Stress and Le Chatelier's Principle

Advancement, Stress, and Chemical Equilibrium

The Equilibrium Expression of the reaction:

This relationship is valid only when the reaction mixture is at equilibrium. A reaction mixture may be displaced from equilibrium by changing either the left hand side or the right hand side of the equation. Changes in the left hand side require a change in the temperatiure, since the equilibrium constant is a function of temperature only. Changes in the right hand side require a change in the concentration of one or more species in the reaction, which in turn means a change in either the number of moles of something or the volume of the reactor.

Changes in temperature, number of moles, or volume which cause as system to be disturbed from equilibrium are sometimes called STRESS. How reactant and product concentrations change as a response to a stress is has long been a topic of study by Chemists. Below is a qualitative(no numbers) treatment of this topic.

Stress is something we all like to avoid. In fact, Le Chatelier (1850-1936) said it best (in the context of Chemical Equilibrium) in his much paraphrased statement :

["Le Chatelier's Principle", J. Chem. Ed., 57 417 (1980)]

"General Chemistry" P&H 6^th

"When an equilibrium system is subjected to a change in temperature, pressure, or concentration of a reacting species, the system reacts in a way that partially offsets the change while reaching a new state of equilibrium"

"The Extraordinary Chemistry of Ordinary Things" C.H. Snyder

"Placing a stress on an equilibrium causes the equilibrium to shift so as to relieve the stress"

"Principles of Modern Chemistry", Oxtoby & Natrieb 2^nd

"A system in equilibrium that is subjected to stress will react in a way that counteracts the stress"

"Chemistry the Molecular Science", Olmstead & Williams

"When a change is imposed on a system at equilibrium, the system will react, if possible, in a direction that reduced the amount of change"

Consider the equilibrium between nitrogen, hydrogen and ammonia

N₂ + 3H₂ <--> 2NH₃

If an equilibrium is established at a given temperature and more reactant is added to the vessel at constant volume, the rate of ammonia formation collisions will increase, but the rate of destruction of product will reamin the same, so the equilibrium concentration of ammonia (product) will go up. If we add H₂ to a reaction at equilibrium, the concentrations will change like this: