Equilibrium

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Equilibrium
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Equilibrium

• State of balance.
• Condition in which opposing forces exactly
  balance or equal each other.
• Need a 2-way or reversible situation.
• Need a closed system.

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Dynamic Equilibrium

• Macroscopic level looks like nothing is
  happening.
• Microscopic level lots going on.

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Equilibrium

• Rate of forward process rate of reverse
  process.
• Hallmark Looks like nothing is happening.
  Variables describing system are constant.

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3 Kinds of Equilibria

• Phase equilibrium physical
• Solution equilibrium physical
• Chemical equilibrium - chemical

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Phase Equilibrium

• Phase changes are reversible processes.
• H2O(l) ? H2O(g)
• H2O(l) ? H2O(s)
• Same substance on both sides. Phase is different.

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Examples - Phase Equilibrium

• Water water vapor in a sealed water bottle.
• Perfume in a partially full, sealed flask.
• Ice cubes water in an insulated container.
• Dry ice CO2(g) in a closed aquarium.

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Solution Equilibrium Solids

• Saturated solution dynamic equilibrium.
• Dissolving Solidification occur at equal rates.

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Solid in Liquid

• NaCl(s) ? NaCl(aq)
• Favored a little bit by higher temperature.

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Solution Equilibrium Gases
CO2 in water unopened. CO2(g) ? CO2(aq) Favored
by high pressure low temperature.
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Reversible Reactions

• N2(g) 3H2(g) ? 2NH3(g)
• Forward N2 H2 consumed. NH3 produced.
• 2NH3(g) ? N2(g) 3H2(g)
• Reverse NH3 consumed. N2 H2 produced.

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Reversible Reactions, 1 Equation

• N2(g) 3H2(g) ? 2NH3(g)
• Forward rxn, reactants are on left. Read left to
  right.
• Reverse rxn, reactants are on right. Read in
  reverse right to left.
• Rxns run in both directions all the time.

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N2(g) 3H2(g) ? 2NH3(g)
Why is this point significant?
Concentration
H2
NH3
N2
Time
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Reaction Rate

• Depends on concentration of reactants.
• As concentration of reactants decreases, rate
  decreases.
• As concentration of NH3 increases, rate of
  reverse rxn increases.

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Chemical Equilibrium

• State in which forward reverse rxns balance
  each other.
• Rateforward rxn Ratereverse rxn
• Does it say anything about the concentrations of
  reactants products being equal?

NO!
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Chemical Equilibrium

• Rateforward rxn Ratereverse rxn
• At equilibrium, the concentrations of all species
  are constant. They stop changing.
• They are hardly ever equal.

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Reversible Reactions vs. Reactions that Go to
Completion

• If your goal is to maximize product yield
• Easier in a reaction that goes to completion.
• Use up all the reactants.
• Left with nothing but product.
• Reversible reactions are different.
• Look at ?Conc/?time picture again.

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N2(g) 3H2(g) ? 2NH3(g)
Original Equilibrium Point
Concentration
H2
NH3
N2
Time
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Reversible Reactions

• Once you reach equilibrium, you dont produce any
  more product.
• This is bad news if the product is what youre
  selling.
• How can you change the equilibrium
  concentrations? For example, how can you
  maximize product?

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How can you get from here
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New equilibrium point
Lots of product as fast as possible.
To here?
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Affecting Equilibrium

• Equilibrium can be changed or affected by any
  factor that affects the forward and reverse
  reactions differently.

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What factors affect rate of rxn?

• Concentration/Pressure
• Temperature
• Presence of a catalyst

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Catalyst

• Has the same effect on the forward reverse
  reactions.
• Equilibrium is reached more quickly, but the
  equilibrium point is not shifted.
• The equilibrium concentrations are the same with
  or without a catalyst.

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Concentration, Pressure, Temperature

• Changes in concentration, pressure, temperature
  affect forward reverse rxns differently.
• Composition of equilibrium mixture will shift to
  accommodate these changes.

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LeChateliers Principle

• If a system at equilibrium is subjected to a
  stress, the system will act to reduce the
  stress.
• A stress is a change in concentration, pressure,
  or temperature.
• System tries to undo stress.

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N2(g) 3H2(g) ? 2NH3(g)
Original Equilibrium
New Equilibrium
H2
NH3
N2
Stress Increased N2
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System

• Only 2 possible actions
• Shift to the right form more product. The
  forward rxn speeds up more than the reverse rxn.
• Shift to the left form more reactant. The
  reverse reaction speeds up more than the forward
  rxn.

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A B ? C D, at equil.

• If I increase the concentration of A, how will
  the system react?
• How does the new equilibrium mixture compare to
  the original equilibrium mixture?
• Use logic. If you increase A, the system wants
  to decrease A. It has to use A up, so it
  speeds up the forward reaction.

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A B ? C D
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Changes in Temp

• Exothermic rxn
• A B ? C D heat
• If you increase the temperature, the system
  shifts to consume heat. So here, it shifts to
  the left.
• Endothermic rxn
• A B heat ? C D
• If you increase the temperature, the system
  shifts to consume heat. So here, it shifts to
  the right.

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Changes in Pressure

• N2(g) 3H2(g) ? 2NH3(g)
• If you increase pressure, the system shifts to
  the side with fewer moles of gas. Here, the
  right hand side has only 2 moles of gas while the
  LHS has 4. Increasing pressure will cause a
  shift to the right.
• If you decrease pressure, the system shifts to
  the side with more moles of gas.

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H2(g) I2(g) ? 2HI(g)

• This system has 2 moles of gas on the LHS 2
  moles of gas on the RHS.
• Systems with equal moles of gas on each side
  cannot respond to pressure changes.