Equilibrium is dynamic.

A ⇌ B molecules flip at rates set by kf and kb; forward and backward rate bars meet while concentrations level off, from either starting side, reaching K = kf/kb.

Set rates so K = 4, then start from pure B and reach the same 80% A-to-B split as from pure A.

Equilibrium is static: reactions stop.

• Chemical equilibrium can be reached from either side: starting with reactants alone or with products alone leads to the same equilibrium mixture at a given temperature.
• Guldberg and Waage's law of mass action: for aA + bB ⇌ cC + dD, Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ at equilibrium, with concentrations in mol L⁻¹.
• Kc is constant at a fixed temperature, whatever the starting concentrations.
• The equilibrium constant of the reverse reaction is 1/Kc.
• Multiplying a balanced equation by n raises K to the power n; dividing by n gives the nth root of K.
• Adding two reactions multiplies their equilibrium constants.
• Units of Kc are (mol L⁻¹)^Δn; K is dimensionless when Δn = 0, and in thermodynamic treatments K is often taken as dimensionless using standard states.
• In calculations, convert equilibrium moles to mol L⁻¹ before substituting; the volume cancels out only when Δn = 0.

Teaching this idea? Put the simulation on your own page or LMS with this code.