TOPIC 7 TOPIC 7 EQUILIBRIUM EQUILIBRIUM

TOPIC 7 TOPIC 7

EQUILIBRIUM EQUILIBRIUM

7.1EQUILIBRIUM

By: Merinda Sautel Alameda Int’l Jr/Sr High

School Lakewood, CO [email protected]

ESSENTIAL IDEA

Many reactions are reversible. These reactions will reach a state of equilibrium when the rates of

the forward reaction and reverse reaction are equal. The position of equilibrium can be

controlled by changing the conditions.

NATURE OF SCIENCE (1.8)

Obtaining evidence for scientific theories – isotopic labeling and its use in defining equilibrium.

NATURE OF SCIENCE (5.5)

Common language across different disciplines – the term dynamic equilibrium is used in other contexts, but not

necessarily with the chemistry definition in mind.

INTERNATIONAL-MINDEDNESS

The Haber process has been described as the most important chemical reaction on Earth as it

has revolutionized global food production.

However, it also had a large impact on weaponry in both world wars.

THEORY OF KNOWLEDGE

Scientists investigate the world at different scales; the

macroscopic and microscopic. Which ways of knowing allow us to move from the macroscopic to the microscopic?

Chemistry uses a specialized vocabulary; a closed system is one in which no matter is exchanged with the surroundings.

Does our vocabulary simply communicate knowledge, or does it shape what we can know?

The career of Fritz Haber coincided with the political upheavals of two world wars. He supervised the release of chlorine on the

battlefield in World War I and worked on the production of

explosives. How does the social context of scientific work affect the methods and findings of science? Should scientists be held

morally responsible for the applications of their discoveries?

UNDERSTANDING/KEY IDEA 7.1.A

A state of equilibrium is reached in

a closed system when the rates of

the forward and reverse reactions

are equal.

• The system is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction and the

concentration of products and reactants remains unchanged.

• A reversible reaction is a chemical reaction in which the products can react to re-form the reactants.

Arrows going both directions (  ) indicates equilibrium in a chemical equation.

2HgO_(s)  2Hg_(l) + O_2(g)

APPLICATION/SKILLS

Be familiar with the characteristics

of chemical and physical systems

in a state of equilibrium.

EQUILIBRIUM IN

PHYSICAL SYSTEMS

• A physical system is one in which a chemical reaction is not occurring.

• Equilibrium will only occur in a “closed” system.

• A “closed” system is one in which no matter can be exchanged with the surroundings.

• A typical example is the evaporation-

condensation cycle of a volatile liquid in a closed container.

– When the rate of evaporation equals the rate of condensation, the system is at equilibrium.

EQUILIBRIUM IN

CHEMICAL SYSTEMS

• A chemical system is one in which a chemical reaction is occurring.

• For the system to reach equilibrium, the forward reaction must equal that of the reverse reaction.

• See the graphic on the next slide.

2NO 2NO

(g) (g)   2NO(g) + O 2NO(g) + O

(g) (g)

CHARACTERISTICS OF THE EQUILIBRIUM STATE

• 1. Equilibrium is dynamic – the reaction has not stopped but both reactions or processes are still occurring on the microscopic level even though it looks like the reaction or process has

stopped.

• 2. Equilibrium is achieved in a closed system so that no matter can escape to the

surroundings.

• 3. The concentrations of reactants and

products remain constant at equilibrium – they are being produced or consumed at an equal rate.

• 4. There is no macroscopic change in properties – meaning the observable properties do not change at equilibrium because they depend on the

concentration of the mixture components.

• 5. Equilibrium can be reached from either direction – meaning that at the same conditions equilibrium will be reached whether the reaction started with all

reactants, all products or a mixture of both.

• 6. It is important to note that even though the concentrations of reactants and products are

constant at equilibrium, this does not imply they are equal.

Equilibrium Position

• The equilibrium position is the proportion of the reactants and products in the equilibrium mixture.

• A reaction with an equilibrium mixture with mostly products is said to “lie to the right”.

• A reaction with an equilibrium mixture with mostly reactants is said to “lie to the left”.

• We can quantitatively compare equilibrium mixtures with different conditions by using the equilibrium

constant K_c.

• There can be numerous equilibrium positions, but only one equilibrium constant, K_c, at a specified temperature.

