Factors affecting chemical equilibrium

If an outside change (in concentration, pressure or temperature) is applied to a system at equilibrium, the equilibrium shifts in the direction that reduces that change — Le Chatelier’s principle (1884). If a reagent is added the equilibrium shifts to the side that consumes it; if a product is removed it shifts toward forming the product; K does not change. If the pressure rises, the equilibrium shifts to the side with fewer moles of gas; if the number of gas molecules is equal, or there are no gases, pressure has no effect (solids are not counted). If the temperature rises, the equilibrium shifts to the side that absorbs heat (endothermic); if it falls, to the side that releases heat (exothermic); only temperature changes the value of K. A catalyst speeds up the forward and reverse reactions equally, so it does not shift the equilibrium, only helps it to be reached sooner. In industry (ammonia, sulfuric acid) a compromise is made between rate and yield: a lower temperature favours yield, a higher one favours speed.