chapter 6: phenomena - university of california, santa...
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Chapter 6: Chemical Equilibrium
Chapter 6: Phenomena
Reaction 1: A(aq) ⇌ B(aq) Reaction 2: B(aq) ⇌ 2C(aq)
Trial InitialAfter 10
Minutes
After 20
Minutes
After 30
MinutesTrial Initial
After 10
Minutes
After 20
Minutes
[A]
(M)
[B]
(M)
[A]
(M)
[B]
(M)
[A]
(M)
[B]
(M)
[A]
(M)
[B]
(M)
[B]
(M)
[C]
(M)
[B]
(M)
[C]
(M)
[B]
(M)
[C]
(M)
1 0.60 0.00 0.48 0.12 0.48 0.12 0.48 0.12 1 0.60 0.00 0.30 0.60 0.23 0.75
2 0.70 0.70 0.90 0.50 1.12 0.28 1.12 0.28 2 0.70 0.70 0.49 1.12 0.49 1.12
3 0.00 0.60 0.20 0.40 0.40 0.20 0.48 0.12 3 0.00 0.60 0.08 0.44 0.08 0.44
4 1.10 0.00 0.90 0.20 0.88 0.22 0.88 0.22 4 1.10 0.00 0.80 0.60 0.53 1.14
5 0.00 1.20 0.20 1.00 0.40 0.80 0.60 0.60 5 0.00 1.20 0.23 0.75 0.23 0.75
6 1.40 0.35 1.40 0.35 1.40 0.35 1.40 0.35 6 1.40 0.35 1.10 0.95 0.85 1.45
7 1.00 1.58 1.20 1.38 1.40 1.18 1.60 0.98 7 1.00 1.58 1.00 1.58 1.00 1.58
Phenomena: Scientists studied the following reactions by putting
different amounts of substances (reactants and products) into a
sealed rigid vessel and measuring the concentration as a
function of time. What patterns do you notice about the data?
Chapter 6
Chemical
Equilibrium
• Dynamic Equilibrium
• Equilibrium Constant
• Reaction Quotient
• Determining Equilibrium
Concentration /
Pressure
• Le Chatelier’s Principle• Determining Equilibrium
Constants from Other
Systems
2
Big Idea: Reactions proceed to
an equilibrium
condition. When
equilibrium is reached
the rate of formation of
products equals the
rate of formations of
reactants. When a
system at equilibrium is
disturbed the reaction
responds by minimizing
the disturbance.
Chapter 6: Chemical Equilibrium
Student Question
Dynamic Equilibrium
Is the following statement true or false?
At equilibrium the amount of products always
equals the amount of reactants.
a) True
b) False
c) Not Enough Information
3
Chapter 6: Chemical Equilibrium
Dynamic Equilibrium
Data from all day Chemistry Club beach BBQ
4
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4:0
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dynamic
equilibrium
dynamic
equilibrium
dynamic
equilibriumdynamic
equilibrium
Chapter 6: Chemical Equilibrium
Dynamic Equilibrium
Dynamic Equilibrium: The condition in which the
rate of the forward reaction equals the rate of the
reverse reaction.
5
Chapter 6: Chemical Equilibrium
A⇌2B+C
Equilibrium Expressions
Activity (aJ): The effective concentration or
pressure of a species J expressed as the partial
pressure or concentration of the species relative to
its standard value.
Equilibrium Constant
Full Simplified
𝐾 =𝑎𝐵2𝑎𝐶𝑎𝐴
𝐾 =𝐵 2 𝐶
𝐴
6
Chapter 6: Chemical Equilibrium
Liquids and Solids
Solids and liquids only have one concentration, therefore, this
is also the reverence concentration. Any number divide by
itself is 1 which results in solids and liquids not being included
in the equilibrium constant.
Note: Since the equation for activity is concentration over concentration or pressure
over pressure, many books do not put units on equilibrium expressions.
Equilibrium Constant
Aqueous Solutions Gas Solutions
𝑎𝑗 =𝐽
𝑐°𝑎𝑗 =
𝑃𝑗
𝑃°
[J] = concentration of J PJ = partial pressure of J
c° = reference concentration (1 M) P° = reference pressure (1 atm)
7
Chapter 6: Chemical Equilibrium
What does Keq tell us?
Keq << 1
lots of reactants, not many products at equilibrium
reverse reaction is favored/faster*
Keq ~1
equal amounts of reactants and product at
equilibrium
neither direction favored/faster*
Keq >> 1
lots of products, not many reactants at equilibrium
forward reaction is favored/faster*
*when equal amounts of products and reactants are present
Equilibrium Constant
8
Chapter 6: Chemical Equilibrium
Student Question
Before Class Question
How are K and KP related for the following
reaction?
