cmc chapter 17

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Chemical Equilibrium

Section 17.1 A State of Dynamic Balance

Section 17.2 Factors Affecting Chemical Equilibrium

Section 17.3 Using Equilibrium Constants

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Section 17-1

Section 17.1 A State of Dynamic Balance

• List the characteristics of chemical equilibrium.

free energy: the energy that is available to do work—the difference between the change in enthalpy and the product of the entropy change and the absolute temperature

• Write equilibrium expressions for systems that are at equilibrium.

• Calculate equilibrium constants from concentration data.

Section 17-1

Section 17.1 A State of Dynamic Balance (cont.)

reversible reaction

chemical equilibrium

law of chemical equilibrium

equilibrium constant

homogeneous equilibrium

heterogeneous equilibrium

Chemical equilibrium is described by an equilibrium constant expression that relates the concentrations of reactants and products.

Section 17-1

What is equilibrium?

• Chemical reactions often reach a balancing point, or equilibrium.

Section 17-1

What is equilibrium? (cont.)

• 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)

Section 17-1

What is equilibrium? (cont.)

• How does reversibility affect the production of ammonia?

• Decreases in the concentrations of N2 and H2 cause the reaction to slow.

Section 17-1

What is equilibrium? (cont.)

• The reaction reaches equilibrium in figure d.

Section 17-1

What is equilibrium? (cont.)

• 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.

Section 17-1

Equilibrium Expressions

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

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

Section 17-1

Equilibrium Expressions (cont.)

• 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.

• The Equilibrium Constant Expression

Section 17-1

Equilibrium Expressions (cont.)

• The equilibrium constant, Keq, is the numerical value of the ratio of product concentrations to reactant concentrations, with each concentration raised to the power equal to its coefficient in the balanced equation.

Keq > 1: Products are favored at equilibrium

Keq < 1: Reactants are favored at equilibrium

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

Section 17-1

Equilibrium Expressions (cont.)

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.

Section 17-1

Equilibrium Expressions (cont.)

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

• Ethanol in a closed flask is represented by C2H5OH(l) ↔ C2H5OH(g).

Section 17-1

Equilibrium Constants

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

A. A

B. B

C. C

D. D

Section 17-1

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Section 17.1 Assessment

A reaction is in equilibrium when:

A. there are more products than reactants

B. the amount of products equals the reactants

C. the rate of the forward reaction is greater than the reverse reaction

D. the rate of the forward and reverse reactions are equal

A. A

B. B

C. C

D. D

Section 17-1

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Section 17.1 Assessment

The value of the equilibrium constant is constant for a given ____.

A. temperature

B. pressure

C. volume

D. density

End of Section 17-1

Section 17-2

Section 17.2 Factors Affecting Chemical Equilibrium

• Describe how various factors affect chemical equilibrium.

reaction rate: the change in concentration of a reactant or product per unit time, generally calculated and expressed in moles per liter per second.

Le Châtelier’s principle

• Explain how Le Châtelier’s principle applies to equilibrium systems.

When changes are made to a system at equilibrium, the system shifts to a new equilibrium position.

Section 17-2

Le Châtelier’s Principle

• Le Châtelier’s Principle was proposed in 1888 and states that if stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

• Stress is any kind of change in a system that upsets the equilibrium.

Section 17-2

Le Châtelier’s Principle (cont.)

• Adjusting the concentrations of either the reactants or the products puts stress on a system in equilibrium.

• Adding reactants increases the number of effective collisions between molecules and upsets the equilibrium.

Section 17-2

Le Châtelier’s Principle (cont.)

• The equilibrium shifts to the right to produce more products.

• Stress is relieved by shifting to the left, converting products to reactants.

Section 17-2

Le Châtelier’s Principle (cont.)

• Increasing pressure shifts the system to the left, and more products are formed.

• Changing the volume (and pressure) of an equilibrium system shifts the equilibrium only if the number of moles of gaseous reactants is different from the moles of gaseous products.

• If the number of moles is the same on both sides of the balanced equation, changes in pressure and volume have no effect on the equilibrium.

Section 17-2

Le Châtelier’s Principle (cont.)

