chemical equillibrium

Equillibrium !

●
What are the characteristics of chemical equilibrium?

●
How are equilibrium expressions written for systems that are at equilibrium?

●
How are equilibrium constants calculated from concentration data?

A State of Dynamic Balance

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Essential Questions

Vocabulary
New

•reversible reaction

•chemical equilibrium

•law of chemical equilibrium

•equilibrium constant

•homogeneous equilibrium

•heterogeneous equilibrium

Reactions

Irreversible

Reversible

The one goes completion.

Explosions, burning

processes, decay of

leaves, etc.

The one that can occur in both the forward and the reverse

directions. Formation of NH3 ,

HCl, H2SO4.

Chemical Equilibrium

1. Physical Equilibrium

• An equilibrium established by changing states of matter is

called physical equilibrium.

H2O(l) <----->H2O(g)

1. Physical Equilibrium

H2O(l)  H2O(g)

Vaporization and condensation of water in a closed container at constant temperature is a physical equilibrium.

When rate of vaporization and rate of condensation become equal an equilibrium state is established.

At equilibrium the amounts of water vapor and liquid water remain constant. But evaporation and condensation

continues on microscopic scale. This is called dynamic

equilibrium.

• Chemical equilibrium is a state of balance in which the

rate of a forward reaction equals the rate of the reverse reaction and the concentrations of products and reactants remain unchanged.

2

H ( )

I₂( )

2HI( )

⎯⎯→

+

⎯⎯

g

g

g

• Only a very small fraction of the collisions between H2

and I2 result in the formation of HI.

• As more HI molecules are made, they collide more often

and form H2 and I2 by the reverse reaction

2. Chemical Equilibrium

2. Chemical Equilibrium

• Rate Comparison for H2(g) + I2(g)  2HI(g)

• When the forward rate and the reverse rate are equal, the system is at chemical equilibrium.

2. Chemical Equilibrium

• Rate Comparison for H2(g) + I2(g)  2HI(g)

• When the reaction reaches

equilibrium concentration of substances become constant.

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2. Chemical Equilibrium

Fundamentals of Dynamic Equilibrium

• It does not show any visible change.

• It reaches through spontaneous change.

• Forward and reverse reactions take place continuously.

• It must be in a closed system.

• Temperature must be kept constant through equilibrium.

Example 1

a. H2(g) + Cl2(g)  2HCl(g)

(homogeneous)

b.

Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) (heterogeneous)

What is Chemical Equilibrium?

A State of Dynamic Balance

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Equilibrium happens when a chemical
reaction does not convert all reactants to
products: many reactions reach a state of
balance or dynamic equilibrium in which both
reactants and products are present. Another
way of defining equilibrium is to say that a
system is in equilibrium when the forward and
reverse reactions occur at equal rates.

Equilibrium does not necessarily mean that
reactants and products are present in equal
amounts. It means that the reaction has reached
a point where the concentrations of the reactant
and product are unchanging with time because
the forward and backward reactions have the
same rate

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What happens when 1 mol of nitrogen and 3 mol of
hydrogen react together under standard conditions?

A State of Dynamic Balance

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•The concentration of the product ammonia is zero at the start and gradually increases with time.

•Hydrogen and nitrogen
reactants are consumed in the
reaction, so their Concentrations
gradually decrease.

•After a period of time, the
concentrations of hydrogen,
nitrogen, and ammonia no
longer change. All
concentrations become
constant, as shown by the
horizontal lines on the right
side of the diagram.

A State of Dynamic Balance

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The concentrations of hydrogen
and nitrogen are not zero, so not
all of the reactants were converted to products even though the reaction is spontaneous.

What happens when 1 mol of nitrogen and 3 mol of
hydrogen react together under standard conditions?

•When a reaction results in an almost complete conversion of reactants to products, chemists say that the reaction goes to completion, but most reactions do not go to completion.

•The reactions appear to stop because they are reversible.

A reversible reaction is a chemical reaction that can occur in
both the forward and the reverse directions.

•

N2(g) + 3H2(g) 2NH3(g)

A State of Dynamic Balance

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•Chemists combine the forward and reverse equations into a single equation that uses the double arrow to show that both reactions occur.

•The reactants in the forward reaction are on the left of the
arrows and the reactants in the reverse reaction are on the right of the arrows.

•

In The forward reaction, hydrogen and nitrogen combine to form the product ammonia and in the reverse reaction,
ammonia decomposes into products hydrogen and nitrogen.

What happens when 1 mol of nitrogen and 3 mol of
hydrogen react together under standard conditions?

Forward: N2(g) + 3H2(g) → 2NH3(g)

Reverse: N2(g) + 3H2(g)  2NH3(g)

●Chemical reactions often reach a balancing point or equilibrium.

A State of Dynamic Balance

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What is equilibrium?

•A reversible reaction is a chemical reaction that can occur in both the forward and reverse directions, such as the formation of ammonia.

N2(g) + 3H2(g) 2NH3(g)

How does reversibility affect the production of
ammonia?
Decreases in the concentrations of N2 and H2 cause the reaction to
slow. As soon as ammonia is present, the reverse reaction can
occur, slowly at first, but at an increasing rate as the concentration
of ammonia increases.

