IB Topic 7: Equilibrium 7'1: Dynamic equilibrium

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IB Topic 7 Equilibrium7.1 Dynamic equilibrium

• 7.1.1 Outline the characteristics of chemical and
  physical systems in a state of equilibrium.

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7.1.1 Outline the characteristics of chemical and
physical systems in a state of equilibrium.

• The reactions we have studied so far have gone to
  
• completion. In other words the reaction proceeds
  until one
• or more of the reactants runs out.
• Mg 2HCl ? MgCl2 H2
• There are reactions that are reversible. In other
  words, the
• reactions occur simultaneously in both directions
• 2SO2(g) O2(g) ? 2SO3(g)
• 2SO3(g) ? 2SO2(g) O2(g)

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7.1.1 Outline the characteristics of chemical and
physical systems in a state of equilibrium.

• Reversible Reactions
• Reactions occurring simultaneously in both
  directions
• 2SO2(g) O2(g) 2SO3(g)
• Reaction 1 Sulfur dioxide reacts with oxygen to
  produce sulfur trioxide. SO2(g) O2(g) are
  reactants, SO3(g) is the product.
• Reaction 2 Sulfur trioxide decomposes to sulfur
  dioxide oxygen. SO3(g) is the reactant and
  SO2(g) O2(g) are products.

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7.1.1 Outline the characteristics of chemical and
physical systems in a state of equilibrium.

• Chemical Equilibrium
• A state in which the rate of the forward reaction
  equals the rate of the reverse reaction.
• Once equilibrium is reached, the concentrations
  of the reactants and the concentrations of the
  products do not change.
• The reaction continues but no change in
  concentrations

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7.1.1 Outline the characteristics of chemical and
physical systems in a state of equilibrium.

• 2HI H2 I2
• Reaction 1 (Graphs 1 2) Starting with a
  concentration of 2.0 HI and 0 H2 or I2. HI begins
  to decompose, forming H2 I2. As H2 I2 form,
  they begin to react forming HI. Eventually the
  rates become equal so the amount of HI reacted
  the amount of HI produced. The reaction continues
  but no change in concentrations.

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7.1.1 Outline the characteristics of chemical and
physical systems in a state of equilibrium.

• 2HI H2 I2
• Reaction 2 (Graphs 3 4)
• Explain what is happening.

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7.1.1 Outline the characteristics of chemical and
physical systems in a state of equilibrium.

• Physical System at Equilibrium
• Liquid water evaporates to form water vapor. At a
  given temperature in a closed system, water will
  evaporate until the vapor reaches a certain
  pressure. When that occurs, equilibrium is
  reached. Water still evaporates but at the same
  rate as water condensing.

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IB Topic 7 Equilibrium7.2 The position of
equilibrium

• 7.2.1 Deduce the equilibrium constant expression
  (Kc) from the equation for a homogeneous
  reaction.
• 7.2.2 Deduce the extent of a reaction from the
  magnitude of the equilibrium constant.
• 7.2.3 Apply Le Chateliers principle to predict
  the qualitative effects of changes of
  temperature, pressure and concentration on the
  position of equilibrium and on the value of the
  equilibrium constant.
• 7.2.4 State and explain the effect of a catalyst
  on an equilibrium reaction.
• 7.2.5 Apply the concepts of kinetics and
  equilibrium to industrial processes.

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7.2.1 Deduce the equilibrium constant expression
(Kc) from the equation for a homogeneous reaction.

• Equilibrium Constant (Kc)
• means concentration expressed in mol dm-3
• When a system reaches equilibrium, the
  reactants stays the same and the products
  stays the same. There is a mathematical
  relationship between the rcts and prod.
• aA bB cC dD
• Kc Cc x Dd
• Aa x Bb

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7.2.1 Deduce the equilibrium constant expression
(Kc) from the equation for a homogeneous reaction.

• Write the equilibrium constant expression for the
  following
• Contact Process (manufacture of sulfuric acid)
• 2SO2(g) O2(g) 2SO3(g)
• Kc SO32
• SO22 x O2
• A homogeneous reaction is one in which all the
  reactants and products are in the same phase. We
  can write an equilibrium expression if the
  substances are all gases, all liquids or all in
  aqueous solution.

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7.2.1 Deduce the equilibrium constant expression
(Kc) from the equation for a homogeneous reaction.

• Write the equilibrium constant expression for the
  following
• Haber Process (manufacture of ammonia)
• 3H2(g) N2(g) 2NH3(g)
• The dissociation of hydrogen iodide
• 2HI(g) H2(g) I2(g)

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7.2.2 Deduce the extent of a reaction from the
magnitude of the equilibrium constant.

• The equilibrium constant is a measure of the
  amount of
• products at equilibrium compared with the amount
  of
• reactants.
• More products than reactants at equilibrium. The
  reaction goes almost to completion.
• Kc gtgt1
• H2(g) I2(g) 2HI(g) Kc 794

Products
Reactants
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7.2.2 Deduce the extent of a reaction from the
magnitude of the equilibrium constant.

• The equilibrium constant is a measure of the
  amount of
• products at equilibrium compared with the amount
  of
• reactants.
• b) More reactants than products at equilibrium.
  The reaction hardly proceeds.
• Kc ltlt1
• N2(g) O2(g) 2NO(g) Kc 1 x 10-30

Reactants
Products
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7.2.2 Deduce the extent of a reaction from the
magnitude of the equilibrium constant.

