Bell work

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Bell work

• What comes to mind when you hear equilibrium?

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Chemical equilibrium
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12-1 and 12-2 Chemical Equilibrium

• Equilibrium
• There is an equal amount of reactants and
  products present
• Total amount of particles remain the same
• Law of conservation of mass
• It is dynamic
• When a state of equilibrium is reached, the rate
  of the forward process is equal to the rate of
  the reverse process
• As reactants are converted to products, the same
  amount of products are converted back to reactants

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Chemical Equilibrium

• Reactions usually flow from left to right
• Some reactions can flow either way
• Call reversible reactions
• If the reactants and products are kept in a
  closed system where nothing can escape, the
  reactants may go in either direction
• Write a reversible reaction with two arrows
• Explains why cold packs can be reused
• Reversing the reaction breaks apart the products
  and reforms the reactants

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At chemical equilibrium

• 2SO2 (g) O2 (g) 2 SO3 (g)
• Forward rate reverse rate
• What happens to the reaction rate?
• Does the reaction stop completely?

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Liquid-vapor equilibrium

• A volatile liquid in a closed system will be in
  equilibrium with its vapor above it.
• H2O (l) ?? H2O (g)
• Molecules change form back and forth at the same
  rate
• The reaction does not stop
• Rates are the same of the opposite processes
• Amount of liquid and gas molecules will remain
  constant

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How is equilibrium measured?

• The equilibrium constant Keq
• Remember at equilibrium, the reactants continue
  to form the products, and the products are
  reacting to re-form the reactants at the same
  rate
• At equilibrium, the forward rate (ratef) equals
  the reverse rate (rater)
• Keq Kforward product
• ----------- -------------
• Kreverse reactant

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12.2 and 12.3 Keq quantitatively describes
equilibrium

• The equilibrium constant is the ratio (at
  equilibrium) of the mathematical product of the
  concentrations of the products to the
  mathematical product of the concentrations of the
  reactants
• The concentrations (mols/L) are raised to the
  power of the coefficients in a balanced equation
• aA bB ?? cC dD
• Keq C c D d products /reactants
• -----------------------
• A a B b

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About the Keq

• Used to tell which direction the reaction is
  going
• Does not include solids or liquids!
• Gases and aqueous only

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What does the Keqtell us?

• Keq products / reactants
• Tells us which direction the reaction is
  traveling
• If the value is 1, the products and the reactant
  concentrations are the same (it is at
  equilibrium)
• In reactions that have Keq values greater than
  one, the reverse reaction is favored
• More product is present than reactants
• In reactions that have Keq values less than one,
  the forward reaction is favored
• More reactants are present than product

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Lets do some practice using concentrations

• H2 (g) O2(g) ? H2O (g)
• Calculate the Keq if H2 1.0 M
• O2 2.0 M
• H2O 1.5 M

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Practice

• Nitrogen pentachloride decomposes at 7600C.
• NCl5(g) ?? NCl3(g) Cl2(g)
• Keq 3.00 x 10-2
• What is the concentration of each of the products
  (NCl3 and Cl2) at equilibrium if the
  concentration of NCl5 is 1.5 x 10-3 mol/L and the
  concentration of NCl3 and Cl2 are equal?

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Answer

• NCl5(g) ? NCl3(g) Cl2(g)
• Keq 3.00 x 10-2
• Keq NCl3 Cl2
• --------------
• NCl5
• 3.00 x 10-2 x x
• --------------
  
• 1.5 x 10-3

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Answer continued

• 3.00 x 10-2 x2
• --------------
  
• 1.5 x 10-3
• x2 (3.00 x 10-2) (1.5 x 10-3)
• x2 (4.5 x 10-5)
• X 0.0067 mol/L

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Bell Work

• Calculate Keq for the synthesis of ammonia at
  4000C if the following concentrations are present
  at equilibrium.
• N2(g) H2(g) ? NH3(g)
• N2 4.5 mol/L
• H2 1.80 mol/L
• NH3 3.28 mol/L

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practice

• Formamide, HCONH2 is used to manufacture
  agricultural chemicals, dyes, and
  pharmaceuticals. At 1250C formamide decomposes
• HCONH2(g) ? NH3(g) CO(g)
• Keq 4.8
• if the equilibrium concentrations of NH3 and CO
  are both 0.60 mol/L, what is the
• equilibrium concentration of formamide?