A State of Dynamic Balance

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•This continues until the two rates, the
forward slowing and reverse increasing, are equal.

•At that point, the system has reached a state of equilibrium, figure “d”.

How does reversibility affect the

production of ammonia?

•Chemical equilibrium is a state in
which the forward and reverse
reactions balance each other because they take place at equal rates.

•Equilibrium is a state of action, not
inaction.

A State of Dynamic Balance

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•Some chemical systems have little tendency to react, while others go to completion.

•The majority reaches a state of equilibrium with some of the reactants unconsumed.

•The law of chemical equilibrium states that at a given
temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

aA + bB cC+ dD

Equilibrium Expressions

A State of Dynamic Balance

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The value of Keq is constant only at a specified temperature.

Keq > 1: Products are favored at equilibrium

Keq < 1: Reactants are favored at equilibrium

Equilibrium Expressions

H2(g) +I2(g) 2HI(g)

This reaction is a
homogeneous equilibrium,
which means that all the
reactants and products are in
the same physical state.

When the reactants and products
are present in more than one
physical state, the equilibrium is
called a heterogeneous
equilibrium.

Ex: Ethanol in a closed flask.

C2H5OH(l) C2H5OH(g).

Open Ended

Millions of tons of ammonia (NH3) are
produced each year for use in the
manufacture of products such as explosives,
fertilizers, and synthetic fibers. You might have
used ammonia in your home as a household
cleaner, which is particularly useful for
cleaning glass. Ammonia is manufactured
from its elements, hydrogen, and nitrogen,
using the Haber process. Write the equilibrium
constant expression for the following reaction.

N2(g) + 3H2(g) ⇋ 2NH3(g)

A State of Dynamic Balance

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EQUILIBRIUM CONSTANT EXPRESSIONS FOR

HOMOGENEOUS EQUILIBRIA

Problem
Millions of tons of ammonia (NH3) are
produced each year for use in the
manufacture of products such as explosives,
fertilizers, and synthetic fibers. You might have
used ammonia in your home as a household
cleaner, which is particularly useful for
cleaning glass. Ammonia is manufactured
from its elements, hydrogen, and nitrogen,
using the Haber process. Write the equilibrium
constant expression for the following reaction.

N2(g) + 3H2(g) ⇋ 2NH3(g)

Response
The equation for the reaction provides the
information needed to write the equilibrium
constant expression. The equilibrium is
homogeneous because the reactants and
products are in the same physical state.

The general form of the equilibrium constant
expression is

Keq = [C]c

[A]a[B]b

KNOWN

[A] = [N2], coefficient N2 = 1

[B] = [H2], coefficient H2 = 3

[C] = [NH3], coefficient NH3 = 2

UNKNOWN

Keq = ?

A State of Dynamic Balance

Copyright © McGraw-Hill Education

Form a ratio of product concentration to reactant concentrations.

•State the general form of the equilibrium constant expression.

Keq = [C]c

[A]a[B]b

•Substitute A = N2 , B = H2 , and C = NH3.

Keq = [NH3]c

[N2]a[H2]b

•Substitute a = 1, b = 3, and c = 2.

Keq = [NH3]2

[N2][H2]3

EQUILIBRIUM CONSTANT EXPRESSIONS FOR

HOMOGENEOUS EQUILIBRIA

Open Ended

Problem

In addition to its uses in baking and as an antacid and cleaning agent, baking soda is often placed in open boxes in refrigerators to freshen the air. Write the equilibrium constant expression for the decomposition of baking soda (sodium hydrogen carbonate).

2NaHCO3(s) ⇋ Na2CO3(s) + CO2(g) + H2O(g)

A State of Dynamic Balance

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EQUILIBRIUM CONSTANT EXPRESSIONS

FOR HETEROGENEOUS EQUILIBRIA

Problem
In addition to its uses in baking and as an
antacid and cleaning agent, baking soda is
often placed in open boxes in refrigerators to
freshen the air. Write the equilibrium constant
expression for the decomposition of baking
soda (sodium hydrogen carbonate).

2NaHCO3(s) ⇋ Na2CO3(s) + CO2(g) + H2O(g)

Response
You are given a heterogeneous equilibrium
involving gases and solids. Solids are omitted
from the equilibrium constant expression.

KNOWN

[C] = [Na2CO3], coefficient Na2CO3 = 1

[D] = [CO2], coefficient CO2 = 1

[E] = [H2O], coefficient H2O = 1

[A] = [NaHCO3], coefficient NaHCO3 = 2

UNKNOWN

equilibrium constant expression = ?

SOLVE FOR THE UNKNOWN

Form a ratio of product concentrations to
reactant concentrations.

•State the general form of the equilibrium
constant expression.

Keq = [C]c[D]d[E]e

[A]a[B]b

A State of Dynamic Balance

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SOLVE FOR THE UNKNOWN (continued)

•Substitute A = NaHCO3, C = Na2CO3, D = CO2, and E = H2O.