• The equilibrium constant is a measure of the
  amount of
• products at equilibrium compared with the amount
  of
• reactants.
• c) Reactants and products present in somewhat
  equal amounts.
• Kc 1
• C2H6O C2H4O2 C4H8O2 H2O Kc 4

Products
Reactants
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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• Henri-Louis Le Chatelier
• (1850-1936)
• French industrial chemist
• If a system at equilibrium is disturbed by a
  change in temperature, pressure, or the
  concentration of one of the components, the
  system will shift its equilibrium position so as
  to counteract the effect of the disturbance.

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• Effects of Concentration Change
• If a chemical system is at equilibrium and we add
  a substance (either a reactant or a product) the
  reaction will shift to reestablish equilibrium by
  consuming part of the added substance. Removal of
  a substance will result in a shift that forms
  more of the substance.
• The value of Kc does not change (think paper
  clips)

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• Added H2
• Reaction will shift to use up some of the added
  H2.
• Forward reaction temporarily speeds up.
• N2 used up as it reacts with some of the extra
  H2.
• More NH3 is being produced.
• Eventually a new equilibrium is reached.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92.0 kJ

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• What effect will removing NH3 have on the
  equilibrium?
• System will shift to make more NH3 so it will
  temporarily speed up to the right.
• Some N2 H2 will react to produce more NH3.
• At the new equilibrium there will be less N2,
  less H2, and less NH3 than the original
  equilibrium.
• The value of Kc remains the same.

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• What effect will adding NH3 have on the
  equilibrium?
• What effect will removing N2 have on the
  equilibrium?

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• Effects of Pressure/Volume Change
• If a chemical system is at equilibrium and we
  increase the pressure (reduce the volume), the
  reaction will shift toward the side having the
  fewest moles of gas. Decreasing the pressure
  (increasing the volume) causes a shift in the
  direction that produces more gas molecules.
• Only affects systems containing gas molecules.
• The value of Kc does not change.

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• What effect does increasing the pressure have on
  the
• equilibrium?
• Reaction will shift toward the side with the
  fewest gas molecules.
• The left side has 4 gas molecules (1N2 3H2).
  The right side has 2 gas molecules. Reaction will
  shift to the right.
• N2 H2 will react and more NH3 will be produced.
• Kc does not change

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• What effect does decreasing the pressure have on
  the
• equilibrium?
• If a reaction has equal numbers of gas molecules
  on the left and on the right, changing the
  pressure has no effect.
• 2HI(g) H2(g) I2(g)

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• Effects of Temperature Change
• Increasing temperature causes the equilibrium
  position to shift in the direction that absorbs
  heat (endothermic).
• Decreasing temperature causes the equilibrium
  position to shift in the direction that produces
  heat (exothermic).
• The value of Kc will change with a change in
  temp. If the reaction shifts right, the value of
  Kc increases. If the reaction shifts left, the Kc
  value decreases.

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• What effect does increasing the temperature have
  on the equilibrium?
• Increasing the temperature causes the reaction to
  shift to use up some of the added heat
  (endothermic rx).
• The reaction as written is exothermic so the
  endothermic rx is from right to left. The rx will
  shift left.
• N2 increases, H2 increases, NH3 decreases.
• Kc value will decrease

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• What effect does decreasing the temperature have
  on the equilibrium?

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7.2.3 Apply Le Chateliers principle to predict
the qualitative effects of changes of
temperature, pressure and concentration on the
position of equilibrium and on the value of the
equilibrium constant.

• N2O4(g) 2NO2(g) ?Ho 58.0 kJ
• Both gases are present in a flask at equilibrium.
  N2O4 is a colorless gas while NO2 is brown.
• What color will the contents of the flask be if
  the pressure is increased? Explain.
• State and explain three (3) ways the amount of
  NO2 production can be increased.

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7.2.4 State and explain the effect of a catalyst
on an equilibrium reaction.

• A catalyst lowers the activation energy barrier
  for both the forward and the reverse reactions.
• Therefore a catalyst increase the rates of both
  reactions by the same factor.
• A catalyst increases the rate at which
  equilibrium is achieved, but does not change the
  final composition of the substances.
• The Kc value is not affected by the presence of a
  catalyst.

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7.2.5 Apply the concepts of kinetics and
equilibrium to industrial processes.

• N2(g) 3H2(g) 2NH3(g) ?Ho -92 kJ
• The Haber process to manufacture ammonia
• Ammonia is an important starting point for the
  production of fertilizers, nitric acid,
  explosives and polymers (nylon).
• Under what conditions can will an industrial
  chemist run this reaction to increase the yield
  of ammonia?
• An optimum temperature must be found
• Read pg. 133-134

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7.2.5 Apply the concepts of kinetics and
equilibrium to industrial processes.

• 2SO2(g) O2(g) 2SO3(g) ?Ho -192 kJ
• The contact process to manufacture sulfuric acid
• SO3(g) H2O(l) H2SO4(l)
• Sulfuric acid is used in many chemical processes
• Under what conditions can an industrial chemist
  run this reaction to increase the yield of sulfur
  trioxide?
• An optimum temperature must be found.
• Read pg. 134

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