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Chemical Equilibrium

Chemical Equilibrium

The Concept of Equilibrium (15.1) Ways of Expressing Equilibrium Constants

(15.2) What Does the Equilibrium Constant Tell

Us? (15.3) Factors that Affect Chemical Equilibrium

(15.4)

General Chemistry I – Concepts Representations of matter (1.3 and1.4) Formula calculations and stoichiometry

(3.6 through 3.9) Weak acids, molar concentrations of

solutions and solution stoichiometry (4.4through 4.6)

Pressure of a gas, the ideal gas law, and partial pressures (5.2 through 5.5)

Enthalpy and energy diagrams (6.4 and 6.6)

15.1 The Concept of Equilibrium What is equilibrium?◦ Discussed this previously – chs

4 and 12

Do reactions occur in both directions?

What does this mean in terms of reversibility?◦ Review: what is an elementary step?

What does reversibility mean in terms of rate?

Margin Figure p. 514

p. 68-69 of LA book

15.1 The Concept of Equilibrium

Let’s consider a reaction from Chapter 14:

Looking at this:◦ just for the rate determining step and◦ using a possible activated complex in an

energy diagram.

1

2

2 2

2 2 3

3 2 2

Overall reaction: NO g CO g NO g CO g

Elementary step 1: NO g NO g NO g NO g

Elementary step 2: NO g CO g NO g CO g

k

k

p. 69-70 of LA book

p. 70 of LA book

Or more general…p. 70 of LA book


What does reversibility mean in terms of rate?

What is the rate law for the forward reaction?

What is the rate law for the reverse reaction?

What happens when these rates are the same?

p. 70 of LA book


The equilibrium constant:◦ Definition:

Relates the concentrations of reactants and products at equilibrium

◦ Now consider the derivation where these rates are equal

What do we notice about the equilibrium constant?◦ Amounts?◦ Direction of reaction?◦ Mechanism?

p. 71 of LA book


The equilibrium constant:◦ What does this mean – what does this

number tell us? Consider the example in the

textbook:

Does it matter if we start with N2O4 only, NO2 only or a mixture?

Margin Figure p. 514

p. 72 of LA book

2 4 2N O g 2NO g


How did you know which direction the first two reactions will go?

Figure 15.1 p. 515

p. 73 of LA book

2 4 2N O g 2NO g 0.00463 at 298 KK

15.1 The Concept of Equilibriump. 73 of LA book

2 4 2N O g 2NO g



number tell us?

Consider a simpler example:

p. 74 of LA book

X g Y g



number tell us? K is large

Figure 15.2 p. 516

p. 74 of LA book



number tell us? K is small

Figure 15.2 p. 516

p. 74 of LA book


So, how did you know which direction this reaction went?

Figure 15.1 p. 515

p. 75 of LA book

2 4 2N O g 2NO g 0.00463 at 298 KK

15.2 Ways of Expressing Equilibrium Constants

Homogeneous equilibrium◦ What does this mean?◦ How do we identify this in a balanced equation?◦ Can we use other concentration units than molar

concentration?

Review: What is partial pressure? How does KP relate to Kc? Can KP = Kc?

Hom

ogen

eous

equ

ilibr

ium

p. 75-76 of LA book


PracticeSuppose start with 1.00 atm N2O4(g) and no NO2(g). At 100oC and at equilibrium, the partial pressure of N2O4(g) is 0.22 atmand the partial pressure of NO2 is 1.56 atm.What is KP and what is Kc?What does this look like?H

omog

eneo

us e

quili

briu

mp. 77 of LA book


Solution phase equilibrium – how do we identify this in a balanced equation?

Practice:What is the equilibrium expression for Kc for the reaction of hydrofluoric acid with water?What is the equilibrium expression for Kc for the reaction of hydrofluoric acid with sodium hydroxide?

How do we deal with [H2O]?

Hom

ogen

eous

equ

ilibr

ium

p. 77 of LA book


Heterogeneous equilibrium◦ What does this mean?◦ How do we identify this in a balanced

equation?◦ What does this mean in terms of writing the

equilibrium expression?

In general, what is not included in an equilibrium expression?

Het

erog

eneo

us e

quili

briu

mp. 78 of LA book


Figure 15.3 p. 522

Het

erog

eneo

us e

quili

briu

mp. 78 of LA book


Practice:

What are the equilibrium expressions for Kc for the reactions:a. b.c.

Is Kc = KP for a and b?

p. 78 of LA book

3 4HCl g NH g NH Cl s

2 2 2FeS s 2H g Fe s 2H S g

22PbCl s Pb aq 2Cl aq


What are the rules and forms for the equilibrium constant and writing the equilibrium expression?

1. How do we express concentration? How does KP relate to Kc?

2. What is included in the equilibrium expression? What is excluded?

3. What are the units on KP or Kc?4. Why do we have to give a balanced equation

with the equilibrium constant?

Rul

es a

nd fo

rms

for

the

equi

libri

um e

xpre

ssio

np. 79 of LA book


What are the rules and forms for the equilibrium constant and writing the equilibrium expression?

5. How is K affected by the direction of the reaction?

6. How is K affected by the stoichiometric amounts of products/reactants?

7. What is K for multiple steps for an overall reaction?

Rul

es a

nd fo

rms

for

the

equi

libri

um e

xpre

ssio

np. 79 of LA book

15.3 What Does the Equilibrium Constant Tell Us?

Review: ◦ K>>1 means?◦ K<<1 means?◦ K<0?

Fundamentally important:◦ K is a measure of equilibrium concentrations

for a reaction written in specific stoichiometric amounts and for a certain direction.

p. 82-83 of LA book

15.3 What Does the Equilibrium Constant Tell Us? K is a measure of equilibrium concentrations for

a reaction written in specific stoichiometric amounts and for a certain direction.

Calculations at equilibrium include:◦ Calculating K given equilibrium concentrations◦ Calculating equilibrium concentrations given K◦ Calculating both given other measures for the

system

p. 84 of LA book

15.3 What Does the Equilibrium Constant Tell Us? K is a measure of equilibrium concentrations

for a reaction written in specific stoichiometric amounts and for a certain direction.

Calculations at equilibrium must include:◦ Balanced equation (for which equilibrium is

represented)◦ Equilibrium concentrations

Sometimes these concentrations are given and sometimes these are calculated.

p. 84 of LA book


Practice:

Under certain conditions, begin with 1.00 atm of each N2 and O2 and at equilibrium obtain partial pressure of NO to be 0.20 atm. What is KP?

Cal

cula

ting

Kgi

ven

equi

libri

um c

once

ntra

tions

p. 85 of LA book


Practice:

If the equilibrium constant of the previous reaction changes to 2.5×105 as the temperature changes, what is the concentration of all substances if the initial concentration of nitrogen and oxygen are still 1.00 atm?

Cal

cula

ting

equi

libri

um c

once

ntra

tions

giv

en K

p. 86 of LA book


Practice

At 100oC, KP for the reaction of N2O4reacting to form NO2 is 11. If the reaction begins with 1.5 atm of N2O4, what is the equilibrium pressures of both gases?

Cal

cula

ting

equi

libri

um c

once

ntra

tions

giv

en K

p. 87 of LA book


Calculating equilibrium concentrations given K – sometimes the calculations can get more difficult.

But, are there approximations we make if we know about the value of K?◦ Review: K>>1 means? K<<1 means?

p. 88 of LA book


Sometimes the calculations be simplified through an approximation:

Practice:At 25oC, KP for the reaction of nitrogen and oxygen to form NO is 1.0x10-30. If the reaction begins with 1.0 atm of nitrogen and 0.750 of oxygen, what is the equilibrium pressures of all gases?C

alcu

latin

g eq

uilib

rium

con

cent

ratio

ns g

iven

Kp. 89 of LA book


Calculating both given other measures for the system

Practice:At 150oC, 15.5 g of N2O4 is placed in a 7.5 L container without any NO2. The reaction proceeds to equilibrium with a final pressure in the container of 1.49 atm. What is KP at this temperature?

Cal

cula

ting

both

giv

en o

ther

mea

sure

sp. 90 of LA book


What if the system is not yet at equilibrium or the equilibrium is perturbed in some way?

