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Name _________________________
LeChatallier’s Principle
Some chemical reactions do not completely convert the reactants into products. This is
generally the case when there is not a large difference in the potential energy of the
reactants and products. At some point in time, the rate of the forward and reverse
reactions will become the same and the reaction is said to have reached equilibrium.
However it is possible to disturb this equilibrium by changing the conditions.
LeChatallier’s Priniciple is often used to describe the effect of changing the conditions
of an equilibrium system. Simply stated, when a stress is applied to a system at
equilibrium, the system will respond in such a manner as to reduce the stress. Typically
the stresses in question are things that will selectively speed up the forward reaction
preferentially as compared to the reverse reaction, or visa versa. For example, shown
below is a hypothetical chemical equilibrium in which all species are in the gas phase.
A (g) + 2 B (g)  C (g) + heat
If this reaction is done in a closed container and some additional amount of species A is
added to the container, then the rate of the forward reaction will become faster than the
reverse. To compensate, some of the A species will react with species B and be
converted into C. The position of the chemical equilibrium is said to shift to the right to
reestablish equilibrium. Conversely, if some of species A were removed in some way,
the reaction would shift to the left (the forward reaction would become slower than the
reverse). Similar effects can be seen when studying the concentration of an equilibrium
in aqueous solution. Pressure can also have an effect on a gas phase equilibrium. In
general, increases in pressure will cause an equilibrium to shift to the side with the fewer
number of gas molecules, and likewise decreases in pressure will cause a shift to the side
with the greater number of gas molecules. Heat is a major factor that can disturb an
equilibrium system. In general, if the temperature of an equilibrium system is increased,
a shift will occur in the direction that will absorb some of that heat and if the temperature
of the system is decreased, the equilibrium will shift in the direction the will produce
some heat. In cases where one of the reactants or products are pure solids or liquids,
since the material is not evenly distributed throughout the system and reaction can only
occur at the surface, perturbations involving these species have virtually no effect on the
position of the equilibrium compared with other factors. The factors that have an effect
on an equilibrium system are summarized in Table 1.
Table 1. Responses to applied stresses in an equilibrium system.
Stress
Response
1. Add reactants
Convert reactants into products ()
2. Remove reactants
Convert products into reactants ()
3. Add products
Convert products into reactants ()
4. Remove products
Convert reactants into products ()
5. Increase pressure
Shift the equilibrium to the side with the
least number of molecules
6. Decrease pressure
Shift the equilibrium to the side with the
greatest number of molecules
7. Increase temperature
Shift in the direction that will absorb heat
8. Decrease temperature
Shift in the direction that will release heat
© Van Der Sluys, 2004
Name _________________________
LeChatallier’s Principle
Answer questions 1-5 based on
2 NO2 (g)  N2O4 (g) + heat
1. In which direction will the equilibrium shift if the temperature of the system is
reduced?
2. What will happen to the relative amount of N2O4 if the temperature decreases.
3. What will happen to the relative amount of NO2 if the temperature decreases.
4. In which direction will the equilibrium shift if the volume of the container is
reduced.
5. In which direction will the equilibrium shift if some N2O4 is added to the system?
Answer questions 6-10 based on
Fe3+ (aq) + SCN- (aq)  FeSCN2+ (aq)
6. In which direction will the equilibrium shift if the concentration of Fe3+ is
increased by adding FeCl3 (s).
7. In which direction will the equilibrium shift if KSCN (s) is added to the solution?
8. What happens to the concentration of SCN- (aq) when Fe3+ is added to the
system?
9. What happens to the concentration of Fe3+ (aq) when SCN- is added to the
system?
10. How will the equilibrium shift is NaCl is added to the solution?
Answer questions 10-15 based on the fact that CrO42- is yellow, whereas Cr2O72- is red
and the equilibrium in aqueous solution between the two species is
2 H+ (aq) + 2 CrO42- (aq)  Cr2O72- (aq) + H2O (l)
11. What would the color of a solution be if a large amount of hydrochloric acid were
added to the system?
12. What would the color of the solution be if a large amount of sodium hydroxide
were added to the solution?
13. What would be the effect of diluting the solution with pure water? Explain your
reasoning.
© Van Der Sluys, 2004
Name _________________________
Answers
1. right
2. increase
3. decrease
4. right
5. left
6. right
7. right
8. decreases
9. decreases
10. no effect
11. red
12. yellow
13. shifts left
© Van Der Sluys, 2004
LeChatallier’s Principle