CO(g) + ½O2(g) ⇌ CO2(g)
a) 𝐾𝑃 = K
b) 𝐾𝑃 = 𝑅𝑇𝐾
c) 𝐾𝑃 =1
RTK
d) 𝐾𝑃 = RTK
e) None of the above
9
Chapter 6: Chemical Equilibrium
What does Q tell us?
Q > K
There are more products present than there should
be at equilibrium. Therefore, the reverse reaction is
spontaneous and products tend to decompose
into reactants.
Q = K
The system is at equilibrium.
Q < K
There are more reactants present than there
should be at equilibrium. Therefore, the forward
reaction is spontaneous and reactants tend form
into products.
Reaction Quotient
10
Chapter 6: Chemical Equilibrium
Student Question
Reaction Quotient
The K for the following reaction at 1000.°C is
1.17. For a system with the concentrations
[CO2] = 0. 342 M and [CO] = 0.100 M one can
conclude that:
CO2(g) + C(s) ⇌ 2CO(g)
a) The system is not at equilibrium and the
reaction will proceed to the reactants.
b) The system is at equilibrium and no net
change will occur.
c) The system is not at equilibrium and the
reaction will proceed to the products.
11
Chapter 6: Chemical Equilibrium
Determining Equilibrium Concentration/Pressure
Step 1: Write a balanced chemical reaction.
Step 2: Write the expression for K or KP.
Step 3: Make a chart.
Step 4: Plug in equilibrium values into equilibrium
expression.
Step 5: Use guess and check, quadratic formula,
or an assumption to solve for x.
Step 6: Calculate equilibrium concentrations/
pressures.
12
Species 1 Species 2
Initial (Concentration/Pressure)
Change
Equilibrium
Chapter 6: Chemical Equilibrium
Example 1:
3.4 mol of SO3 is put in an otherwise empty 1.0 L
container. What are the concentrations of the
species at equilibrium if the following reaction
occurs:
2SO2(g) + O2(g) ⇌ 2SO3(g)
K=3.43 (at 1000. K)
Determining Equilibrium Concentration/Pressure
13
Chapter 6: Chemical Equilibrium
Example 2:
The following data was collected at equilibrium
for the system:
H2(g) + I2(g) ⇌ 2HI(g)
𝑃𝐻2 = 0.639𝑎𝑡𝑚
𝑃𝐼2 = 0.0586𝑎𝑡𝑚
𝑃𝐻𝐼 = 0.135𝑎𝑡𝑚
If 0.300 atm of each gas was added to the
container after the equilibrium was established.
What are the new equilibrium pressures?
Determining Equilibrium Concentration/Pressure
14
Chapter 6: Chemical Equilibrium
Example 3:
The following equilibrium is known to occur:
H2CO3(aq) ⇌ 2H+(aq) + CO32-(aq)
K=2.4×10-17
What are the concentrations at equilibrium if
initially there is only 0.14 M H2CO3 present?
Determining Equilibrium Concentration/Pressure
15
Chapter 6: Chemical Equilibrium
Checking Assumptions:
Assumption are good when the amount
subtracted off is less than 5% of the number it is
being subtracted from. (5% Rule)
Calculating the Percentage Lost/Gained𝑎𝑚𝑜𝑢𝑛𝑡 𝑙𝑜𝑠𝑡/𝑔𝑎𝑖𝑛𝑒𝑑
𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑎𝑟𝑖𝑜𝑛100%
Determining Equilibrium Concentration/Pressure
16
Note: This assumption is usually good when K or KP is less than 10-5.
Note: For test you mist check your assumption to receive full credit.
Chapter 6: Chemical Equilibrium
Le Chatelier’s Principle
Le Chatelier’s Principle: When a chemical system at
equilibrium is disturbed, the system shifts in a direction
that minimizes the disturbance.
Concentration
If the concentration of a species is increased, to restore
equilibrium, the reaction must proceed in the opposite
direction of the added species.
If the concentration of a species is decreased, to restore
equilibrium, the reaction must proceed towards the
direction of the removed species.
Volume
If the volume is decreased, to restore equilibrium, the
reaction must proceed towards the side of the equation
with fewer moles of gas. then
If the volume is increased , to restore equilibrium, the
reaction must proceed towards the side of the equation
with more moles of gas.17
Chapter 6: Chemical Equilibrium
Endothermic Reaction: A reaction that absorbs
heat (reactants + energy ⇌ products)
Exothermic Reaction: A reaction that releases
heat (reactants ⇌ products + energy)
Le Chatelier’s Principle
18
Chapter 6: Chemical Equilibrium
Le Chatelier’s Principle
Temperature
Exothermic Reactions (R ⇌ P + heat)
If the temperature is raised,
equilibrium shifts to the reactants.
