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.

I₂(g) <--> 2I(g)

Which way with the equilibrium shift if the pressure increases? Decreases?

Real example

N₂O₄ (g) colorless<==> 2 NO₂(g) Brown

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

The equilibrium will not shift. There needs to be an imbalance in the number of moles of product gasses to reactant gases cause a stress. Moles of products and reactant gases are equal, no shift.

How does temperature shift a system at equilibrium?

First determine if the reaction is endothermic or exothermic.

Place that energy term in the reaction

Exo A --> B + energy

Endo Energy + A --> B

Increasing temperature will make the system absorb the heat.

Exothermic will shift the equilibrium to the left. Endothermic will shift right.

Decreasing the temperature will make the system replace the missing heat.

Exothermic will shift the equilibrium to the right. Endothermic will shift left.

ANIMATION-

See how the change in temperature changes the equilibrium position.

Energy + N₂O₄ (g)<==> 2 NO₂(g)

The real experiment (they use the reverse reaction).

2 NO₂(g)<==> N₂O₄ (g) + Energy

Regents Questions

Jan 2011-46 Given the equation representing a reaction at equilibrium:
N_2(g) + 3H_2(g) <==>2NH_3(g)
What occurs when the concentration of H_2(g) is increased?
(1) The equilibrium shifts to the left, and the concentration of N_2(g) decreases.
(2) The equilibrium shifts to the left, and the concentration of N_2(g) increases.
(3) The equilibrium shifts to the right, and the concentration of N_2(g) decreases.
(4) The equilibrium shifts to the right, and the concentration of N_2(g) increases.

June 2010-43 Given the equation representing a system at equilibrium:
N_2(g) + 3H_2(g)<==>2NH_3(g) + energy

Which changes occur when the temperature of this system is decreased?
(1) The concentration of H_2(g) increases and the concentration of N_2(g)increases.
(2) The concentration of H_2(g) decreases and the concentration of N_2(g)increases.
(3) The concentration of H_2(g) decreases and the concentration of NH_3(g)decreases.
(4) The concentration of H_2(g) decreases and the concentration of NH_3(g)increases.

June 2007-44 Given the equation representing a reaction at equilibrium:
N_2(g) + 3H_2(g)==> 2NH_3(g) + energy
Which change causes the equilibrium to shift to the right?
(1) decreasing the concentration of H_2(g)
(2) decreasing the pressure
(3) increasing the concentration of N_2(g)
(4) increasing the temperature

Jan 2007-44 Given the system at equilibrium:
2POCl_3(g) + energy <==> 2PCl_3(g) + O_2(g)
Which changes occur when O_2(g)is added to this system?
(1) The equilibrium shifts to the right and the concentration of PCl_3(g)increases.
(2) The equilibrium shifts to the right and the concentration of PCl_3(g)decreases.
(3) The equilibrium shifts to the left and the concentration of PCl_3(g) increases.
(4) The equilibrium shifts to the left and the concentration of PCl_3(g) decreases.

Aug 2006-45 Given the reaction at equilibrium:
N_2(g) + 3H_2(g) -->2NH_3(g)+ 91.8 kJ
What occurs when the concentration of H_2(g) is increased?
(1) The rate of the forward reaction increases and the concentration of N_2(g)decreases.
(2) The rate of the forward reaction decreases and the concentration of N_2(g)increases.
(3) The rate of the forward reaction and the concentration of N_2(g) both increase.
(4) The rate of the forward reaction and the concentration of N_2(g)both decrease.

Aug 2004-45 Given the system at equilibrium:
N_2(g) + O_2(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 N_2(g).
(3) increasing the concentration of O_2(g) .
(4) increasing the temperature

Aug 2003-46. Given the reaction at equilibrium:

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:
H_2(g) + I_2(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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