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Le Chatelier ‘s Principle

Chemical Equilibrium - System in Permanent Balance A. Concentration of reactants and products is constant B. Rate for the forward reaction equals the rate of the reverse reaction. Le Chatelier ‘s Principle

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Le Chatelier ‘s Principle

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  1. Chemical Equilibrium - System in Permanent Balance A. Concentration of reactants and products is constant B. Rate for the forward reaction equals the rate of the reverse reaction Le Chatelier ‘s Principle If a change is imposed on a system at equilibrium, the equilibrium positions will shift in a direction that tends to minimize the change

  2. System at equilibrium: N2 + 3 H2 2 NH3 at 1000 C N2 = 0.4 M H2= 1.2 M NH3 = 0.2 M K = 0.0579 Change: Add N2 to the system – upsets equilibrium A. To restore equilibrium, reaction shifts to the right. B. Decreasing N2 and H2 concentration and increasing NH3 concentration

  3. System at equilibrium: N2 + 3 H2 2 NH3 at 1000 C N2 = 0.4 M H2= 1.2 M NH3 = 0.2 M K = 0.0579 Change: Removal of the product, NH3 Reaction will shift to the right More product, NH3 , will be produced N2 and H2 concentrations will decrease

  4. System at equilibrium: N2 + 3 H2 2 NH3 at 1000 C N2 = 0.40 atm H2= 0.80 atm NH3= 0.0036 atm Kp= 6.2 x 10 -5 Change: Reduce volume of reaction chamber, increasing overall pressure Increasing the pressure will have the most effect on the side with the most moles of gas. Reaction will shift to the right More product, NH3 , will be produced N2 and H2 concentrations will decrease

  5. System at equilibrium: N2 + 3 H2 2 NH3 at 1000 C Kc = 0.0579  H = - 92.0 kJ Change: Temperature is increased by 100  C Reaction is exothermic so the reaction equation can be written: N2 + 3 H2 2 NH3 + 92 kJ Increasing the temperature is adding energy. Energy is also a product so the addition of heat will shift the equilibrium to the left The concentration of N2 and H2 will increase Since energy is not a concentration, the shift to the left will result in a smaller equilibrium constant value.

  6. Changing temperature is the most important perturbation of a system at equilibrium that produces a change in the value of the equilibrium constant. Increasing the temperature of an exothermic reaction results in less product being formed. Increasing the temperature of an exothermic reaction also decreases the value of the equilibrium constant. For the reaction: N2 + 3 H2 2 NH3 + 92 kJ Temperature (K) Kc 500 90 600 3 800 0.04

  7. Given the following reaction: PCl3(g) + 3 NH3 (g)  P(NH2)3 (s) + 3 HCl (g) H = 890 J 1. What would happen if ammonia is added to the reaction at equilibrium? Reaction would shift to the right, producing more product. 2.What would happen if P(NH2)3 is removed from a reaction at equilibrium? Nothing since P(NH2)3 is a solid and is not part of the equilibrium expression.

  8. Given the following reaction: PCl3(g) + 3 NH3 (g)  P(NH2)3 (s) + 3 HCl (g) H = 890 J 1. If PCl3 is added to the reaction at equilibrium, how would the concentration of each of the other three substances in the reaction change? The reaction would shift to the right. NH3 would decrease and HCl would increase in concentration. P(NH2)3 would not change since it is not part of the equilibrium expression. 2.What would happen if the volme of the container is increased? The would result in decreased pressure for all gases. Since the reactants include more moles f gases, the relative pressure of the reactants is decreased to a greater extent. The reaction would shift to the left.

  9. Given the following reaction: PCl3(g) + 3 NH3 (g)  P(NH2)3 (s) + 3 HCl (g) H = 890 J 1. What would happen if the temperature of the reaction is increased? Since the reaction is endothermic, energy is a reactant. Increasing reactant shifts the reaction to the right and produces more product. The equilibrium constant for the reaction would also increase.

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