If the temperature is lowered,
equilibrium shifts to the products.
Endothermic Reactions (R + heat ⇌ P)
If the temperature is raised,
equilibrium shifts to the products.
If the temperature is lowered,
equilibrium shifts to the reactants.
19
Chapter 6: Chemical Equilibrium
Student Question
Le Chatelier’s Principle
The following equilibrium is going on in the 3
tubes:2NO2(g brown) ⇌ N2O4(g colorless) + heat
What color is the gas in the hot water?
a) Dark Brown (mainly reactant but some
products)
b) Light Brown (mainly products but some
reactants)
c) Not Enough Information
20
Chapter 6: Chemical Equilibrium
Student Question
Le Chatelier’s Principle
The following KP values were collected for a
system.
KP=6.8×105 25°C
KP=1.9×10-4 400°C
What side of the equation is the heat on?
a) Reactants
b) Products
c) Not enough information
21
Chapter 6: Chemical Equilibrium
Determining Equilibrium Constants From Other Systems
If you knew the equilibrium constants of the first
two reactions, could you determine the
equilibrium constant for the third reaction?
Reaction 1
2GeO(g) + W2O6(g) ⇌ 2GeWO4(g) 𝐾1 =𝐺𝑒𝑊𝑂4
2
𝐺𝑒𝑂 2 𝑊2𝑂6
Reaction 2
GeO(g) + W2O6(g) ⇌ GeW2O7(g) 𝐾2 =𝐺𝑒𝑊2𝑂7
𝐺𝑒𝑂 𝑊2𝑂6
Reaction 3
GeO(g) + GeW2O7(g) ⇌ 2GeWO4(g) 𝐾3 =𝐺𝑒𝑊𝑂4
2
𝐺𝑒𝑂 𝐺𝑒𝑊2𝑂7
22
Chapter 6: Chemical Equilibrium
Student Question
Determining Equilibrium Constants From Other Systems
Given the following two reactions what is the equilibrium constant of 2A(aq)⇌D(aq) +E(aq)?
A(aq) + B(aq) ⇌ D(aq) + C(aq) K1 = 5.0
2C(aq) + D(aq) ⇌ 2B(aq) + E(aq) K2 = 2.0
a) 0.40
b) 2.5
c) 10.
d) 20.
e) None of the above
23
Chapter 6: Chemical Equilibrium
Take Away From Chapter 6
Big Idea: Reactions proceed to a equilibrium condition.
When equilibrium is reached the rate of formation of
products equals the rate of formations of reactants. When a
system at equilibrium is disturbed the reaction responds by
minimizing the disturbance.
Dynamic Equilibrium
Know what it means to be at dynamic equilibrium. (10)
Equilibrium Constant
Be able to calculate the equilibrium expression for a
reaction.(21,31,35)
Solids and liquids not included in equilibrium constant.
Be able to convert between the equilibrium expression in
concentration and in pressure.(24,25,33)
24
Numbers correspond to end of chapter questions.
Chapter 6: Chemical Equilibrium
Take Away From Chapter 6
Equilibrium Constant (Continued)
Know what the value of the equilibrium constant represents.
(2,11,13)
K=1 Similar number of product and reactant at equilibrium.
K<1 More reactant than product at equilibrium.
K>1 More product than reactant at equilibrium.
Reaction Quotient
Know what the value of Q represents when compare to
K.(27,28,37,38)
Q>K Too many products, reaction proceeds to reactant to reach
equilibrium.
Q=K at equilibrium
Q<K Too many reactants , reaction proceeds to products to reach
equilibrium.
25
Numbers correspond to end of chapter questions.
Chapter 6: Chemical Equilibrium
Take Away From Chapter 6
Determining Equilibrium Concentration/Pressure
Be able to determine the equilibrium concentration or
pressures, give initial concentration/pressure, and equilibrium
constant.(43,44,45,46,49,50,54,75,76,79,82)
Guess and check
Quadratic equation
Assumption (usually good when K<10-5)
Le Chatelier’s Principle
Be able to determine what a system will do if equilibrium is
disturbed.(56,58,59,61,62,63,65)
Determining Equilibrium Constants from Other Systems (26,72,73)
26
Numbers correspond to end of chapter questions.