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The world of Equilibrium

Screen Grab of interactive anim - Le Chatlier's principle from Physical Chemistry Book

Chemistry would be a whole lot simpler if all the reactions would just make their expected products (or even some by-products) and then be done. Reactants and products have a mind of their own though and are influenced by temperature, concentration, some even by the weather (pressure)! Worse, some products can’t seem to make up their mind and turn back into reactants.How can any useful data be gained when so many variables seem to throw them off course from forming the true and awaited final product? Simple, come up with a new equation that takes all those variables into consideration and enter the world of equilibrium.

In a previous post I discussed equilibrium and the equilibrium constant K, here I would like to explore the relationship between products and reactants and how they shift to balance each other out.

For example, if I knew that adding A to B would make C, the following equation could be written: A + B → C.

In a wonderfully perfect world, they would react completely and give a 100% yield of C.  Unfortunately, that isn’t usually the case. More often, as the concentration of C builds up, it decomposes back into A and B and creates an equilibrium system. When the forward and reverse processes are occurring at the same rate, it is called dynamic (or dynamical) equilibrium. The equation should now be written as:  A + B  ⇄ C.

This equilibrium can be agitated by change, for instance, if some of the product C was removed, more A and B would react together to form more C. In other words, equilibrium would shift to the right or to products to counteract the disturbance.

If either A or B was removed, some of the product C would need to decompose to recreate more A and/or B and shift left or to the reactant side. This balance between products and reactants is expressed using Le Châtelier’s principle, section 4.6 of the physical chemistry book.

The beauty of this principle is that it doesn’t rely on numbers but on the simple relationship between both sides of the equation. It also takes into account different variables, mainly concentration, heat/temperature and pressure.

As mentioned above, the direction to reach equilibrium can be changed by increasing or decreasing the concentrations of the different species. In order to truly force a reaction to completion, simply remove the desired product as it is formed and equilibrium will continue to shift right until the reactants are consumed.

For example, if the reaction is at equilibrium and more A is added or the concentration increased.  The reaction must try and “get rid of” some of the extra A by using a bit of the B in consequence, making more C. Vice versa, if the concentration of A is decreased, the reaction must try and replace it by decomposing some of the product C and gaining both A and B.

Altering the pressure will only change the equilibrium if there is at least one gaseous species. One simplistic way to visualize this change is to consider the amount of space needed for the gases present.

For A (aq) + B (aq)  ⇄ C (g), if the pressure was increased (volume decreased), equilibrium would shift to the left because there would be less “space” for the gaseous product C so equilibrium favors the side with less/or no gases.

For A (g) + B (g)  ⇄ C (aq), if the pressure was increased (volume decreased), equilibrium would shift greatly to the right because both reactants are gases.

For A (aq) + B (aq)  ⇄ C (s), changing the pressure would have little or no change on equilibrium because there are no gaseous species.

Temperature is treated the same way. If the reaction is endothermic, adding heat shifts equilibrium to the right (towards products) and subtracting heat or cooling will shift left.  This can easily be seen from a simple equation.

A (aq) + B (aq) + Δ  ⇄ C (g)

In an endothermic reaction, heat (the delta sign) is simply a reactant and if it’s “concentration” is increased shift until it “uses up” some of the extra. Exothermic reactions are identical except that heat is a product instead.

The following table identifies how equilibrium will correct changes made to a system.  For example:  The first column can be read this way. If more A is added, equilibrium shifts right and uses some of the original B (thereby decreasing its concentration) to create more C and heat/Δ, increasing their overall concentrations.

A + B  ⇄ C (g) + Δ Change Resulting Shift
Dec Inc Inc Increase A Shifts Right
Dec Inc Inc Increase B Shifts Right
Inc Inc Dec Increase C Shifts Left
Inc Inc Dec Increase Δ Shifts Left
Inc Dec Dec Decrease A Shifts Left
Inc Dec Dec Decrease B Shifts Left
Dec Dec Inc Decrease C Shifts Right
Dec Dec Inc Decrease Δ Shifts Right
Inc Inc Dec Dec Inc Pressure Shifts Left
Dec Dec Inc Inc Dec Pressure Shifts Right

Of course, more than one variable can be changed simultaneously, and this simplistic model will give a good indication of the shift as long as the changes aren’t extreme or cause changes in other variables not mentioned.

Visualizing what is occurring is the first step to understanding equilibrium and how it responds to disturbances. Le Châtelier’s principle is a great tool in doing just that.

Of course, the next step is calculating the exact quantities both before and after the changes, which is a bit trickier but chapter four does just that for each of these variables.

Luckily, equilibrium is one area that a visual aid can greatly help with understanding how the math behind the system works.


Written by: jollshar

Sharlene Jolley has authored 20 more articles.

I received my graduate degree in organic chemistry from Kansas State University and have been teaching undergraduate chemistry courses for over 15 years. I strongly believe that you are never too old or young to learn and appreciate science and have had students ranging in ages from 5 to 65. Along with my college classes, I regularly teach science in K-12 classes and for special interest groups.

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