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Chemical Equilibrium

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Chemical Equilibrium Temperature Changes: Changes in temperature can change the equilibrium constant. Endothermic processes are favored when temperature increases. – PowerPoint PPT presentation

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Title: Chemical Equilibrium


1
Chemical Equilibrium
2
The Equilibrium State
Chemical Equilibrium The state reached when the
concentrations of reactants and products remain
constant over time.
Brown
Colorless
3
  • Chemical equilibrium is achieved when
  • the rates of the forward and reverse reactions
    are equal and
  • the concentrations of the reactants and products
    remain constant

Physical equilibrium
Chemical equilibrium
brown
colorless
4
Demo
Start with NO2
Start with N2O4
Start with NO2 N2O4
5
(No Transcript)
6
Using Equilibrium Constants 01
  • We can make the following generalizations
    concerning the composition of equilibrium
    mixtures
  • If Kc gt 103, products predominate over reactants.
    If Kc is very large, the reaction is said to
    proceed to completion.
  • If Kc is in the range 103 to 103, appreciable
    concentrations of both reactants and products are
    present.
  • If Kc lt 103, reactants predominate over
    products. If Kc is very small, the reaction
    proceeds hardly at all.

7
Using the Equilibrium Constant
Kc 4.2 x 10-48
(at 500 K)
Kc 57.0
(at 500 K)
Kc 2.4 x 1047
(at 500 K)
8
4.63 x 10-3
Law of Mass Action
9
216
10
Write the Equilibrium Constant Kc
If you multiply both side of an equilibrium
reaction by n the equilibrium constant should be
raised to the power of n.
Kc

Kc


Kc
Kc
2
11
Homogenous equilibrium applies to reactions in
which all reacting species are in the same phase.
Kp Kc(RT)Dn
Dn moles of gaseous products moles of gaseous
reactants
(c d) (a b)
12
Homogeneous Equilibrium
H2O constant
Kc
The concentration of pure liquids are not
included in the expression for the equilibrium
constant.
13
The equilibrium concentrations for the reaction
between carbon monoxide and molecular chlorine to
form COCl2 (g) at 740C are CO 0.012 M, Cl2
0.054 M, and COCl2 0.14 M. Calculate the
equilibrium constants Kc and Kp.
Kc
220
Kp Kc(RT)Dn
Dn 1 2 -1
R 0.0821
T 273 74 347 K
Kp 220 x (0.0821 x 347)-1 7.7
14
The equilibrium constant Kp for the reaction is
158 at 1000K. What is the equilibrium pressure
of O2 if the PNO 0.400 atm and PNO 0.270 atm?
2
347 atm
15
Heterogenous equilibrium applies to reactions in
which reactants and products are in different
phases.
CaCO3 constant CaO constant
Kc CO2
The concentration of pure solids and pure liquids
are not included in the expression for the
equilibrium constant.
16
Kp
17
Consider the following equilibrium at 295
K The partial pressure of each gas is 0.265
atm. Calculate Kp and Kc for the reaction?
0.265 x 0.265 0.0702
Kp Kc(RT)Dn
Kc Kp(RT)-Dn
Dn 2 0 2
T 295 K
Kc 0.0702 x (0.0821 x 295)-2 1.20 x 10-4
18
Writing Equilibrium Constant Expressions
  • The concentrations of the reacting species in the
    liquid phase are expressed in M. In the gaseous
    phase, the concentrations can be expressed in M
    or in atm.
  • The concentrations of pure solids and pure
    liquids, do not appear in the equilibrium
    constant expressions.
  • The equilibrium constant is a dimensionless
    quantity.
  • In quoting a value for the equilibrium constant,
    you must specify the balanced equation and the
    temperature.
  • If a reaction can be expressed as a sum of two or
    more reactions, the equilibrium constant for the
    overall reaction is given by the product of the
    equilibrium constants of the individual reactions.

19
Demo
Review
Start with NO2
Start with N2O4
Start with NO2 N2O4
20
The reaction quotient (Qc) is calculated by
substituting the initial concentrations of the
reactants and products into the equilibrium
constant (Kc) expression.
  • IF
  • Qc gt Kc system proceeds from right to left to
    reach equilibrium
  • Qc Kc the system is at equilibrium
  • Qc lt Kc system proceeds from left to right to
    reach equilibrium

21
Use reaction quotient to predict the direction of
shift when the volume is halved in the following
equilibrium N2O4(g) æ 2 NO2(g),
  • Consider the reaction N2O4(g) æ 2 NO2(g), taking
    place in a cylinder with a volume 1 unit.

