Chemical Equilibrium and Le
Chatlier's Principle
By: Elaina Foster-Allen
Discovery
• The law of mass action, was actually what led
to the chemical equilibrium concept
• It was discovered in Egypt by Claude
Berthollet.
What is Chemical Equilibrium
• Chemical equilibrium is the condition which
occurs when the concentration of reactants
and products participating in a chemical
reaction exhibit no net change over time.
• This could Also be called "steady state
reaction
Continued
Chemical equilibrium applies to reactions
that can occur in both directions. In a
reaction such as:
CH4(g) + H2O(g) <--> CO(g) + 3H2(g)
What Happens
• The reaction happens both ways.
• After some products are created the products
begin to react. This reaction causes reactants
to form
• Even though the reactants are constantly
forming products and vice-versa the amount
of reactants and products does become
steady.
The Equilibrium Expression
• The equilibrium expression for a chemical
reaction may be expressed in terms of the
concentration of the products and reactants.
• The concentration of liquid and solid does not
change.
• jA + kB → lC + mD
Le Chatelier's Principal
• Henry-Louis Le Chatelier was a French chemist
and engineer.
• In 1884 he proposed one of the central
concepts of chemical equilibria.
• His Principal states A change in one of the
variables that describe a system at equilibrium
produces a shift in the position of the
equilibrium that counteracts the effect of this
change.
What does the principal describe?
• what happens to a system when something
momentarily takes it away from equilibrium.
Three ways in which we can change the
equilibrium:
conditions of a chemical reaction at
• (1) changing the concentration of one of the
components of the reaction
• (2) changing the pressure on the system
• (3) changing the temperature at which the
reaction is run.
Example involving change of
concentration
2NO(g) + O2(g) <--> 2NO2(g)
• If you add more NO(g) the equilibrium shifts to the
right producing more NO2(g)
• If you add more O2(g) the equilibrium shifts to the
right producing more NO2(g)
• If you add more NO2(g) the equilibrium shifts to the
left producing more NO(g) and O2(g)
Example involving pressure change
2SO2(g) + O2(g) <--> 2SO3(g)
• an increase in pressure will cause the reaction to
shift in the direction that reduces pressure,
• This is the side with that has the least amount of
gas molecules. Therefore an increase in pressure
will cause a shift to the right in result more
product is produced
• A decrease in volume is one way of increasing
pressure.
Example involving temperature change
N2(g) + 3H2(g) <--> 2NH3 + 91.8 kJ,
• The increase in temperature will cause a shift to
the left because the reverse reaction uses the
excess heat.
• An increase in forward reaction would produce
more heat since the forward reaction is
exothermic.
• Therefore the shift caused by a change in
temperature depends upon whether the reaction
is exothermic or endothermic.
References
• 1.
http://library.thinkquest.org/10429/low/equil/equi
l.htm
• 2.
http://www.learner.org/workshops/chemistry/wor
kshop4/4_5.html
• 3.
http://chemistry.about.com/od/equilibrium/a/Che
mical-Equilibrium.htm
• 4.
http://chemed.chem.purdue.edu/genchem/topicre
view/bp/ch16/lechat.html