Chemical equilibrium is achieved for constant temperature and pressure systems when the rate of change of concentration goes to zero for all species.[^1] Chemical equilibrium can happen fast or slow relative to the problem studied. In a converging diverging rocket nozzle, you assume two types of equilibria - reaction continues to happen as the gasses pass through the nozzle. This is called *shifting equilibrium* - flow passes through the nozzle so fast that after the combustion chamber no reaction happen. This is called *frozen equilibrium* The reality is somewhere in between. [[Gibbs Free Energy]] of a system in equilibrium does not change. [^1]: Combustion Engineering, Ragland & Bryden