Equilibrium constant
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
K = [products]^coeff / [reactants]^coeff (gases use partial pressures, Kp); K > 1 favors products.
Equilibrium constant in plain English
Imagine a reversible reaction as a busy dance floor where couples keep pairing up (making products) and breaking apart (going back to reactants). Eventually the rate of pairing equals the rate of splitting, so the amounts stop changing even though molecules never stop moving — that's equilibrium. The equilibrium constant K is just a single number that tells you the final 'balance point': you multiply the product concentrations (each raised to its coefficient from the balanced equation) and divide by the reactant concentrations the same way. If K > 1 there are more products than reactants at balance (the reaction 'leans right'), and if K < 1 it leans back toward reactants; for gas reactions you can use partial pressures instead, which people label K_p.
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