22
Le Châteliers Principle 10
  • The Volume is then halved, which is equivalent to
    doubling the pressure.
  • Since Q gt K, the product is too high and the
    reaction progresses in the reverse direction.

23
At 12800C the equilibrium constant (Kc) for the
reaction Is 1.1 x 10-3. If the initial
concentrations are Br2 0.063 M and Br
0.012 M, calculate the concentrations of these
species at equilibrium.
Let (x) be the change in concentration of Br2
Initial (M)
0.063
0.012
ICE
Change (M)
-x
2x
Equilibrium (M)
0.063 - x
0.012 2x
Solve for x
24
4x2 0.048x 0.000144 0.0000693 0.0011x
4x2 0.0491x 0.0000747 0
ax2 bx c 0
x -0.00178
x -0.0105
At equilibrium, Br 0.012 2x -0.009 M
or 0.00844 M
At equilibrium, Br2 0.063 x 0.0648 M
25
Calculating Equilibrium Concentrations
  1. Express the equilibrium concentrations of all
    species in terms of the initial concentrations
    and a single unknown x, which represents the
    change in concentration.
  2. Write the equilibrium constant expression in
    terms of the equilibrium concentrations. Knowing
    the value of the equilibrium constant, solve for
    x.
  3. Having solved for x, calculate the equilibrium
    concentrations of all species.

26
Predicting the direction of a Reaction
  • The reaction quotient (Qc) is obtained by
    substituting initial concentrations into the
    equilibrium constant. Predicts reaction
    direction.Qc gt Kc System proceeds to form
    reactants.Qc Kc System is at equilibrium.Qc
    lt Kc System proceeds to form products.

27
Le Châteliers Principle 01
  • Le Châteliers principle If an external stress
    is applied to a system at equilibrium, the
    system adjusts in such a way that the stress
    is partially offset.

28
Le Châteliers Principle 02
  • Concentration Changes
  • The concentration stress of an added reactant or
    product is relieved by reaction in the direction
    that consumes the added substance.
  • The concentration stress of a removed reactant or
    product is relieved by reaction in the direction
    that replenishes the removed substance.

29
Le Châteliers Principle Haber process
N2(g) 3 H2(g) æ 2 NH3(g)
Exothermic
Cat  iron or  ruthenium 
30
Le Châteliers Principle 06
  • The reaction of iron(III) oxide with carbon
    monoxide occurs in a blast furnace when iron ore
    is reduced to iron metal
  • Fe2O3(s) 3 CO(g) æ 2 Fe(l) 3 CO2(g)
  • Use Le Châteliers principle to predict the
    direction of reaction when an equilibrium mixture
    is disturbed by
  • (a) Adding Fe2O3 (b) Removing CO2 (c) Removing CO

31
Le Châteliers Principle 07
  • Volume and Pressure Changes Only reactions
    containing gases are affected by changes in
    volume and pressure.
  • Increasing pressure Decreasing volume
  • PV nRT tells us that increasing pressure or
    decreasing volume increases concentration.

32
Le Châteliers Principle 08
  • N2(g) 3 H2(g) æ 2 NH3(g) Kc 0.291 at
    700 K

33
Le Châteliers Principle 11
  • Does the number of moles of reaction products
    increase, decrease, or remain the same when each
    of the following equilibria is subjected to a
    decrease in pressure by increasing the volume.
  • PCl5(g) æ PCl3(g) Cl2(g)
  • CaO(s) CO2(g) æ CaCO3(s)
  • 3 Fe(s) 4 H2O(g) æ Fe3O4(s) 4 H2(g)

34
Le Châteliers Principle 13
  • Temperature Changes Changes in temperature can
    change the equilibrium constant.
  • Endothermic processes are favored when
    temperature increases.
  • Exothermic processes are favored when
    temperature decreases.

35
Le Châteliers Principle 14
  • Example
  • The reaction N2(g) 3 H2(g) æ 2 NH3(g) which is
    exothermic by 92.2 kJ.

36
Le Châteliers Principle 15
  • In the first step of the Ostwald process for
    synthesis of nitric acid, ammonia is oxidized to
    nitric oxide by the reaction
  • 4 NH3(g) 5 O2(g) æ 4 NO(g) 6 H2O(g) ?H
    905.6 kJ
  • How does the equilibrium amount vary with an
    increase in temperature?

37
Le Châteliers Principle 16
  • The following pictures represent the composition
    of the equilibrium mixture at 400 K and 500 K for
    the reaction A(g) B(g) æ AB(g).
  • Is the reaction endothermic or exothermic?

38
Effect of Catalysis, Reduction of Activation
Energy
No effect
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