ie-Chemistry P13
Chemical Equilibrium

one of nature's essential balancing acts

in development


In our world there are situations where there are little or no changes.  It seems as if everything has stopped.  This might indeed by the case.  But it is generally more likely that the situation is actually a steady state, or at equilibrium.  In both of these situations change continues to occur but there is compensation for each change.

recognizing steady states and equilibrium

Both steady state and equilibrium require the following observable condition:
  1. there are no observable or measurable changes.  In chemical situations this means there are no change in mass, volume, temperature, pressure, color or other appearance, electrical potential (Voltage), radioactivity, or such.

A steady state can be achieved when material is added to a situation if equal material is simultaneously removed.  For example a steady state may exist in a section of water pipe or smoothly flowing river when water leaving the section is replaced by new water entering.

An equilibrium situation had the additional requirement that:
  1. the system be closed.  No material is allowed to enter or leave a system at equilibrium.

Perhaps there is value in emphasizing that steady states and equilibrium are recognized by constant macroscopic properties.


While there are no observable changes occurring in a chemical equilibrium, in all cases non-observable changes continue to occur.  This might at first seem trivia, it turns out that continuing processes are crucial for explaining several aspects of chemical equilibrium.  Processes with involve chemical change continue to occur, but compensating changes immediately restore the original conditions.

explaining equilibrium

The mathematics of the opposing reactions provides considerable insight.  Consider a hypothetical reaction where substances A, B and C form products P and Q in the proportions:
A + 2B + C ↔ P + 3Q.
The rate of the forward reaction, Rf, depends on the concentrations of the molecules which must collide together in order to react.  Since B is involved twice, its concentration affects the rate of reaction two times.  Because we arbitrarily choose to measure concentrations as Molarities and the rates in other arbitrary units, the equation requires a constant, kf, which often is temperature dependent:
Rf = kf [A]•[B]•[B]•[C] = kf [A]•[B]2•[C].
Likewise the rate of the reverse reaction, Rr, depends on the concentration of P and Q.  And since three Q must collide to complete the reverse reaction, the concentration of Q has an effect three times:
Rr = kr [P]•[Q]3.
Now we realize that for equilibrium to exist, the rates of the forward and reverse reactions much be equal in order to cancel out the changes each creates:
Rf = Rr
which by substitution requires
kf [A]•[B]2•[C] = kr [P]•[Q]3.
If we now gather the constants on one side of the equation and the variables on the other:  (The convention is to place concentrations of products over reactants.)
kf / kr = [P]•[Q]3 / [A]•[B]2•[C]
which, if we introduce a new symbol becomes
Keq = [P]•[Q]3 / [A]•[B]2•[C]

So we notice that the ratio of the concentrations of products divided by concentrations of reactions is constant at equilibrium.  We shall call this Keq the equilibrium constantWe note that coefficients in the chemical reaction become exponents in this equilibrium equation.  Since the constants in the rate equations vary with temperature, the equilibrium constant also is generally dependent on temperature.  But at constant temperature, while one concentration may vary, the others must compensate so that the ratio remains the same.  This is a very useful relationship since once an equilibrium constant is know, the effects of changing equilibrium concentrations can be successfully predicted.  It allows for optimizing production of desired materials, minimizing unwanted biproducts, and estimating the amount of product.

Le Chatelier's principle

Often changes are imposed on a situation formerly at equilibrium.

The Le Chatelier's principle (called the Le Chatelier-Braun principle by some) is used to predict the effect of a change in conditions on a chemical equilibrium.  The principle is named after the French chemist Henry Louis Le Chatelier (b1850 Paris, d1936, shown at right→) who proposed the principle in 1884.  The German physicist Karl Ferdinand Braun (b1850, d1918) also independently suggested the concept:  If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or total pressure, then the equilibrium shifts to partially counter-act the imposed change.  The chemical system adjusts to minimize that change.  When stated more broadly, the principle applies in many fields, going by a variety of names such as Lenz's law for electrical circuits, homeostasis in biology and price equilibrium in economics.




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created 1 January 2009
revised 2 January 2009
by D Trapp
Mac made