What is the Reaction Quotient (Q)The quantity obtained by substituting the initial concentrations into the equilibrium constant expression◦ How is this the same as the equilibrium constant?◦ How is this different from the equilibrium

constant?

p. 91 of LA book


What is the Reaction Quotient (Q)

Figure 15.4 p. 525

p. 91 of LA book

15.3 What Does the Equilibrium Constant Tell Us? What is the Reaction Quotient (Q)

Practice:

If for the preceding reaction (KP = 11), it is determined that at some point, the partial pressure of NO2 is 2.5 atm and N2O4 is 0.5 atm, is the reaction at equilibrium? If not, which way will the reaction shift in order to achieve to equilibrium?

p. 91 of LA book


Practice: Hydrogen reacts with ethylene to

produce ethane (all gases). Calculate the equilibrium constant if initially begin with 1.00 atm of each reactant (no product) and at equilibrium have 0.6 atm of product.

p. 92 of LA book

15.3 What Does the Equilibrium Constant Tell Us? Practice: Calculate Q if have 0.5 atm of all reactants

and products. Indicate the direction the reaction will proceed to reach equilibrium.

If the reaction is begun with 2.00 atm of ethane (and nothing else), what will the partial pressures of all substance be at equilibrium and what is the total pressure at equilibrium?

p. 92 of LA book

15.4 Factors That Affect Chemical Equilibrium What is Le Châtelier’s 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 as it tries to reestablish equilibrium

What stresses will we consider?◦ Changing concentrations◦ Changing volumes◦ Changing pressures◦ Changing temperatures

p. 92 of LA book

15.4 Factors That Affect Chemical Equilibrium

Changing concentrations This includes:◦ Adding more reactant◦ Adding more product◦ Removing reactant◦ Removing product

p. 93 of LA book


Changing concentrations Consider the reaction of N2O4 and NO2:

If the partial pressure of N2O4 is increased, what is Q? What will happen to the reaction?

p. 93 of LA book

2 4 2N O g 2NO g



If the partial pressure of N2O4 is decreased, what is Q? What will happen to the reaction?

p. 93 of LA book

2 4 2N O g 2NO g



If the partial pressure of NO2 is increased, what is Q? What will happen to the reaction?

p. 93 of LA book

2 4 2N O g 2NO g



If the partial pressure of NO2 is decreased, what is Q? What will happen to the reaction?

p. 93 of LA book

2 4 2N O g 2NO g


Figure 15.7 p. 531

p. 94 of LA book

2 2 33H g N g 2NH g


Changing concentrations Consider an equilibrium of an insoluble

salt in water:

If sodium bromide is added and the silver ion reacts with the bromide ion, what is Q and what will happen?

p. 94 of LA book

AgCl s Ag aq Cl aq


Changing volumes◦ Will changing volume of a container affect: Gases Solutions

Consider the concentration of a gas and how this will be affected by a change in volume

p. 94 of LA book


Changing pressure◦ Will changing the pressure affect: Gases Solutions

Consider the concentration of a gas and how this will be affected by a change in pressure

p. 95 of LA book


Consider the concentration of a gas and how this will be affected by a change in pressure◦ Adding more gases or removing gases◦ Increasing the volume or decreasing the

volume◦ Adding an inert gas to the container where

the volume of the container is unchanged

p. 95 of LA book


Changing temperatures◦ Think again in terms of the energy diagram:

p. 95 of LA book


p. 95 of LA book


Changing temperatures◦ Think again in terms of the energy diagram:

Review: What is the rate of the forward and

reverse reactions in terms of activation energy?

p. 95-96 of LA book


Two possibilities: Endothermic system:

p. 96 of LA book


Two possibilities: Exothermic system:

p. 97 of LA book

15.4 Factors That Affect Chemical Equilibrium Practice:

Give the expression for Kc What direction will reaction go if:◦ Add hydrogen◦ Add ammonia◦ Remove nitrogen◦ Remove ammonia◦ Increase volume◦ Decrease volume◦ Increase temperature◦ Decrease temperature

p. 98 of LA book

52 2 3 253 2 92.9 kJ; 6.0 10CN g H g NH g H K

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