UNDERSTANDING/KEY IDEA 7.1.B

The equilibrium law describes how the equilibrium constant (K

) can be determined for a particular

chemical reaction.

Equilibrium Constant K

[ ] [ ] [ ] [ ]

l m

j k

K C D

 A B

For the reaction For the reaction:

Where

Where KK_cc is the equilibrium constant, is the equilibrium constant, and is unitless. The only thing that and is unitless. The only thing that

can change K

can change K_cc for a reaction is a for a reaction is a change in temperature.

change in temperature.

jA + kB  lC + mD

APPLICATION/SKILLS

Be able to deduce the equilibrium

constant (K

) from an equation for a

homogeneous reaction.

Writing an Equilibrium Expression

2NO 2NO

(g) (g)   2NO(g) + O 2NO(g) + O

(g) (g)

Remember that concentrations are written with Remember that concentrations are written with square brackets [ ]. K

square brackets [ ]. K_cc is calculated using molar is calculated using molar concentrations. Solids and liquids are

concentrations. Solids and liquids are notnot included in the K expression.

included in the K expression.

K

= ^???

2

2 2 2

[ ] [ ]

[ ]

K NO O

 NO

Write the equilibrium expression for the reaction:

UNDERSTANDING/KEY IDEA 7.1.C

The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is

temperature dependent.

Product Favored Equilibrium

Large values for K signify the reaction is

“product favored”

When equilibrium is achieved, most of the reactants have been converted to products.

Reactant Favored Equilibrium

Small values for K signify the reaction is

“reactant favored”

When equilibrium is achieved, very little reactant has been converted to product.

UNDERSTANDING/KEY IDEA 7.1.D

The reaction quotient (Q) measures the

relative amount of products and reactants present during a reaction at a particular

point in time. Q is the equilibrium expression with non-equilibrium concentrations. The

position of the equilibrium changes with changes in concentration, pressure and temperature.

The Reaction Quotient

For some time, t, when the system is not at equilibrium, the reaction quotient, Q takes the place of K, the equilibrium constant, in the

law of mass action.

[ ] [ ] [ ] [ ]

l m

j k

Q C D

 A B

jA + kB  lC + mD

Significance of the Reaction Quotient

 If Q = K, the system is at equilibrium

 If Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved

 If Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved

APPLICATION/SKILLS

Be able to determine the

relationship between different

equilibrium constants for the same

reaction at the same temperature.

• The equilibrium constant K_c has a fixed value at the same temperature.

• You can derive K_c from different sets of data for the same reaction at constant temperature.

• All K_c values should be equal no matter how you arrived at the value since only temperature affects K_c.

GUIDANCE

Be familiar with the relationship

between K

values for reactions

that are multiples or inverses of

each other.

Conclusions about Equilibrium Expressions

 If an equation is written in reverse, the K If an equation is written in reverse, the K expression is the reciprocal of the original K.

expression is the reciprocal of the original K.

2

2 2 2

[ ] [ ]

[ ]

K NO O

 NO

2NO2NO₂₂(g) (g)  2NO(g) + O 2NO(g) + O₂₂(g(g

) )

2NO(g) + O

2NO(g) + O₂₂(g) (g)  2NO 2NO₂₂(g)(g)

2 2 2

2

[ ]

' 1

[ ] [ ] K NO

K NO O

 

Conclusions about Equilibrium Expressions

 When the balanced equation for a reaction When the balanced equation for a reaction is multiplied by a factor

is multiplied by a factor nn, the equilibrium , the equilibrium expression for the new reaction is the

expression for the new reaction is the original expression, raised to the

original expression, raised to the nthnth power. _power.

2

2 2 2

[ ] [ ]

[ ]

K NO O

 NO

2NO2NO₂₂(g) (g)  2NO(g) + O 2NO(g) + O₂₂(g(g

) )

NONO₂₂(g) (g)  NO(g) + ½O NO(g) + ½O₂₂(g(g

) )

1 12

2 2

2

[ ][ ]

' [ ]

K K NO O

  NO

• If you add two reactions together, you will multiply the two K expressions to obtain the new K’.

UNDERSTANDING/KEY IDEA 7.1.E

A catalyst has no effect on the

position of equilibrium or the

equilibrium constant.

ADDITION OF A CATALYST

• When a catalyst is added, both the rate of the forward and the rate of the reverse

reactions are increased by the same amount so there is no effect on the equilibrium position.

