Transcript PowerPoint
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)