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practice

• Example. For the reaction, CO 3H2 ? ? CH4
  H2O, calculate Keq from the following equilibrium
• concentrations CO 0.0613 M H2 0.1839 M
  CH4 0.0387 M H2O 0.0387 M.

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review
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Direction of reaction

• Reaction Quotient (Q)
• Same application of Keq to us so far
• Same expression
• Q products / reactants

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Characteristics about K

• Equilibrium can be approached from either
  direction.
• K does not depend on the initial concentrations
  of reactants and products.
• K does depend on temperature.
• The equilibrium constants for forward and reverse
  reactions are the reciprocals of each other
• K 1/K
• Example, K 11, then K 1/11 0.091
• If a reaction can be expressed as the sum of two
  or more reactions, K for the overall reaction is
  the product of the equilibrium constants of the
  individual reactions
• Reaction 3 Reaction 1 Reaction 2
• K(Reaction 3) K(reaction 1) x K(Reaction 2)

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One more thing about Keq

• Keq is the equilibrium constant
• It tells us which way the reaction is occurring
• Has other names depending on what is happening
• Also called Kc
• Constant for molar concentrations
• Also called Ka
• When the reaction involves an acidic situation
• Acid dissociation constant for weak acids
• Also called Kb
• When the reaction involves a basic situation
• Base dissocaition constant for weak bases
• Also called Kp
• When all of the reactants and products in the
  reaction are gases so Keq can be expressed in
  terms of partial pressure (atm)
• Gases can be or pp
• Also called Ksp
• When the reaction involves solubility (solubility
  product)
• Note that no denominator will be used in these
  types of reactions because the reactants are
  solids

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practice

• Write the expression for Kp for the following
  reaction
• CuO (s) H2 (g) ??Cu (s) H2O (g)

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example

• For the reaction, 2SO2(g) O2(g) ?? 2SO3(g)
• Write the equilibrium constant expression, Kp.
• What is the value for Kp if Kc 2.8x102 at 1000
  K?

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We can use Ksp to determine solubility, s

• Solubility the concentration in a saturated
  solution
• Tells us the maximum amount of solute you can
  dissolve
• If soluble, high concentration of ions and low
  concentration of reactants
• Ksp is larger than 1
• The bigger the ksp, the more soluble the stuff is
• The smaller the ksp, the less soluble the stuff

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The position of equilibrium

• Equilibrium constants vary with temperatures
• At what temperature are they mostly products at
  equilibrium?
• T 470 oC
• N2 3H2 ?? 2 NH3 K 1.0 x 10-1
• T 25 oC
• N2 3H2 ?? 2 NH3 K 4.0 x 108
• The second temperature shows us that they are
  most products at equilibrium

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Which solid dissolves the most in water (most
soluble)?

• PbCl2 (s) ?? Pb2 (aq) 2Cl- (aq)
• K 1.6 x 10-8
• PbI2 (s) ?? Pb2 (aq) 2I- (aq)
• K 8.5 x 10-9
• The first one would dissolve most number is
  bigger

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Example for you

• At a temperature of 1000 K, carbon monoxide can
  react with oxygen gas to form carbon dioxide
• Write a balanced equation for this reaction
• Write the Keq expression for this reaction
• The equilibrium constant for this reaction is 2.2
  x 1022. is this reaction expected to be at
  equilibrium or to go to completion?

