Chemical Equilibrium

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Chemical Equilibrium

• L. Scheffler

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Chemical Equilibrium

• Chemical equilibrium occurs in chemical
  reactions that are reversible. In a reaction such
  as
• CH4(g) H2O(g) ? CO(g) 3H2 (g)
• The reaction can proceed in both directions
• CO(g) 3H2 (g) ? CH4(g) H2O(g)

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An Equilibrium System

• CH4(g) H2O(g) ? CO(g) 3H2 (g)
• After some of the products are created the
  products begin to react to form the reactants
• At equilibrium there is no net change in the
  concentrations of the reactants and products
• The concentrations do not change but they are not
  necessarily equal

4
Chemical Equilibrium

• CH4(g) H2O(g) ltgt CO(g) 3H2 (g)
• At the beginning of the reaction, the rate that
  the forward direction is higher
• As the reactants decrease the rate in the forward
  direction slows
• As the products form, the rate in the reverse
  direction increases
• When the two rates are the same equilibrium is
  achieved

5
Dynamic Equilibrium

• An equilibrium is Dynamic
• CH4(g) H2O(g) ? CO(g) 3H2 (g)
• The amount of products and the reactants are
  constant.
• (Note The concentrations are not necessarily
  equal but constant. Both reactions are still
  occurring, but at the same rate)

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The Equilibrium Constant

• aA bB ? cC dD
• The upper case letters are the molar
  concentrations of the reactants and products. The
  lower case letters are the coefficients that
  balance the equation.

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The Equilibrium Constant
aA bB ? cC dD
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Equilibrium Constant Calculations

• Example
• N2(g) 3H2 (g) ? 2NH3 (g)
• At equilibrium, a one-liter container has 1.60
  moles NH3, .800 moles N2, and 1.20 moles of H2.
  What is the equilibrium constant?

9
Equilibrium Constant Calculations

• At equilibrium, a one-liter container has 1.60
  moles NH3, .800 moles N2, and 1.20 moles of H2.
  What is the equilibrium constant?

10
Reaction quotient

• The equilibrium constant is a constant ratio only
  when the system is in equilibrium.
• If the system it not at equilibrium the ratio is
  known as a Reaction Quotient
• If the reaction quotient is equal to the
  equilibrium constant then the system is at
  equilibrium

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Using Equilibrium Constants for other calculations

• If a solution is not at equilibrium the ratio
  of the right side over the left is called a
  reaction quotient.
• aA bB ? cC dD

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Equilibrium Constants and calculations

• If Q gt Keq , the product side is too high and the
  equilibrium will shift to the left to restore
  equilibrium
• If Q lt Keq , the product side is too low and the
  equilibrium will shift to the right to restore
  equilibrium

13
Equilibrium Calculations Using I. C. E. Models

• Equilibrium constants and concentrations can
  often be deduced by carefully examining data
  about initial and equilibrium concentrations
• Initial Change Equilbrium

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Equilibrium CalculationsICE Model problem 1

• Hydrogen and iodine are in equilibrium with
  Hydrogen iodide to this reaction
• H2 I2 ? 2HI
• Suppose that 1.5 mole of H2 and 1.2 mole of
  I2 are placed in a 1.0 dm3 container. At
  equilibrium it was found that there were 0.4 mole
  of HI. Calculate the equilibrium concentrations
  of H2 and I2 and the equilibrium constant.

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Equilibrium CalculationsICE Model Problem 1
Solution

• Hydrogen and iodine are in equilibrium with
  Hydrogen iodide to this reaction
• H2 I2 ? 2HI
• Suppose that 1.5 mole of H2 and 1.2 mole of
  I2 are placed in a 1.0 dm3 container. At
  equilibrium it was found that there were 0.4 mole
  of HI. Calculate the equilibrium concentrations
  of H2 and I2 and the equilibrium constant.
• I C E Since 2x 0.4, x 0.2
• H2 1.5 -x 1.5- x H2
  1.5 0.2 1.3
• I2 1.2 -x 1.2 x I2
  1.2 0.2 1.0
• HI 0 2x 0.4
• Keq HI2 .
  (0.4)2 0.123
• H2 I2 (1.3) (1.0)

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Equilibrium CalculationsICE Model Problem 2

• Sulfur dioxide reacts with oxygen to produce
  sulfur trioxide according to this reaction
• 2 SO2 O2 ? 2SO3
• Suppose that 1.4 mole of SO2 and 0.8 mole of
  SO3 are placed in a 1.0 dm3 container. At
  equilibrium it was found that there were 0.6 dm3
  of SO3. Calculate the equilibrium concentrations
  of SO2 and O2 and the equilibrium constant.

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Equilibrium CalculationsICE Model Problem 2
Solution

• Sulfur dioxide reacts with oxygen to produce
  sulfur trioxide according to this reaction
• 2 SO2 O2
  ? 2SO3
• Suppose that 1.4 mole of SO2 and 0.8 mole of
  SO3 are placed in a 1.0 dm3 container. At
  equilibrium it was found that there were 0.6 dm3
  of SO3. Calculate the equilibrium concentrations
  of SO2 and O2 and the equilibrium constant.
• I C E Since 2x 0.6, x 0.3
• SO2 1.4 -2x 1.4-2x SO2 1.4
  2( 0.3) 0.8
• O2 0.8 -x 0.8 x O2
  0.8 0.3 0.5
• SO3 0 2x 0.6
• Keq SO32 .
  (0.6)2 0.281
• SO2 2 O2 (0.8)2(0.5)

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Le Chateliers Principle

• Le Chatelier's Principle states When a system in
  chemical equilibrium is disturbed by a change of
  temperature, pressure, or a concentration, the
  system shifts in equilibrium composition in a way
  that tends to counteract this change of variable.
  
• A change imposed on an equilibrium system is
  called a stress
• The equilibrium always responds in such a way so
  as to counteract the stress

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Le Chateliers Principle

• Types of stresses
• Change in concentration of one or more reactants
  or products
• Change in temperature
• Change in pressure
• Addition of a catalyst

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Effect of a Change in Temperature

• An increase in the temperature causes the
  equilibrium to shift in the direction of the
  endothermic reaction
• N2 (g) 3 H2 (g) ? 2NH3 (g) DH -92 kJ mol-1
• Since DH is negative the endothermic reaction is
  the reverse direction. An increase in
  temperature causes the reaction to shift to the
  left, resulting in an increase in N2 and H2 and a
  decrease in NH3

21
Effect of a Change in Pressure

• Pressure affects only gases in an equilibrium
• PV nRT
• An increase in pressure causes the equilibrium to
  shift in the direction that has the fewer number
  of moles
• N2 (g) 3 H2 (g) ? 2NH3 (g) DH -92 kJ mol-1
• An increase in pressure results in a an decrease
  in N2 and H2 and an increase in NH3

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Effect of a Change in one of the reactants or
products

• The equilibrium responds in such a way so as to
  diminish the increase
• Substances on the same side of the arrow respond
  in opposite directions.
• Substances on the opposite side of the arrow move
  in the same direction
• N2 (g) 3 H2 (g) ? 2NH3 (g)
• An increase in N2 results in a decrease in N2
  and H2 and an increase in NH3

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Effect of a Catalyst

• Catalysts affect both the forward and reverse
  directions equally
• A catalyst does not change the concentrations but
  reduces the time required for the system to come
  to equilibrium