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LeChatelier Principle

 

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Le Chatelier's principle can be used to predict the effect of a change in conditions on a chemical equilibrium

This principle basically states that if stress is applied to a system at equilibrium, the position of the equilibrium will shift in the direction that reduces the stress to reinstate equilibrium. For example, if more reactants are added to the system, the reaction will shift in the forward direction, and if more products are added, the reaction will shift in the reverse direction. If heat is added to the system and the reaction is exothermic, heat should be thought of as a product and the reaction will shift to the left; if the reaction is endothermic and heat is added, the reaction will shift to the right. 

 

Pressure Changes- The addition of pressure will cause a shift in the direction that results in the fewer number of moles of a gas, while if pressure is relieved, the reaction will shift in the direction that produces more moles of a gas.

Start the animation and see for yourself. The pressure increase shifts the equilibrium to the side with less gases (N2O4).

 

What if the number of reacting gases are equal to the number of product gases?

The equilibrium will not shift. Start the animation and see for yourself.

Regents Questions

Jan 2011-46 Given the equation representing a reaction at equilibrium:
N2(g) + 3H2(g) <==>2NH3(g)
What occurs when the concentration of H2(g) is increased?
(1) The equilibrium shifts to the left, and the concentration of N2(g) decreases.
(2) The equilibrium shifts to the left, and the concentration of N2(g) increases.
(3) The equilibrium shifts to the right, and the concentration of N2(g) decreases.
(4) The equilibrium shifts to the right, and the concentration of N2(g) increases.

June 2010-43 Given the equation representing a system at equilibrium:
               N2(g) + 3H2(g)<==>2NH3(g) + energy

Which changes occur when the temperature of this system is decreased?
(1) The concentration of H2(g) increases and the concentration of N2(g)increases.
(2) The concentration of H2(g) decreases and the concentration of N2(g)increases. 
(3) The concentration of H2(g) decreases and the concentration of NH3(g)decreases.
(4) The concentration of H2(g) decreases and the concentration of NH3(g)increases.

June 2007-44 Given the equation representing a reaction at equilibrium:
             N2(g) + 3H2(g)==> 2NH3(g) + energy
Which change causes the equilibrium to shift to the right?
(1) decreasing the concentration of H2(g)
(2) decreasing the pressure
(3) increasing the concentration of N2(g)
(4) increasing the temperature

Jan 2007-44 Given the system at equilibrium:
2POCl3(g) + energy <==> 2PCl3(g) + O2(g)
Which changes occur when O2(g) is added to this system?
(1) The equilibrium shifts to the right and the concentration of PCl3(g)increases.
(2) The equilibrium shifts to the right and the concentration of PCl3(g)decreases.
(3) The equilibrium shifts to the left and the concentration of PCl3(g) increases.
(4) The equilibrium shifts to the left and the concentration of PCl3(g) decreases.

Aug 2006-45 Given the reaction at equilibrium:
N2(g) + 3H2(g) -->2NH3(g) + 91.8 kJ
What occurs when the concentration of H2(g) is increased?
(1) The rate of the forward reaction increases and the concentration of N2(g)decreases.
(2) The rate of the forward reaction decreases and the concentration of N2(g)increases.
(3) The rate of the forward reaction and the concentration of N2(g) both increase.
(4) The rate of the forward reaction and the concentration of N2(g) both decrease.

Aug 2004-45 Given the system at equilibrium:
N2(g) + O2(g) + energy <--> 2 NO(g)
Which changes
will result in a decrease in the amount of NO(g) formed?
(1)
decreasing the pressure .
(2)
decreasing the concentration of N2(g).
(3)
increasing the concentration of O2(g) .
(4)
increasing the temperature

Aug 2003-46. Given the reaction at equilibrium:

46.gif

The concentration of A(g) can be increased by

A.  lowering the temperature
B.  adding a catalyst
C. increasing the concentration of AB(g)
D.  increasing the concentration of B(g)

Aug 2002-38 Given the equilibrium reaction in a closed system:
H2(g) + I2(g) + heat-->2 HI(g)
What will be the result of an increase in temperature?
(1) The equilibrium will shift to the left and [H2] will increase.
(2) The equilibrium will shift to the left and [H2] will decrease.
(3) The equilibrium will shift to the right and [HI] will increase.
(4) The equilibrium will shift to the right and [HI] will decrease.

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