Le Chatelier's Principle and Factors Affecting Equilibrium

Le Chatelier's Principle states that when a system at equilibrium is subjected to a change (a stress) in any of the factors determining the equilibrium condition -- concentration, pressure, volume, or temperature -- the system responds in the direction that minimises the effect of that change, establishing a new equilibrium. (a) Change of concentration: for the Haber equilibrium $N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)+\text{Heat}$, adding extra H2 disturbs the equilibrium (makes $Q_c<K_c$); the system responds by favouring the forward reaction, consuming the added H2 (and some N2) to form more NH3, until a new equilibrium is reached with an increased NH3 yield -- in general, increasing a species' concentration shifts equilibrium in the direction that consumes it, while the value of $K_c$ itself stays unchanged. (b) Change of pressure: only affects equilibria where the number of gas moles differs between reactants and products (from $P\propto n/V$, i.e. concentration). For $N_2O_4(g)\rightleftharpoons 2NO_2(g)$ (Table 12.1), decreasing pressure deepens the colour (shifts right, toward the side with more gas molecules, NO2), while increasing pressure lightens it to almost colourless (shifts left, toward fewer molecules, N2O4); for $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$, where gas moles are equal on both sides, pressure changes have no effect on the equilibrium at all. In general, a reaction with a decrease in volume (fewer gas moles on the product side) is favoured by increasing pressure, and vice versa; $K_c$ itself is unaffected by pressure. (c) Change of temperature: raising the temperature of a system at equilibrium always shifts it in the ENDOTHERMIC direction, since that is the direction that absorbs the added heat -- for a reaction whose forward direction is exothermic, an increase in temperature therefore shifts equilibrium in the reverse direction. For $CO(g)+2H_2(g)\rightleftharpoons CH_3OH(g)+\text{Heat}$ (exothermic forward reaction, Table 12.2), $K_c$ falls steadily as temperature rises ($1.7\times10^{17}$ at 298 K, $1.1\times10^{11}$ at 500 K, $2.1\times10^{6}$ at 1000 K) -- unlike concentration or pressure, a temperature change genuinely changes the numerical VALUE of $K_c$, not just the position of equilibrium. (d) Effect of catalyst: for the esterification $CH_3COOH(l)+C_2H_5OH(l)\rightleftharpoons CH_3COOC_2H_5(l)+H_2O(l)$, adding H+ as a catalyst reduces the time to reach equilibrium from many days to a few hours, but a catalyst …