Keq = [Na2CO3]c[CO2]d[H2O]e

[NaHCO3]a

•Substitute a = 2, c = 1, d = 1, and e = 1.

Keq =[Na2CO3]1[CO2]1[H2O]1

[NaHCO3]2

•Omit terms involving solid substances.

Keq = [CO2][H2O]

EQUILIBRIUM CONSTANT

EXPRESSIONS FOR

HETEROGENEOUS EQUILIBRIA

A State of Dynamic Balance

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•For a given reaction at a given temperature, Keq will always be the same regardless of the initial concentrations of reactants and
products.

Equilibrium Constants

Open Ended

Calculate the value of Keq for the equilibrium constant expression         Keq =  "[NH3]2" /"[N2][H2]3"  given concentration    data at one equilibrium position: [NH3] = 0.933 mol/L, [N2] = 0.533 mol/L, [H2] = 1.600 mol/L.

A State of Dynamic Balance

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THE VALUE OF EQUILIBRIUM CONSTANTS

Problem

Calculate the value of Keq for the
equilibrium constant expression

Keq = [NH3]2

[N2][H2]3 given concentration

data at one equilibrium position: [NH3]
= 0.933 mol/L, [N2] = 0.533 mol/L,
[H2] = 1.600 mol/L.

Response
You have been given the equilibrium
constant expression and the concentration
of each reactant and product. You must
calculate the equilibrium constant.

KNOWN

Keq = [NH3]2

[N2][H2]3
[N2] = 0.533 mol/L

[NH3] = 0.933 mol/L

[H2] = 1.600 mol/L

UNKNOWN

Keq = ?

SOLVE FOR THE UNKNOWN

•Substitute [NH3] = 0.933 mol/L, [N2] = 0.533
mol/L, and [H2] = 1.600 mol/L.

Keq = [0.933]2

[0.533][1.600]3 = 0.399

Multiple Choice

This symbol indicates that a reaction is ____________

• reversible
• irreversible

Multiple Choice

What are the two factors to look for when determining if the reaction is at equilibrium?

• Forward reaction rate is faster than the reverse and concentrations are equal
• Forward and reverse reaction rates are equal and concentration is constant
• Forward and revers reaction rates are equal and concentration is equal
• Forward reaction rate is faster than the reverse and concentration is equal

Multiple Choice

2A + 3B  <---->  2AB
The forward reaction forms the substance __________.

• A
• B
• AB
• A + B

Multiple Choice

At what time the reaction reached equilibrium?

• t₁
• t₂
• t₃
• t₄

Multiple Choice

If the equilibrium constant is less than 1, it means that

• Products are favored

• Reactants are favored

Multiple Choice

When all reactants and products have the same physical state, then the reaction is called

• Heterogeneous equilibrium

• Homogeneous equilibrium

Multiple Choice

What is the K_eq expression for this reaction?

2 NO_(g) + O_2(g) ⇌ 2 NO_2(g)

• K_eq = [NO₂]² / [NO]² [O₂]

• K_eq = [NO]² [O₂] / [NO₂]²

• K_eq = [NO]² [O₂] [NO₂]²

• K_eq = [NO₂]² / [NO]² + [O₂]

Multiple Choice

What are [A] and [B]?

• concentration of reactants
• concentration of products
• energy of reactants
• energy of products

Multiple Choice

What is the equilibrium expression for:
Fe₃O₄(s) + 4H₂(g) <=> 3Fe(s) + 4H₂O(g)
K_c =

• [Fe]³ [H₂O]⁴  / [Fe₃O₄] [H₂]⁴
• [Fe₃O₄] [H₂]⁴ /  [Fe]³ [H₂O]⁴
• [H₂O]⁴ / [H₂]⁴
• [Fe] [H₂O] / [Fe₃O₄] [H₂]

Multiple Choice

Consider the following reaction :    
SO_{2 (g)} +  NO_{2 (g)} ↔ SO_{3 (g)} + NO (g)  
had reached a state of equilibrium, was found to contain   0.40 M SO₃ , 0.30 M NO, 0.15 M NO₂ , and 0.20 M SO₂. Calculate the equilibrium constant  for this reaction.

• 4
• .42
• .25
• 1

Multiple Select

Which of the following statements is CORRECT for a reaction at an equilibrium?

• The rate of forward and reverse reactions are equal.

• The final concentrations of product and reactant are equal.

• The initial concentrations of product and reactant are equal.

• The rate constant of the forward and reverse reactions are equal.

Multiple Choice

For the reaction, choose the CORRECT equilibrium constant, K_c.

Multiple Choice

At elevated temperatures ammonium carbonate, NH₂COONH₄, is in equilibrium with NH₃ and CO₂ according to the equation;

NH₂COONH₄(s) ↔ 2 NH₃(g) + CO₂(g)

What is the equilibrium expression for this reaction?

• K = 2 [NH₃][CO₂]

  [NH₂COONH₄]

• K = [NH₃]²[CO₂]

  [NH₂COONH₄]

• K = 2 [NH₃][CO₂]

• K = [NH₃]²[CO₂]