Advancement, Stress, and Chemical Equilibrium


The Equilibrium Expression of the reaction:
is
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 6th

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

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

"Chemistry the Molecular Science", Olmstead & Williams


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:


Let's consider the observed effects of stress on a simple gas phase equilibrium

What happens if I2 (gas) is added at constant total volume?


Consider the same reaction at slightly lower temperature (different phases present)

What happens if I2 (solid) is added at constant total volume?

Reactants or products in a pure phase do not enter into the equilibrium expression.

Example:

Liquid Water <--> Water Vapor

Keq = Kp = pvap
The vapor pressure of a liquid is independent of the amount of the liquid (pure phase)


Consider the dimerization of NO2

What happens to the system if the volume is increased at constant temperature?

What happens to the system if the temperature is increased at constant volume?

Of course, each of the above examples can be treated quantitatively...


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