Equilibrium in Chemical Process

A chemical reaction carried out in a closed system, so that neither reactants nor products can escape, often does not give a 100% yield of products -- some reactants remain once concentrations stop changing, at which point the reaction has attained equilibrium, with the forward and reverse reaction rates equal. Chemical equilibrium at a fixed temperature is marked by the constancy of measurable properties such as pressure, concentration or density, and can be approached starting from either side (from pure reactants, or from pure products). Two examples confirm this. First, colourless $N_2O_4(g)$ placed in a closed flask partly converts to reddish-brown $NO_2(g)$: $N_2O_4(g)\rightleftharpoons 2NO_2(g)$. As soon as any NO2 forms, the reverse reaction (NO2 recombining to N2O4) begins; the flask's colour deepens to a stable light brown and then stops changing, because the rate of N2O4 decomposing equals the rate of NO2 recombining. Second, starting from hydrogen iodide alone in a closed vessel, $2HI(g)\rightleftharpoons H_2(g)+I_2(g)$: the violet colour of I2 forming increases at first, then stops increasing once equilibrium is reached, and analysis shows HI, H2 and I2 all present together with constant concentrations. Starting instead from H2 and I2 vapour, $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$: the initially deep violet colour of I2 fades and then stabilises, again leaving HI, H2 and I2 all present together at equilibrium -- with, at a given temperature, essentially …