Dynamic Equilibrium
Dynamic 
Static 
In Dynamic Equilibrium the rate of the forward reaction is the same as
the rate of the backward
 the concentration of reactants and products is unchanged
Eg.
Factors that affect equilibria
1.
2.
3.
♫ any change in these, results in a shift in the
concentration of the products or reactants ♫
Types of Equilibria
Type of Equilibrium
Description
A dynamic equilibrium between a solute and a
solvent in a saturated solution in a closed
system
A dynamic equilibrium between different
physical states of a pure substance in a closed
system
A dynamic equilibrium between the products
and reactants of a chemical reaction in a
closed system
Dynamic equilibrium in Chemical Systems
There are 5 experimental conditions that must be met for dynamic
equilibrium to take place in a chemical system
1.
2.
3.
4.
5.
Percent reaction
 Similar to percent yield
 The yield of product measured at equilibrium compared
with the maximum possible yield of product
Percent Reaction = Concentration at Equilibrium x 100
Theoretical yield
Example 1
In a closed system the following equilibrium system sets its self up
2Na+(aq) + I2(aq) ↔ 2NaI(g)
If initially there is 2.0 M of Na+ and 3.0M of I2 in the reaction vessel and
the equilibrium concentration is found to be 1.56 [HI]. What is the %
reaction?
If Percent reaction is…
<1%
_
_
>99%
_
_
1% - 99%
_
_
Ice Tables
I
C
E
- Initial concentration
- Change in Concentration
- Equilibrium Concentration
 At equilibrium the amount of products and reactants are
constant (This does not mean equal concentrations just
constant)
 Therefore we can express this constant ratio of products
and reactants for an equilibrium reactions using the
equilibrium constant, Keq
Example 2
Consider the following equation for the formation of hydrogen chloride
from its elements at SATP.
H2(g) + Cl2(g) ↔ 2 HCl(g)
If the reaction begins with 1.00 M of H2(g), and Cl2(g) with no HCl in the
reaction vessel. Calculate the concentration of H2(g) and HCl at
equilibrium if the concentration of Cl2(g) is 0.15 mol/L at equilibrium.
Example 3
Consider the decomposition of CaCO3 by HCl:
2HCl(aq) + CaCO3(aq) ↔ CaCl2(aq) + CO2(g) + H2O(l)
If the initial concentrations of HCl(aq) and CaCO3(aq) were 4.0M and 2.0M
respectively; determine the equilibrium concentrations of CaCO3, CaCl2,
CO2, and H2O, if the equilibrium concentration of [HCl]is 0.25M
Equilibrium constant
For the general reaction:
aA + bB ↔ cC + dD
The value for Keq = [C]c[D]d
[A]a[B]b
♫
all concentrations are equilibrium concentrations
♫
numerical coefficients used to balance the equation appear as
exponents in Keq
♫
Keq is temperature dependent
Example 4
2NO2(g) ↔
N2O4(g)
Pure Liquids and Solids
The concentration of pure solids and pure liquids do not appear in the
equilibrium constant expression since these concentrations are constant.
They are accounted for in the equilibrium constant.
Example 5
S(s) + O2(g)

SO2(g)
2NO(g) ↔2NO2(g)
4HCl(g) + O2(g) ↔ 2H2O(l) + 2Cl2(aq)
2NOCl(g) ↔ 2NO(g) + Cl2(g)
Equilibrium equation constants for the reverse reaction are indicated as
follows:
Keq= [A]a[B]b reactants
[C]c[D]d products
K1eq= 1/Keq
Keq – What does it tell us????
1.
2.
3.
Calculation Of Keq
Keq can be calculated if equilibrium reactant and product concentrations
are provided.
Example 6
CO2(g) + H2(g) ↔
CH3OH(g) + O2(g)
Carbon dioxide reacts with hydrogen to form methanol and oxygen.
Calculate the equilibrium constant at 327oC if an equilibrium mixture
contains the following concentrations of reactants and products:
(CO2) = 0.51 M (H2) = 0.09 M [CH3OH)= 0.015 M [O2] = 0.21 M
Equilibrium Problems
Determining Equilibrium Concentrations, given the initial
concentrations- Perfect Square Problem
Example 7
Find the equilibrium concentrations of H2 , I2 and HI if the initial
concentrations of H2 (g) and I2 (g) is 2.0 mol/L.
H2 (g) + I2 (g) –> 2HI (g) Keq
Step 1 Set up an ICE table
Step 2 Determine Keq expression and sub the equilibrium concentrations
into the expression.
Determining Equilibrium concentrations given initial
concentrations- Imperfect Square problem
Example 8
Given the following reaction has the equilibrium constant 25 @ 1100 K.
H2 (g) + I2 (g) –> 2HI (g)
2.O mol of H2 (g) and 3 mol of I2 (g) are placed in a 1 L reaction vessel @
1100K. What is the equilibrium concentration of each gas?
Step 1: Set up an ICE table
Step 2: Determine the Keq expression and sub in the equilibrium
concentrations into the expression.
Example 9
At 20000C the K is 6.40 x 10-7 for the decomposition of carbon dioxide
into carbon monoxide and oxygen. Calculate the concentration of all
entities at equilibrium if 0.250 mol of CO2 is placed in a closed 1.00L
container and heated to 20000C.
Approximation Rule:
If the value of the equilibrium constant is very small, it is often possible
to use an approximation that will SIMPLIFY the arithmetic .
What is very small?
An approximation can be made if the concentration from which x is
subtracted, or to which x is added, is at least 100 times the value of the
equilibrium constant.