Title: Chemical Equilibrium
1Chemical Equilibrium
2The 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
4Demo
Start with NO2
Start with N2O4
Start with NO2 N2O4
5(No Transcript)
6Using 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.
7Using 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
10Write 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
11Homogenous 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)
12Homogeneous Equilibrium
H2O constant
Kc
The concentration of pure liquids are not
included in the expression for the equilibrium
constant.
13The 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
14The 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
15Heterogenous 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
17Consider 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
18Writing 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.
19Demo
Review
Start with NO2
Start with N2O4
Start with NO2 N2O4
20The 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
21Use 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.
22Le 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.
23At 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
244x2 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
25Calculating Equilibrium Concentrations
- Express the equilibrium concentrations of all
species in terms of the initial concentrations
and a single unknown x, which represents the
change in concentration. - Write the equilibrium constant expression in
terms of the equilibrium concentrations. Knowing
the value of the equilibrium constant, solve for
x. - Having solved for x, calculate the equilibrium
concentrations of all species.
26Predicting 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.
27Le 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.
28Le 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
30Le 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
31Le 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.
32Le Châteliers Principle 08
- N2(g) 3 H2(g) æ 2 NH3(g) Kc 0.291 at
700 K
33Le 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)
34Le 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.
35Le Châteliers Principle 14
- Example
- The reaction N2(g) 3 H2(g) æ 2 NH3(g) which is
exothermic by 92.2 kJ.
36Le 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?
37Le 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?
38Effect of Catalysis, Reduction of Activation
Energy
No effect