• It only speeds up how fast K_c is achieved.

APPLICATION/SKILLS

Be able to apply Le Chatelier’s

principle to predict the qualitative

effects of changes of temperature,

pressure, and concentration on the

position of equilibrium and on the

value of the equilibrium constant.

LeChatelier’s Principle

When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress.

Translated:

Translated: The system undergoes a temporary The system undergoes a temporary shift in order to restore equilibrium.

shift in order to restore equilibrium.

1. When you add a substance or heat, the 1. When you add a substance or heat, the

system shifts to the opposite side.

system shifts to the opposite side.

2. When you take out a substance or heat, the 2. When you take out a substance or heat, the

system shifts to the side of the take out.

system shifts to the side of the take out.

3.3. When you increase pressure, the system When you increase pressure, the system

shifts to the side with the least number of shifts to the side with the least number of

gaseous molecules.

gaseous molecules.

LeChatelier Example #1

A closed container of ice and water is at

equilibrium. Then, the temperature is raised.

Ice + Energy  Water

The system temporarily shifts to the _______ to restore equilibrium.right

LeChatelier Example #2

A closed container of N₂O₄ and NO₂ is at equilibrium. NO₂ is added to the container.

N₂O₄ (g) + Energy  2 NO₂ (g) The system temporarily shifts to the _______ to restore equilibrium.left

LeChatelier Example #3

A closed container of water and its vapor is at equilibrium. Vapor is removed from the system.

water + Energy  vapor

The system temporarily shifts to the _______ to restore equilibrium.right

LeChatelier Example #4

A closed container of N₂O₄ and NO₂ is at equilibrium. The pressure is increased.

N₂O₄ (g) + Energy  2 NO₂ (g) The system temporarily shifts to the

_______ to restore equilibrium, because there are fewer moles of gas on that side of the equation.

left

Industrial Applications

• ***Remember the only thing that can change the value of K_c is a change in temperature.

• In reactions involving the manufacture of a chemical, the goal is to obtain the highest yield possible.

• We use our knowledge of Le Chatelier’s

principle and kinetics to maximize the yield.

HABER PROCESS: Production of NH₃

• N_2(g) + 3H_2(g)  2NH_3(g) ∆H = -93 kJ/mol

• First we need to get information from the balanced equation.

– All products and reactants are in the gaseous state with 4 moles on the left and 2 on the right.

– The reaction is exothermic so we want to re- write the equation as:

– N_2(g) + 3H_2(g)  2NH_3(g) + heat

N

+ 3H

 2NH

+ heat

• To maximize the amount of NH₃ obtained we can do several things:

– 1. Remove NH3 as it is produced so the reaction will continue to the right producing more NH₃ to achieve equilibrium.

– 2. Increasing the pressure pushes the equilibrium to the side of the least gaseous molecules which is to the right;

therefore, producing more NH3.

– 3. We want a lower temperature since the reaction is

exothermic, but not too low which would slow things down too far.

– 4. Adding a catalyst will speed up the reaction and compensate for the lower temperature.

CONTACT PROCESS: Production of sulfuric acid, H₂SO₄

• Sulfuric acid has the highest production of any chemical in the world.

• There are three steps in the production of H₂SO₄ with the second step being the rate determining step.

• 1. Combustion of sulfur to form sulfur dioxide.

• 2. Oxidation of sulfur dioxide to form sulfur trioxide.

• 3. Combination of sulfur trioxide produced in step 2 with water.

2SO_2(g) + O_2(g)  2SO_3(g) ∆H=-196 kJ/mol

• Again, the reaction is exothermic and has fewer

gaseous molecules on the right so we will basically follow the same industrial applications as with the Haber process.

• 1. We will not take out SO₃ because it is what is needed for step 3 of the process.

• 2. Increase pressure to shift equilibrium to the right.

• 3. Lower temperature because the reaction is exothermic.

• 4. Add a catalyst to increase the rate of the reaction.

Citations

International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015.

Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed.

N.p.: Pearson Baccalaureate, 2014. Print.

ISBN 978 1 447 95975 5 eBook 978 1 447 95976 2

Most of the information found in this power point comes directly from this textbook.

The power point has been made to directly complement the Higher Level Chemistry textbook by Brown and Ford and is used for direct instructional purposes only.