EQUILIBRIUM BASICS
Chapter 14.1-14.3
Lesson Objectives
Know
-Factors that affect/don’t affect a reaction reaching equilibrium
-K is equilibrium constant for any reversible reaction
-Kc and Kp are related to each other through Kp = Kc(RT)Dng
-Coefficient Rule
-Reciprocal Rule
-Rule of Multiple Equilibria
Understand
-Equilibrium is based on when RATES of forward and reverse reactions are
equal, not when concentrations of reactants and products are equal
-The equilibrium constant K has its roots in the kinetics rate law constant k
-The size of the equilibrium constant tells us which side (reactants or
products) is favored in an equilibrium reaction
-K may only be calculated with concentration or pressure values when the
system is at EQUILIBRIUM
-Solids and liquids are not used in equilibrium constant expressions
Do
-Write an equilibrium constant expression for a given reaction
-Calculate K based on equilibrium concentrations
-Apply the Coefficient Rule, the Reciprocal Rule, and the Rule of Multiple
Equilibria to calculate a new K from a known K
-Calculate K when given initial concentration or pressure values (ICE Chart)
Which of the following is NOT true
about a reaction at equilibrium?
N2O4  2NO2
A.
B.
C.
D.
E.
[N2O4] = 2 [NO2]
The rate of the forward and reverse reactions
are the same
The equilibrium constant will describe
whether the process favors reactants or
products
[N2O4] and [NO2] remain constant.
For every 2 molecules of NO2 reacted, 1
molecule of N2O4 is produced
AT EQUILIBRIUM
Two reactions are occurring (forward and
reverse) indicated by double arrow.
 Equilibrium definition: a reaction has
reached equilibrium (balance) when the
forward and reverse reactions occur at the
same RATE.
 This means that concentrations of
reactants and products stay CONSTANT
over time

An Equilibrium Example
See this interactive tutorial here
Equilibrium factors:

Equilibrium IS dependent on
◦ TEMPERATURE (affects the equilibrium
constant, K)

It is NOT dependent on
◦ Original concentrations
◦ Volume of the container
◦ Total pressure of the system
 Remember – the partial pressure of a gas in a
mixture is proportional to the mole fraction of that
gas in the mixture!
The following data are for the system
A(g)  2B(g)
Time (s)
0
20
40
60
80
100
PA (atm)
1.00
0.83
0.72
0.65
0.62
0.62
PB (atm)
0.00
0.34
0.56
0.70
0.76
0.76
How long does it take the system to reach equilibrium?
K: equilibrium constant

Tells us the extent to which the reaction will go
until it reaches equilibrium
◦ Large K means the reaction goes mostly to product
◦ Small K means the reaction stays mostly as reactant
For a reaction
2A(g) + B(g)  2C(g)
K = 1 x 1083
which of the following can be concluded?
A.
B.
C.
D.
E.
At equilibrium, [A] = ½[B] = [C]
At equilibrium, the largest concentration is [A]
At equilibrium, the largest concentration is [C]
This reaction favors the reactants
This reaction happens very quickly
Writing K expression (equilibrium expression)
Uses only gaseous or aqueous products and
reactants (no solids or liquids; their
concentrations do not change during reactions)
 Coefficients become exponents
 Products over reactants
 For aA + bB  cC + dD

Kp =

(PC)c x (PD)d
(PA)a x (PB)b
or Kc =
[C]c [D]d
[A]a [B]b
Relate to each other: Kp = Kc(RT)Δng
◦ R = gas law constant (0.0821 atm·L/mol·K)
◦ Δng = change in moles of gas (products – reactants)
Which of the following species should NOT be
included in the equilibrium constant expression
for the reaction?
2A(s) + B(aq)  2C(l) + D(g)
A.
B.
C.
D.
E.
A, B, C
B, C
A, C
A, C, D
B, D
For a reaction 2A + B  2C
When equilibrium is established, the ratio of
products to reactants (K) = 0.5. Which of the
following initial conditions could be changed to
cause a different equilibrium constant value?
A.
B.
C.
D.
Start with more A
Dilute the reaction by adding water
Increase the temperature of the reaction
Add a catalyst
To calculate Kp from Kc for the reaction
A(g) + 2B(g)  C(g)
what would be the exponent to which Kc(RT) is
raised?
A.
B.
C.
D.
E.
-2
-1
0
1
2
Write the equilibrium constant (K) expressions
for the following reactions:
I2(g) + 5F2(g)  2 IF5(g)
SnO2(s) + 2H2(g)  Sn(s) + 2H2O(l)
For the reaction
2 NO(g) + O2(g)  2 NO2(g)
determine the Kp at 298K if Kc is 4.67 x 1013
True or False?

