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/equil.htm

• 2.http://www.learner.org/workshops/chemistry/workshop4/4_5.html

• 3.http://chemistry.about.com/od/equilibrium/a/Chemical-Equilibrium.htm

• 4.http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch16/lechat.html