Lowering the piston decreases the volume

and increases the pressure.

The reaction between CO and H2

is at equilibrium.

As a result, more molecules of the products form. Their

formation relieves the stress on the system.

Section 17-2

Le Châtelier’s Principle (cont.)

• Changes in temperature alter the equilibrium position and the equilibrium constant.

• If heat is added to an equilibrium system, the equilibrium shifts in the direction in which the heat is used up.

Section 17-2

Le Châtelier’s Principle (cont.)

• Any change in temperature results in a change in Keq.

Section 17-2

Le Châtelier’s Principle (cont.)

• A catalyzed reaction reaches equilibrium more quickly, but with no change in the amount of product formed.

A. A

B. B

C. C

D. D

Section 17-2

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Section 17.2 Assessment

Which does NOT result in a shift of the equilibrium to the right?

A. removing products

B. adding reactants

C. increasing concentration of reactants

D. adding products

A. A

B. B

C. C

D. D

Section 17-2

Section 17.2 Assessment

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Any change in ____ results in a change in Keq.

A. temperature

B. pressure

C. volume

D. concentration

End of Section 17-2

Section 17-3

Section 17.3 Using Equilibrium Constants

• Determine equilibrium concentrations of reactants and products.

solubility: the maximum amount of solute that will dissolve in a given amount of solvent at a specific temperature and pressure

• Calculate the solubility of a compound from its solubility product constant.

• Explain the common ion effect.

Section 17-3

Section 17.3 Using Equilibrium Constants (cont.)

solubility product constant

common ion

common ion effect

Equilibrium constant expressions can be used to calculate concentrations and solubilities.

Section 17-3

Calculating Equilibrium Concentrations

• Equilibrium constants can be used to calculate unknown concentrations of products when other concentrations are known.

Section 17-3

The Solubility Product Constant

• Some ionic compounds dissolve readily in water, and some barely dissolve at all.

• The equilibrium constant expression for the dissolving of a sparingly soluble compound is called the solubility product constant, Ksp.

Section 17-3

The Solubility Product Constant (cont.)

• The solubility product constant expression is the product of the concentrations of the dissolved ions, each raised to the power equal to the coefficient of the ion in the chemical equation.

Section 17-3

The Solubility Product Constant (cont.)

Section 17-3

The Solubility Product Constant (cont.)

• The values in the table can be used to determine the solubility of a sparingly soluble compound.

• Ksp can be used to predict whether a precipitate will form when any two ionic solutions are mixed.

Section 17-3

The Solubility Product Constant (cont.)

• Use the initial concentrations of ions in solution in the solubility product constant expression to calculate Qsp.

• If Qsp < Ksp the solution is unsaturated and no precipitate will form.

• If Qsp = Ksp the solution is saturated and no change will occur.

Section 17-3

The Solubility Product Constant (cont.)

• If Qsp > Ksp a precipitate will form, reducing the concentrations of the ions in the solution until the product of their concentrations in the Ksp expression equals the numerical value of Ksp.

Section 17-3

The Solubility Product Constant (cont.)

Section 17-3

The Common Ion Effect

• Why is PbCrO4 less soluble in aqueous solution of K2CrO4 than in pure water?

• The K2CrO4 solution contains CrO4

2– ions before any PbCrO4 dissolves.

Section 17-3

The Common Ion Effect (cont.)

• A common ion is an ion that is common to two or more ionic compounds.

• The lowering of the solubility of a substance because of the presence of a common ion is called the common ion effect.

A. A

B. B

C. C

D. D

Section 17-3

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Section 17.3 Assessment

The presence of a common ion ____ the solubility of the dissolved substance.

A. decreases

B. increases

C. does not change

D. speeds up

A. A

B. B

C. C

D. D

Section 17-3

Section 17.3 Assessment

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If Qsp > Ksp

A. the solution is unsaturated and no precipitate will form

B. the solution is saturated and no precipitate will form

C. a precipitate will form, reducing the concentrations of the ions in the solution

D. a common ion must be present

End of Section 17-3

Resources Menu

Chemistry Online

Study Guide

Chapter Assessment

Standardized Test Practice

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Concepts in Motion

Study Guide 1

Section 17.1 A State of Dynamic Balance

Key Concepts

• A reaction is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.