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Direction of reaction

• So far you have solved for the K for equations
• This tells us what is happening with the reaction
• Magnitude of K
• If the K value is large (K gtgt 1), the equilibrium
  lies to the right and the reaction mixture
  contains mostly products.
• If the K value is small (K ltlt1), the equilibrium
  lies to the left and the reaction mixture
  contains mostly reactants.
• If the K value is close to 1 (0.10 lt K lt 10), the
  mixture contains appreciable amounts of both
  reactants and products.

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12.4 - Direction of reaction

• To solve for the direction of reaction, we need a
  new unit called the reaction quotient (Q)
• Q has the same expression as k
• Q products/reactants

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example
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12.5 - System equilibrium

• Equilibrium can favor the formation of reactants
  or products
• When a system is at equilibrium, it will stay
  that way until something changes this condition
• Stresses alter the equilibrium
• Change the location of the equilibrium

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Le Chateliers principle

• French chemist Henri Louis Le Chatelier
• His principle states that when a system at
  equilibrium is disturbed by applying stress, it
  attains a new equilibrium position to accommodate
  the change and relieve the stress
• What are stresses?
• Concentration, temperature, or pressure

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Changing concentration

• If you add more reactants, the system will shift
  towards the products (right)
• If you remove reactants (means there is more
  products than reactants now), the system will
  shift towards the reactants (left)
• If you remove products (means there is more
  reactants than products now), the system will
  shift towards the products (right)

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Temperature affects equilibrium systems

• Temperature changes can cause equilibrium shifts
• An increase in temperature causes the endothermic
  reaction to occur
• The way the reaction shifts depends on the type
  of reaction
• Endothermic or exothermic

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Endothermic

• Example
• What does it look like (reaction diagram)
• If we lower the temperature, what are we doing?
• By lowering the temperature, one of the products
  (energy) is being removed
• As the temperature continues to drop, the system
  reacts to relieve the stress

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Increase temperature

• Endothermic reaction
• Add temperature, adding heat energy
• The system will react in a way to relieve the
  stress and absorbs the heat

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Exothermic reaction

• Example
• Increasing temperature
• Left shift, more product
• Decreasing temperature
• Right shift, more reactants

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Increase temperature

• Exothermic reaction
• Add temperature, adding heat energy
• The system will react in a way to relieve the
  stress and absorbs the heat

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Now lets decrease temperature

• Exothermic
• Endothermic

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vant Hoff equation

• K2 ?Ho 1 1
• 1n ----- ------ ---- - ----
• K1 R T1
  T2
• K2 and K1 are equilibrium constants at T2 and T1
• ?Ho is the standard enthalpy change for the
  forward reaction
• R is the gas constant (8.31 J/molk)
• If the forward reaction is exothermic, K
  decreases as T increases
• If the forward reaction is endothermic, K
  increases as T increases

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Pressure changes

• Gaseous equilibrium systems
• Compression or expansion
• Count the number of gas moelcules on each side of
  the equation
• Compression
• Increase in pressure will cause the system to
  shift to the side that will form less gas
  moelcules
• Expansion
• Decrease in pressure will cause the system to
  shift to the side that will form more gas
  particles

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Practice

• Answer the following questions
• What does Le Chateliers principle say?
• How does changing the concentration of the
  reactants affect a reaction (according to Le
  Chatelier)?
• How does increasing pressure affect the reaction?

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Practice

• What will happen to the following reactions when
  the various stresses are applied
• 2 H2 (g) O2 (g) ? 2 H2O (l)
• 1. More hydrogen is added to the system
• 2. Oxygen is removed from the system
• 3. This is exothermic the temperature is
  increased
• 4. The pressure is increased
• 5. The pressure is decreased

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recap

• Out of the changes talked about with Le Chatelier
• Adding or removing
• Compressing or expanding
• Changing temperature
• The only one that changes the value of the
  equilibrium constant is a change in temperature
• In the other two cases, K remains constant