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Equilibrium

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Title: Equilibrium


1
Equilibrium
  • Chapter 15

2
Equilibrium
  • Have you ever tried to maintain your balance as
    you walked across a narrow ledge?
  • In a chemical reaction balance or equilibrium is
    also maintained.
  • You can think of the yields ?? sign as the ledge

3
  • Equilibrium systems exist in ocean water, blood,
    urine, and many other biological systems
  • Chemical reactions for the most part are
    reversible
  • You can think of the yield sign ?? as the ledge
    in a chem system

4
I. Chemical Equilibrium Concept
  • Chemical Equilibrium occurs when opposing
    reactions are proceeding at equal rates
  • At equil rate forward rate reverse
  • A??B indicates a molar conc.
  • Fr A?B rate kf A kf A kr B
  • Rr B?A rate kr B B kf cons Kc
  • Rearranging formula A kr

5
A dynamic equilibrium
  • Individual molecules are undergoing change but
    there is no net exchange in the concentration of
    reactants and products
  • Does not mean that the concentrations are not
    changing just that the ratio equals a definite
    value
  • Look at Habber reaction figure 15.6 text.
  • Dihydrogen monoxide
  • is also known as hydric acid, and is the major
    component of acid rain.
  • contributes to the "greenhouse effect."
  • may cause severe burns.
  • contributes to the erosion of our natural
    landscape.
  • accelerates corrosion and rusting of many metals.
  • may cause electrical failures and decreased
    effectiveness of automobile brakes.
  • has been found in excised tumors of terminal
    cancer patients.

6
Law of Mass Action
  • Equilibrium can be reached from either direction
  • Concentrations of reactants and products are
    expressed as
  • aA bB ?? pP qQ
  • equil constant Kc Pp Qq products
  • Aa
    Qb reactants

7
Equilibrium Constant mass action
  • Kc depends on the stoichiometry not on mechanics
  • Doesnt depend on the initial conc of reactants
    and products
  • Doesnt depend on other added sub as long as they
    do not react
  • Varies with tempt
  • Catalyst do not effect just speeds reaching eq.

8
Putting it Together
Question? Main Ideas Details Monitor



9
Writing equilibrium expressions
  • 2O3 (g) ?? 3O2 (g)
  • 2NO (g) Cl2 (g) ?? 2NOCl (aq)
  • AgCl (s) ?? Ag (aq) Cl- (aq)
  • Kc O23 Kc NOCl2
  • O32 NO2 Cl2
  • Kc Ag Cl-
  • Pure solids and liquids do not effect the
    equilibrium because their conc remain unchanged

10
Eq expressed as pressure
  • C3H8(g) O2(g) ? CO2(g) H2O(l)
  • Kp CO2p3
  • C3H8p O2p5
  • P partial pressure of the gas
  • Kp kc(RT)delta n

11
What does kc tell you?
  • Ex CO (g) Cl2 (g) ?? COCl2 (g)
  • Kc COCl2 4.57 X 109
  • CO Cl2
  • Kcgtgtgt1 larger numerator reaction goes almost
    totally to products eq lies right favors
    products
  • Kcltltlt1 larger denominator

12
Le Chateliers Principle
  • Henri-Louis Le Chatelier (1858-1936)
  • If a system at eq is disturbed by a change in
    temperature, pressure, or the concentration of
    one of the components, the system will shift
    its eq pos so as to counter the effects of the
    distrubance

13
Change in Concentration
  • Le Châtelier's principle states that if the
    concentration of one of the components of the
    reaction (either product or reactant) is changed,
    the system will respond in such a way as to
    counteract the effect
  • If a substance (either reactant or product) is
    removed from a system, the equilibrium will shift
    so as to produce more of that component (and once
    again achieve equilibrium)

14
Change in concentration
  • If a substance (either reactant or product) is
    added to a system, the equilibrium will shift so
    as to consume more of that component (and once
    again achieve equilibrium)

15
Putting it Together N2(g) 3H2(g) ltgt 2NH3(g)
Question? Main Ideas Details Monitor
How would inceasing H2 change eq
Predict the relative conc of each reactant and product
16
  • The reaction is driven "to the right" by the
    effects of added H2
  • The eq concs will not be identical to the
    original state. However, Kc will be the same. The
    new equilibrium state contains a slightly higher
    concentration of NH3(g), and slightly lower
    concentration of N2(g) (as well as a slightly
    higher concentration of H2(g).

