Dynamic Equilibrium and the Equilibrium Constant

If the initial mixture contains reactants but no products, the forward reaction initially occurs without a reverse reaction. If both are present initially, both directions can occur from the start. As product builds up the reverse reaction speeds up and the forward reaction slows down. Eventually the two rates become equal. From that moment the measurable concentrations stop changing, but individual molecules are still converting in both directions.

For the elementary idealized pair A ⇌ B, equilibrium requires kf[A] = kr[B]. Thus [B]/[A] = kf/kr, while the two concentrations can be very different. This ratio-of-rate-constants result applies directly to this elementary pair; it is not a rule to take arbitrary overall reaction orders from its coefficients. A constant bulk composition also needs stable external conditions and no net addition, removal or competing process.

For A ⇌ B, K_c = [B]/[A], so K_c = 100 means one hundred B particles per A particle in an ideal mixture. Here a large K directly means B dominates. For A ⇌ 2 B, however, K_c = [B]²/[A]; its size is not a simple mole fraction. Initial amounts, total concentration and stoichiometric powers are needed to calculate the composition. “Products favored” describes the equilibrium activity expression relative to the chosen standard states, not a universal guarantee that every product is more concentrated than every reactant.

Likewise K ≈ 1 means neither side of the activity ratio is strongly favored; it does not require equal concentrations or equal total moles. Always write the balanced reaction and its expression before interpreting a numerical K. Reversing or scaling the equation also changes the reported K.

For a specified reaction, its thermodynamic equilibrium constant depends on temperature. At fixed temperature, changing concentrations or gas volume can change Q and may cause a shift until Q again equals K. A catalyst alone changes neither K nor the equilibrium composition; it provides a faster pathway for both directions. If the mixture is already at equilibrium, it remains there.

At fixed temperature, two sealed preparations with different total inventories can have different equilibrium concentrations while satisfying the same expression. For A ⇌ B with K = 2, total concentration 0.300 M gives [A] = 0.100 and [B] = 0.200 M, while total 0.600 M gives 0.200 and 0.400 M. Position means the resulting composition; K is the constraint on its ratio. A catalyst changes the rates of interconversion, not the value of that ratio. A very large K does not reveal whether equilibrium will be reached in seconds or centuries.