Given the equation below,
N2 + 3H2  2 NH3
if one mole of N2 is mixed with 3 moles of
H2, 2 moles of NH3 will form.
True or False?

Given the equation below,
N2 + 3H2  2 NH3
for every 1 mole of N2 that reacts, 3
moles of H2 will also react and 2 moles of
NH3 will form.
Calculate K for the reaction
NH4CO2NH2(s)  2 NH3(g) + CO2(g)
if at equilibrium there are 0.159 g of NH4CO2NH2 and
pressures of 0.0451 atm of CO2 and 0.0961 atm NH3
K depends on the form (coefficients) of
the balanced equation!

This means that K changes when:
◦ The coefficients are multiplied
◦ Reactions are reversed
◦ Reactions are added together (mechanism)
Coefficient rule:
If coefficients are multiplied by a factor (n),
then K is RAISED TO THE (n) EXPONENT
 A2 (g) + 2B (g)  2AB (g)

2
P
K = AB
PA x PB2

3A2 (g) + 6B (g)  6AB (g)
PAB6
K=
PA3 x PB6
Calculate K for the formation of 2 moles of ICl(g)
ICl (g)  ½I2 (g) + ½Cl2 (g)
K = 2.2x10-3
Reciprocal rule:
If the reaction is reversed, K of the forward
and K of the reverse are reciprocals of each
other (new K = 1/old K)
 A2 (g) + 2B (g)  2AB (g)

PAB2
K=
PA x PB2

2AB (g)  A2 (g) + 2B (g)
2
P
x
P
K= A B
PAB2
Rule of multiple equilibria

If multiple reactions are added together to get an
overall reaction, the overall K is the PRODUCT of the
individual K’s
A2 (g) + 2B (g)  2AB (g)
2AB (g) + C (g)  A2B2C (g)
A2 (g) + 2B (g) + C (g)  A2B2C (g)
Find overall K it by multiplying individual K’s:
Given the reactions below and their constants,
calculate K for the rxn:
Fe(s) + H2O(g)  FeO(s) + H2(g)
H2O(g) + CO(g)  H2(g) + CO2(g)
FeO(s) + CO(g)  Fe(s) + CO2(g)
K = 1.6
K = 0.67
Determining K

K must use values AT EQUILIBRIUM!

When given initial concentrations or pressures in
a problem , you must determine what the values
will be at equilibrium before you can calculate K

Use an ICE Chart to determine how the initial
values will increase or decrease to get to
equilibrium, then solve for K.
◦ Solids and liquids have no effect on equilibrium, so
they don’t need values in your ICE chart.
The ICE Chart
Initial/Change/Equilibrium
 Given initial pressures, find equilibrium constant

A2 = 0.1 atm
B = 0.2 atm
A2(g) + 2B(g)
I
C
E
0.1 atm
-x
0.2 atm
-2x
0.1 - x
0.2 - 2x
 2AB(g)
0.0 atm
+2x
2x
For the decomposition reaction
NH4HS(s)  NH3(g) + H2S(g)

In a sealed flask are 10.0 g of NH4HS, NH3
with a partial pressure of 0.692 atm and
H2S with a partial pressure of 0.0532 atm.
When equilibrium is established, it is found
that the partial pressure of NH3 has
increased by 12.4%. Calculate K for this
reaction.
Lesson Objectives
Know
-Factors that affect/don’t affect a reaction reaching equilibrium
-K is equilibrium constant for any reversible reaction
-Kc and Kp are related to each other through Kp = Kc(RT)Dng
-Coefficient Rule
-Reciprocal Rule
-Rule of Multiple Equilibria
Understand
-Equilibrium is based on when RATES of forward and reverse reactions are
equal, not when concentrations of reactants and products are equal
-The equilibrium constant K has its roots in the kinetics rate law constant k
-The size of the equilibrium constant tells us which side (reactants or
products) is favored in an equilibrium reaction
-K may only be calculated with concentration or pressure values when the
system is at EQUILIBRIUM
-Solids and liquids are not used in equilibrium constant expressions
Do
-Write an equilibrium constant expression for a given reaction
-Calculate K based on equilibrium concentrations
-Apply the Coefficient Rule, the Reciprocal Rule, and the Rule of Multiple
Equilibria to calculate a new K from a known K
-Calculate K when given initial concentration or pressure values (ICE Chart)