17
Change in Volume and Pressure
  • A chemical system in equilibrium can respond to
    the effects of pressure also. According to Le
    Châtelier's Rule, if the pressure is increased on
    a system, it will respond by trying to reduce the
    pressure. How does it do this?
  • We are primarily concerned with homogeneous
    gaseous reactions
  • The stoichiometry of the reaction may lead to a
    greater number of molecules on one side of the
    equation.
  • For example, in the Haber reaction, N2(g)
    3H2(g) ltgt 2NH3(g) there are twice as many moles
    of reactants as products

18
  • If the Haber reaction were in equilibrium, and
    the pressure was increased, the reaction would
    respond to oppose the increase in pressure. It
    could accomplish this by shifting the equilibrium
    to the right (producing NH3(g))
  • This would reduce the overall number of moles in
    the reaction, and therefore, lower the pressure
  • Systems shift to the side with the fewest number
    of moles if both are the same then no change in
    eq cons occurs

19
Changes in Temperature
  • The intrinsic value of K does not change when we
    increase concentrations or pressures of
    components in a reaction. However, almost every
    equilibrium constant (K) changes in response to
    changes in temperature.
  • We will consider reaction conditions under which
    no work is done, and therefore all energy changes
    associated with reactions will be manifested by
    temperature changes)

20
Temperature Changes
  • Exothermic reactions are associated with heat
    release when the reaction proceeds in the forward
    direction
  • Endothermic reactions are associated with heat
    release when the reaction proceeds in the reverse
    direction (i.e. heat is absorbed in the forward
    direction)

21
  • These two types of reactions and their associated
    heat changes can be written as
  • Exothermic Reactants yield Products Heat
  • Endothermic Reactants Heat yield Products
  • If temperature is increased, the equilibrium will
    shift so as to minimize the effect of the added
    heat
  • The reaction will shift in the appropriate
    direction such that the added heat is absorbed

22
  • When heat is added to exothermic reactions at
    equilibrium, products will be consumed to produce
    reactants (shift to the LEFT) May also be
    written delta t is negative.
  • When heat is added to endothermic reactions at
    equilibrium, reactants will be consumed to
    produce products (shift to the RIGHT) May also be
    written delta t is positive.

23
Based on this behavior, what is the effect of T
upon K?
  • Assume K 1.0 for an exothermic reaction at
    equilibrium.
  • Added heat causes the reaction to shift to the
    left. Reactants lt Products Heat
  • Thus, 1.0 must represent a reaction quotient, Q,
    that is too large in comparison to the new value
    of K.
  • Thus, the effect of increasing temperature on an
    exothermic reaction is to lower the value of K.
  • Conversely, the effect of increasing temperature
    on an endothermic reaction is to increase the
    value of K

24
Putting it Together Calc Delta H of formation
for C3H8(g) O2(g) ? CO2(g) H2O(l)
Question? Is the value Exo,or endotherm Main Ideas Details Monitor
How would inc. temp effect eq
How would dec temp eff eq k
25
Calculations with eq K
  • Example calculating unknown concentrations using
    the eq constant
  • CO(g) 3H2(g) lt-gt CH4(g) H20(g)
  • At eq 0.3 mol of CO, 0.1 mol H2 and 0.02 mol of
    H20 are in 1.0 liter of a vessel at 1200 k kC is
    3.92 what is the conc of CH4?

26
  • Kc CH4 H2O
  • CO H23
  • 3.93 CH4 (.020)
  • (0.30) (0.10)3
  • CH4 (0.30)(0.10)3 3.93
  • (0.020)
  • 0.059 mol/l

27
Learning CheckPCl5(g)lt-gt PCl3(g) Cl2(g)
  • A l.0 liter vessel has a unknown amount of PCl5
    at eq Kc at 250 0C is 0.0415. Calc the unknow
    conc. if 0.02moles of PCl3 and Cl2 are in the
    container. (0.0096)

28
Solving linear eq equasions
  • CO(g) H20(g) lt-gt C02(g) H2(g)
  • Given 1.0 mol of CO2 and H20 in a 50.0 l vessel.
    How many moles are in an eq mix at 1000 oC Ec
    0.58 at 1000oC
  • CO(g) H20(g) lt-gt C02(g) H2(g
  • I 0.02 0.02 0 0
  • C -x -x x x
  • E 0.02-x 0.02-x x x

29
  • 0.58 CO2H2 X2
  • CO H2O (0.02-X)2
  • ,- 0.76 X2
  • (0.02-X)
  • (the neg one gives a neg answer x cant be
    neg)
  • _ 0.76(0.02-X)X
  • 0.0152-0.76XX
  • 0.0152 1.76X
  • X 0.0086

30
H2(g) I2(g) lt-gt 2HI(g)
  • What is the eq comp of a reaction mixture
    starting with 0.500 mol each of H2 and I2 in a 1
    l vessel? Kc 49.7 at 458 oC (H2 I2 0.11
    mol/l HI 0.78 mol/l

31
Equil with quadratic expressions
  • Calc the conc. of the previous problem with 1.00
    molar H2 and 2.00 molar I2 as the starting
    concentrations.
  • H2(g) I2(g) lt-gt 2HI(g)
  • I 1.00 2.00 0
  • C -x -x 2x 49.9 (2x)2
  • E 1.00-x 2.00-x 2x (1.00-x)(2.00-x)

32
  • (1.00-x)(2.00-x) (2x)2
  • 49
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