it, per Le ChΓ’telier's principle).