• The equilibrium constant expression is a ratio of the molar concentrations of the products to the molar concentrations of the reactants with each concentration raised to a power equal to its coefficient in the balanced chemical equation.

• The value of the equilibrium constant expression, Keq, is constant for a given temperature.

Study Guide 2

Section 17.2 Factors Affecting Chemical Equilibrium

Key Concepts

• Le Châtelier’s principle describes how an equilibrium system shifts in response to a stress or a disturbance.

• When an equilibrium shifts in response to a change in concentration or volume, the equilibrium position changes but Keq remains constant. A change in temperature, however, alters both the equilibrium position and the value of Keq.

Study Guide 3

Section 17.3 Using Equilibrium Constants

Key Concepts

• Equilibrium concentrations and solubilities can be calculated using equilibrium constant expressions.

• Ksp describes the equilibrium between a sparingly soluble ionic compound and its ions in solution.

• If the ion product, Qsp, exceeds the Ksp when two solutions are mixed, a precipitate will form.

• The presence of a common ion in a solution lowers the solubility of a dissolved substance.

A. A

B. B

C. C

D. D

Chapter Assessment 1

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A change in ____ alters both the equilibrium position and the equilibrium constant.

A. pressure

B. temperature

C. volume

D. density

A. A

B. B

C. C

D. D

Chapter Assessment 2

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A(n) ____ equilibrium is when all reactant and products are in the same physical state.

A. endothermic

B. exothermic

C. heterogeneous

D. homogeneous

A. A

B. B

C. C

D. D

Chapter Assessment 3

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Which is NOT an example of a homogeneous equilibria?

A. H2(g) +I2(g) ↔ 2HI(g)

B. 2H2(g) +O2(g) ↔ 2H2OI(g)

C. HCl(aq) + NaOH(aq) ↔NaCl(aq) + H2O

D. C2H5OH(l) ↔ C2H5OH(g)

A. A

B. B

C. C

D. D

Chapter Assessment 4

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Barium sulfate is toxic to humans, so why can it be ingested for use with gastrointestinal x-rays?

A. It is minimally poisonous.

B. It has high solubility.

C. It has low solubility.

D. Your body readily absorbs it.

A. A

B. B

C. C

D. D

Chapter Assessment 5

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The state in which the forward and reverse chemical reactions take place at the same rate is called ____.

A. Le Châtelier’s Principle

B. Henry’s Law

C. Charles’s Law

D. chemical equilibrium

A. A

B. B

C. C

D. D

STP 1

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Which does NOT describe a system that has reached chemical equilibrium?

A. No new product is formed by the forward reactions.

B. The forward and reverse reactions occur at equal rates.

C. The concentration of products is equal to the concentration of reactants.

D. All the reactants have been used up.

A. A

B. B

C. C

D. D

STP 2

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A(n) ____ equilibrium is when all reactant and products are present in more than one physical state.

A. endothermic

B. exothermic

C. heterogeneous

D. homogeneous

A. A

B. B

C. C

D. D

STP 3

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Adding product to a chemical equilibrium:

A. does nothing

B. creates a stress and shifts the equilibrium to the right

C. creates a stress and shifts the equilibrium to the left

D. causes more product to form

A. A

B. B

C. C

D. D

STP 4

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Ethylene (C2H4) reacts with hydrogen to form ethane (C2H6). C2H4(g) + H2(g) ↔ C2H6(g) + heat. How could you increase the amount of hydrogen in the system?

A. Increase the heat.

B. Decrease the heat.

C. Increase the C2H4.

D. Decrease the C2H6.

A. A

B. B

C. C

D. D

STP 5

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Which does NOT result in a shift of the equilibrium to the right?

A. removing products

B. adding reactants

C. increasing concentration of reactants

D. decreasing the concentration of reactants

IB Menu

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IB 1

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IB 6

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IB 11

CIM

Figure 17.14 Equilibrium Shifts

Figure 17.19 Precipitation Reaction

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