EQUILIBRIUM

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EQUILIBRIUM
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Reversible Reactions
Many reactions occur forward and reverse if the
system is closed.
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Reversible reactions reach chemical equilibrium
when the rate of the forward reaction equals the
rate of the reverse reaction.
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Equilibrium

• Occurs when the rate of the forward reaction is
  equal
• to the rate of the backward reaction

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Applications of Equilibrium

• Phase Changes melting, boiling, freezing,
  condenstation
• Saturated Solutions dissolving and crystallizing
• Acids/Bases
• Various Chemical Reactions Organic chemistry,
  biochemistry

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The rate of evaporation is equal to the rate of
condensation
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Reaction Rate

• How fast or slow a reaction occurs.
• In equilibrium, one must think about forward and
  reverse reactions

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Collision Theory

• Particles must collide in order to react
• Reactants collide, forming products
• Products collide forming reactants

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Reactions with a HIGH activation energy, Ea
proceed at a SLOW rate.
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A B lt--gt AB
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Equilibrium Constant (Kc)

• A indication of how far a reaction goes to the
  right

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• Molar concentration

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The concentrations of solids and pure liquids are
constant. (You cant find the concentration of
something that isnt a solution!)Omit the
concentrations of pure liquids and pure solids in
equilibrium constant expressions.
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Kc gt 1More products than reactants at
equilibrium Kc lt 1More reactants than
products at equilibrium
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You do not need to calculate Kc, but here is an
example
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2 HI(g) ?? H2(g) I2(g)
The value for K, the equilibrium constant is less
than one.
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Once a reaction reaches equilibrium can it ever
change??
If there is a change in either the forward or
reverse rates.
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What changes reaction rate (and therefore
changes the equilibrium) 1. Concentration 2.
Pressure 3. Temperature
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Le Chateliers Principle

• If changes are made to
• a system in equilibrium,
• the reaction will shift in
• the direction that will
• reduce the stress.

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A note about Kc

• In a system at equilibrium, if disturbed, the
  concentrations will always arrange themselves to
  multiply and divide in the Kc equation to give
  the same number (at constant temperature).

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Concentration

• If substance is added, the
• reaction will shift away from
• that side
• If substance is removed, the reaction will shift
  towards that side

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Concentration

• N2(g) 3H2(g) lt---gt 2 NH3(g) energy
• If the concentration of nitrogen is increased,
  which way will the equilibrium shift?
• If ammonia is removed, which way will the
  equilibrium shift?

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Pressure

• If pressure is increased,
• the reaction will shift to the
• side that will produce
• fewer molecules to reduce the pressure
• (compare moles of gas)

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If Pressure increases, which way will the
equilibrium shift?

• 2 H2(g) O2(g) lt---gt 2 H2O(g)
• N2(g) 3H2(g) lt---gt 2 NH3(g) energy
• 2 HI(g) ?? H2(g) I2(g)

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Temperature

• Increased temperature will shift the reaction to
  the side that absorbs heat and vice versa

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Temperature

• Increasing the temperature favors the endothermic
  reaction
• Decreasing the temperature favors the exothermic
  reaction

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If the temperature is increased, which way will
the equilibrium shift?

• 2 H2O(g) energy lt--gt 2 H2(g) O2(g)
• N2(g) 3H2(g) lt---gt 2 NH3(g) energy

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Catalyst

• No change in the equilibrium
• The forward and the reverse reactions are faster.
• The activation energies of the forward and
  reverse reactions are both lower.

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The forward and reverse reactions are faster
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Haber Process

• N2(g) 3H2(g) lt---gt 2 NH3(g) energy
• Fritz Haber figured out to use an iron catalyst
  to increase the production of ammonia, a
  fertilizer. He earned a Noble Prize for his
  work.
• List four ways to shift the equilibrium to the
  right to produce more ammonia.

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Haber process conditions

• A low temperature of 350 - 450 oC
• A high pressure of 150-250 atm
• A catalyst Fe
• Under these conditions, 15 ammonia is achieved
• About 140 millions of tons of ammonia are
  manufactured each year.

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The Contact Process to make Sulfuric Acid

• Combust sulfur
• S (s) O2 (g) ---gt SO2 (g)
• 2. React sulfur dioxide with oxygen
• 2 SO2(g) O2(g) lt--gt 2 SO3(g) energy
• 3. Mix sulfur trioxide with water
• SO3(g) H2O(l) ---gt H2SO4(l)
• What conditions will cause the second reaction to
  shift to the right to produce more sulfur
  trioxide?

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The Contact Process Conditions

• To maximize sulfur trioxide production
• A low temperature of 450 oC
• A high pressure of 2 atm
• A catalyst V2O5