Chemical Equilibrium

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Chemical Equilibrium

• Honors Chapter 16

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Equalized

• All reactions want stability. This is obtained
  by chemicals moving back and forth until the
  reaction is stable.
• Equilibria plural for Equilibrium

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What is Equilibrium Constant

• All reactions contain a limited number of atoms
  in each direction.
• Reactions can go Forward or in Reverse
• A B AB
• Remember Vapor Pressure is noted by the number
  of molecules at equilibrium in the liquid or gas
  state.

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Graph to Equilibrium
A
Concentration
Equilibrium
B
Time
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Rate of the Reactions
Rate of the Forward Reaction
Rate
Rate of the Reverse Reaction
Equilibrium
Time
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What is equilibrium?

• At equilibrium the reaction is still proceeding
• However, it is proceeding at the same rate in
  both directions (note the double arrow)
• rate to the right the rate to the left
• Therefore, while individual molecules will react,
  the overall concentration of any reagent will
  remain constant.
• Some example reactions...
• THE REACTION DOES NOT STOP, keeps moving in both
  directions

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The Law of Chemical Equilibrium

• Remember The reversible reaction achieves
  equilibrium when the rate of the forward reaction
  is equal to the rate of the reverse.
• LAW OF MASS ACTION the relative concentrations
  of reactants and products at equilibrium in terms
  of quantity called equilibrium constant.

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The Equilibrium Constant (K or Keq)

• Numerical (unitless) value used to evaluate the
  status or state of a reaction
• Determine which way it will progress
• Useful in all areas of chemistry and many other
  sciences
• Several different variations

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• (homogenous reactions) Solution concentrations
  are in Molarity
• (homogenous) Gas concentrations are in pressures
  (usually atm, but can be defined in other units)
• (heterogenous) Solids have unitless
  concentrations of 1
• When the calculated K is at the standard value,
  the reaction is at equilibrium.
• There is a unique K for each reaction and each
  temperature
• Unique at pressures too, since they may vary
  concentrations!

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Keq

• aA bB cC dD
• a, b, c, d represent coefficients
• ABCD represent compounds

Keq Cc Dd Aa Bb
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What is the equilibrium expression for the
reaction (homogenous reaction)2CO O2 2CO2

• Keq CO22
• CO2O2

Why no numbers? Because we do not know the
Molarity of the compounds
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What is the equilibrium expression for the
homogenous reaction2CO O2 2CO2

• Keq CO22
• CO2O2

What if we have the molarity Carbon Dioxide
0.0100 M Carbon Monoxide 0.0200 M Oxygen
0.0200 M
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What is the equilibrium expression for the
Homogeneous reaction2CO O2 2CO2

• Keq 0.0100M2
• 0.0200M20.0200M

Keq 12.5
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Heterogenous reaction

• C (s) H2O (g) CO (g) H2
  (g)
• Carbon is a solid and can not be measured in
  molarity. In this case, Carbon has a unit of 1.
• Keq COH2
• H2O

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Manipulating Equilibrium Constants...

• Homogenous equilibrium look at all compounds
• Heterogenous equilibrium all solids have a unit
  of 1 since they can not be measured in M.
• Keq gtgt 1 the reaction is towards the products
• Keq ltlt 1 the reaction is towards the reactants
  (barely moves)

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Le Chateliers Principle

• If we disturb a reaction at equilibrium
• 1. change in Temperature
• 2. Change in Pressure
• 3. change in Concentration
• The reaction rates will shift to try to
  re-establish equilibrium concentrations of all
  reagents
• the rate in one direction will exceed the other

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The Reaction Quotient

• The reaction quotient (Q) is used to determine if
  the reaction is at equilibrium.
• If Q lt Keq then the equation falls toward the
  products. This means that reactant will react
  and produce more product, and less reactants are
  left over compared to equilibrium.
• If Q gt Keq then the equation falls toward the
  reactants. This means that more reactants are
  left over than products that could be made.
• If Q Keq then the equation is at equilibrium.

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Changes in Concentration

• All reactions will shift in reverse or forward to
  achieve equilibrium.
• Forward ? ? Backward
• If you add a concentration of a compound to the
  reaction, the reaction will try to balance out.
• Example NO2 ? N2O4
• If you add N2O4 to a stable reaction, the
  reaction will shift in reverse, changing N2O4
  into NO2 until equalized.

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Change in Pressure

• Take the equation
• 2NO2 ? N2O4 There are 2 moles to 1 mole ratio
• If the equation is at equilibrium, and pressure
  is applied decreasing the volume. Now the
  reaction must move in the direction to decrease
  the number of moles
• The reaction will move in the forward
  dirction.
• If pressure is decrease, there is more volume.
  The reaction will move in the direction to
  increase the number of moles, it will now move in
  reverse.

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Change in Temperature

• Relates to Exothermic or endothermic reactions.
• If a reaction is exothermic in one direction,
  then it is endothermic in the reverse.
• If the reaction is exothermic (gives off heat)
  and heat is added, the reaction will move toward
  the reactants.
• If the reaction is endothermic, and the reaction
  is cooled the reaction will move toward the
  products.