Le Chatelier

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Le Chateliers Principle
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?

• What controls the position of chemical
  equilibrium?
• We can change certain aspects of an equilibrium
  reaction to force the equilibrium to shift to one
  side of the reaction (reactant or product side)
  This is called Le Chateliers Principle.

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LeChateliers Principle

• Le Chateliers Principle A change (stress)
  imposed on a system at equilibrium shifts in a
  direction that tends to reduce the effect of that
  change.
• ???

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What does it mean?

• When a reaction is at equilibrium, we can add a
  stress (change) to the system. The reaction
  shifts its equilibrium position to reduce that
  stress.
• What can we do to cause the equilibrium stress?

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Stresses

• Concentration
• Pressure (for gases)
• Volume (for gases)
• Temperature

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Effect of Change in Concentration

• Look again at the following reaction
• N2(g) 3H2(g)?2NH3(g)
• Lets say we have the following concentrations at
  equilibrium N20.399 M, H21.197 M,
  NH30.203 M
• What is the equilibrium constant?

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What is going to happen if we all of a sudden add
1.00 mol/L of N2?

• What is going to happen if we all of a sudden add
  1.00 mol/L of N2?
• Remember, the system was at equilibrium right
  before the extra N2 was added. Also, when you
  increase concentration, what happens to the rate
  of the reaction?

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• The rate of the forward reaction has increased,
  so what is going to happen to the concentration
  of NH3?
• The system wants to reestablish equilibrium, but
  the equilibrium has shifted now to the right, or
  more products.

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• Take a look at the new equilibrium
  concentrations N21.348 M, H21.044 M,
  NH30.304 M
• What is the equilibrium constant?
• The equilibrium constant does not change when
  reactant or product is added or removed.

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TO SUMMARIZE

• When a reactant or product is added to a system
  at equilibrium, the system shifts away from the
  side with the added component.
• When a reactant or product is removed from a
  system at equilibrium, the system shifts toward
  the side with the removed component.

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As4O6(s) 6C(s) ?As4(g) 6CO(g)

• Predict the shift in the equilibrium position for
  this reaction in response to the following
  changes
• A) Addition of CO
• B) Addition of C(s)
• C) Removal of As4(g)

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Effect of a Change in Volume

• For equilibrium containing a gas only
• When the volume of a gas decreases, what happens
  to the pressure?
• This is a sudden stress on equilibrium. What can
  the reaction do to reduce this stress?

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Consider the following reaction CaCO3(s)
?CaO(s) CO2(g)

• -If the volume containing the above reaction is
  suddenly deceased, the pressure of CO2 suddenly
  increases. To reduce this stress, the reaction
  will shift away from the gas, or to the left.
  Again, no change in the equilibrium constant.

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To SUMMARIZE

• When the volume of a gaseous reaction system at
  equilibrium is decreased (thus increasing the
  pressure), the system shifts in the direction
  that gives the smaller number of gas particles.
• Reverse is trueincreasing volume shifts to the
  side with more gas molecules

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• Predict the shift in equilibrium position that
  occurs in each of the following reactions when
  the volume is reduced
• A) P4(s) 6Cl2(g) ?4PCl3(l)
• B) PCl3(g) Cl2(g)? PCl5(g)
• C) PCl3(g) 3NH3(g) ?P(NH2)3(g) 3 HCl(g)

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Effect of a Change in Temperature

• When the temperature of reaction changes, the
  equilibrium constant changes.
• Reactions can either absorb heat or release heat.
• A reaction that releases heat (heat is a product)
  is called an exothermic reaction.
• A reaction that absorbs heat (heat is a reactant)
  is called an endothermic reaction.

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• What you do is you put energy into the chemical
  equation. For example, the synthesis of ammonia
  is an exothermic reaction, so we would write the
  reaction with energy included like
• N2(g) 3H2(g)?2NH3(g) Energy

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• The decomposition of calcium carbonate is an
  endothermic reaction, written like this
• CaCO3(s) Energy ?CaO(s) CO2(g)
• Once you have energy written as a reactant or
  product, treat it like any other substance.

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In Summary

• Treat energy as a reactant or product. If energy
  is added (increasing temperature), the
  equilibrium will shift away from the position of
  energy.
• Reverse is true

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Examples

• For each of the following reactions, predict how
  the equilibrium will shift if the temperature is
  increased.
• A) N2(g) O2(g)?2NO(g)
• B) 2SO2(g) O2(g) ?2SO3(g)
• C) C2H2(g) 2Br2?C2H2Br4(g)