Le Chatelier’s Principle Worksheet

Le Chatelier’s Principle Worksheet

CK-12 Foundation Chemistry

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Le Chatelier’s Principle is useful in predicting how a system at equilibrium will respond when

certain changes are imposed. Le Chatelier’s Principle does NOT explain why the system

changes, and is not an acceptable explanation for the change. It merely allows you to determine

quickly how the system will change when a disturbance is imposed. The explanation for why the

system changes can be found in your textbook.

There are three common ways a stress may be applied to a chemical system at equilibrium:

? changing the concentration (or partial pressure) of a reactant or product.

? changing the temperature.

? changing the volume of the container (which changes partial pressure of all gases in the

You should be aware that adding a gaseous substance that is not involved in the reaction

changes the total pressure in the system but does not change the partial pressure of any of the

reactants or products and therefore does not affect the equilibrium.

Le Chatelier’s Principle states when a system at equilibrium is disturbed, the equilibrium shifts

so as to partially undo (counteract) the effect of the disturbance.

Changes in Concentration or Partial Pressure

If a system at equilibrium is disturbed by adding a reactant or removing a product, Le Chatelier’s

Principle predicts that the equilibrium will shift forward, thus using up some of the added

reactant or producing more of the removed product. In this way, the equilibrium shift partially

counteracts the disturbance. Similarly, if the disturbance is the removal of a reactant or the

addition of a product, the equilibrium will shift backward, thus producing more of the removed

reactant or using up some of the added product. Once again, the shift tends to “undo” the

disturbance. It should be noted that when the disturbance is an increase or decrease of

concentration of reactant or product, the equilibrium shift tends to partially return the

concentration to its former value but it never gets all the way back to the former value.

The equilibrium constant value, Ke is not changed by the addition or removal of reactants or

products. Since the concentration of solids are constant, they do not appear in the equilibrium

constant expression and their concentrations do not change when disturbances cause

equilibrium shifts, however, the amount of the solid present most certainly does change. The

amount of solid can increase or decrease but the concentration does not change.

Changes in Temperature

Increasing the temperature of a system at equilibrium increases both forward and reverse

reaction rate, but it increases the endothermic reaction more that the exothermic. Therefore, in

an exothermic reaction, the reverse reaction is endothermic and so increasing the temperature

will increase the reverse reaction more than the forward reaction, and the equilibrium will shift

reaction).

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backwards. Since the forward reaction produces heat and the reverse reaction consumes heat,

Le Chatelier’s Principle predicts that when heat is added, the equilibrium will shift backward,

consuming heat, and thus partially countering the disturbance. Cooling an exothermic reaction

slows both reactions but it slows the reverse more than the forward, hence the equilibrium will

shift forward producing more heat, thus partially undoing the stress.

For an endothermic reaction, all the same logic is involved except that the forward reaction is

endothermic and the reverse reaction is exothermic. Therefore, heating an endothermic reaction

causes the equilibrium to shift forward, and cooling an endothermic reaction causes the

equilibrium to shift backward.

When an equilibrium shifts due to a temperature change all the substances on one side of the

equation move in the same direction, that is, they all increase or they all decrease. Therefore,

the equilibrium constant value will also change when the temperature is changed.

Reaction Type Increase Temperature Decrease Temperature

Endothermic increases decreases

Exothermic decreases increases

Changes in Volume

When the volume of a reaction vessel is decreased, the partial pressure (and concentration) of

all gases in the container increase. The total pressure in the vessel will also increase. Le

Chatelier’s Principle predicts that the equilibrium will shift in a direction that tends to counteract

the disturbance. Therefore, the equilibrium will shift to produce fewer moles of gaseous

substances so that the pressure will decrease. Thus, decreasing the volume will cause the

equilibrium to shift toward the side with fewer moles of gaseous substances. The reverse is true

if the volume of the vessel is increased. The partial pressure of all gases will decrease, and the

total pressure will decrease, so the equilibrium shift will be toward the side that contains more

moles of gas, thus increasing pressure and partially counteracting the change.

The Addition of a Catalyst

The addition of a catalyst will increase both forward and reverse reaction rates. In the case of a

catalyst, both reaction rates are increased by the same amount and therefore there will be no

equilibrium shift.

Summary of

3 Le Chatelier’s Principle Worksheet

Exercises

Consider the following reaction.

1. If some is added to this system at equilibrium, which way will the equilibrium

2. When equilibrium is re-established after the is added, how will the concentration

3. When equilibrium is re-established after the is added, how will the concentration

4. When equilibrium is re-established after the is added, how will the amount

5. When equilibrium is re-established after the is added, how will the value of

6. If some is removed from this sytem at equilibrium, which way will the equilibrium

7. When equilibrium is re-established after the is removed, how will the concentration

8. When equilibrium is re-established after the is removed, how will the concentration

shift?

A. Toward the products.

B. Toward the reactants.

C. No shift.

of compare to the original concentration?

A. Increased.

B. Decreased.

C. No change.

of compare to the original concentration?

A. Increased.

B. Decreased.

C. No change.

of compare to the original amount?

A. Increased.

B. Decreased.

C. No change.

compare to the original value of ?

A. Higher.

B. Lower.

C. No change.

shift?

A. Toward the products.

B. Toward the reactants.

C. No shift.

of compare to the original concentration?

A. Increased.

B. Decreased.

C. No change.

of compare to the original concentration?

A. Increased.

B. Decreased.

C. No change.

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9. When equilibrium is re-established after the is removed, how will the value of

10. If the temperature of this system at equilibrium is lowered, which way will the equilibrium

11. When equilibrium is re-established after the temperature was lowered, how will the

12. When equilibrium is re-established after the temperature was lowered, how will the value

13. If the volume of the reaction vessel for this system at equilibrium is decreased, which

14. When equilibrium is re-established after the volume was decreased, how will the

15. When equilibrium is re-established after the volume was decreased, how will the value

compare to the original value of ?

A. Higher.

B. Lower.

C. No change.

shift?

A. Toward the products.

B. Toward the reactants.

C. No shift.

concentration of compare to its original concentration?

A. Increased.

B. Decreased.

C. No change.

of compare to the original value of ?

A. Higher.

B. Lower.

C. No change.

way will the equilibrium shift?

A. Toward the products.

B. Toward the reactants.

C. No shift.

concentration of compare to its original concentration?

A. Higher.

B. Lower.

C. No change.

of compare to the original value of ?

A. Higher.

B. Lower.

C. No change.

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Consider the following reaction.

16. If some is added to this sytem at equilibrium, which way will the equilibrium shift?

17. When equilibrium is re-established after the is added, how will the concentration

18. If the temperature of this system at equilibrium is raised, which way will the equilibrium

19. When equilibrium is re-established after the temperature was raised, how will the

20. When equilibrium is re-established after the temperature was raised, how will the value

A. Toward the products.

B. Toward the reactants.

C. No shift.

of compare to the original concentration?

A. Increased.

B. Decreased.

C. No change.

shift?

A. Toward the products.

B. Toward the reactants.

C. No shift.

concentration of compare to its original concentration?

A. Increased.

B. Decreased.

C. No change.

of compare to the original value of ?

A. Higher.

B. Lower.

C. No change.

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