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Equilibrium
Wade Baxter, Ph.D.
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Printed: April 27, 2014
AUTHOR
Wade Baxter, Ph.D.
EDITORS
Donald Calbreath, Ph.D.
Max Helix
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Chapter 1. Equilibrium
C HAPTER
1
Equilibrium
C HAPTER O UTLINE
1.1
The Nature of Equilibrium
1.2
Le Châtelier’s Principle
1.3
Solubility Equilibrium
1.4
References
Cave explorers are very familiar with stalactites and stalagmites. These picturesque formations occur as water rich in
minerals seeps through the ground and drips from the ceiling of the cave. One of the prominent minerals in the water
is calcium carbonate, an ionic compound that is nearly insoluble in water. As the water evaporates, precipitates of
calcium carbonate, colored by the presence of other ions, form and harden. Stalactites are the icicle-like structures
that hang from the ceiling, while stalagmites grow upward from the ground. When they meet, spectacular columns
form, as seen above. The solubility of ionic compounds involves an equilibrium that occurs between the solid
compound and the ions in a solution. However, equilibrium is a general concept that applies to more than just
precipitation reactions. In this chapter, you will learn about different types of equilibrium and how changes to the
reaction conditions can manipulate that equilibrium, thus increasing or decreasing the amount of products formed in
a reaction.
Oregon Caves. www. f lickr.com/photos/oregoncavesnps/9939448605/. CC BY 2.0.
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1.1. The Nature of Equilibrium
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1.1 The Nature of Equilibrium
Lesson Objectives
•
•
•
•
Describe the nature of a reversible reaction.
Define chemical equilibrium.
Write chemical equilibrium expressions from balanced chemical equations.
Calculate the equilibrium constant for a reaction, and perform calculations involving that constant.
Lesson Vocabulary
• chemical equilibrium
• equilibrium constant
• reversible reaction
Check Your Understanding
Recalling Prior Knowledge
• What is the rate of a chemical reaction?
• How is concentration usually measured?
When a reaction takes place in both the forward and reverse directions, it is said to be reversible. Reversible reactions
can reach a stable state that is referred to as chemical equilibrium. In this lesson, you will learn about the nature of
reversible reactions, chemical equilibrium, and how to calculate and use equilibrium constants.
Reversible Reactions
Up until this point, we have written the equations for chemical reactions in a way that would seem to indicate that
all reactions proceed until all of the reactants have been converted into products. In reality, a great many chemical
reactions do not proceed entirely to completion. A reversible reaction is a reaction in which the conversion of
reactants to products and the conversion of products to reactants occur simultaneously. One example of a reversible
reaction is the reaction of hydrogen gas and iodine vapor to form hydrogen iodide. The forward and reverse reactions
can be written as follows.
Forward reaction: H2 (g) + I2 (g) → 2HI(g)
Reverse reaction: 2HI(g) → H2 (g) + I2 (g)
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Chapter 1. Equilibrium
In the forward reaction, hydrogen and iodine combine to form hydrogen iodide. In the reverse reaction, hydrogen
iodide decomposes back into hydrogen and iodine. The two reactions can be combined into one equation by the use
of a double arrow.
H2 (g) + I2 (g) *
) 2HI(g)
The double arrow indicates that the reaction is reversible.
When hydrogen and iodine gases are mixed in a sealed container, they begin to react and form hydrogen iodide. At
first, only the forward reaction occurs because no HI is present. As the forward reaction proceeds, it begins to slow
down as the concentrations of H2 and I2 decrease. As soon as some HI has formed, it begins to decompose back
into H2 and I2 . The rate of the reverse reaction starts out slow because the concentration of HI is low. Gradually, the
rate of the forward reaction decreases, while the rate of the reverse reaction increases. Eventually, the rate at which
H2 and I2 combine to produce HI becomes equal to the rate at which HI decomposes back into H2 and I2 . When
the rates of the forward and reverse reactions have become equal to one another, the reaction has achieved a state of
balance. Chemical equilibrium is the state of a system in which the rate of the forward reaction is equal to the rate
of the reverse reaction. The Figure 1.1 shows this reaction graphically.
FIGURE 1.1
As a reaction begins, only the forward
reaction occurs. Over time, the forward
reaction rate decreases, while the rate of
the reverse reaction increases. After a
certain amount of time has passed, the
rates of both reactions are equal, and the
reaction has reached equilibrium.
Chemical equilibrium can be attained whether the reaction begins with all reactants and no products, all products
and no reactants, or some of both. Illustrated in the Figure 1.2 are the changes in the concentrations of H2 , I2 ,
and HI for two different initial reaction mixtures. In the situation depicted by the graph on the left, the reaction
begins with only H2 and I2 present. There is no HI initially. As the reaction proceeds toward equilibrium, the
concentrations of H2 and I2 gradually decrease, while the concentration of HI gradually increases. When the curve
levels out and the concentrations all become constant, equilibrium has been reached. After a system has reached
equilibrium, the concentrations of all substances remain constant. In the reaction depicted by the graph on the right,
the reaction begins with only HI and no H2 or I2 . In this case, the concentration of HI gradually decreases, while
the concentrations of H2 and I2 gradually increase until equilibrium is again reached. Notice that in both cases, the
relative position of equilibrium is the same, as shown by the relative concentrations of reactants and products. The
concentration of HI at equilibrium is significantly higher than the concentrations of H2 and I2 . This is true whether
the reaction began with all reactants or all products. The equilibrium position is a property of the particular reversible
reaction and does not depend upon the initial concentrations of the reactants and products.
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1.1. The Nature of Equilibrium
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FIGURE 1.2
(A) The reaction begins with only reactants (H2 and I2 ) and reaches equilibrium when the concentrations of
reactants and products become constant. (B) The same reaction, beginning with only product (HI). The relative
concentrations of the reactants and products are the same in both cases.
Conditions for Equilibrium and Types of Equilibrium
It may be tempting to think that once equilibrium has been reached, the reaction stops. However, chemical equilibrium is a dynamic process. The forward and reverse reactions continue to occur even after equilibrium has been
reached, but, because the rates of the two reactions are equal, there is no net change in the relative amounts of
reactants and products for a reaction that is at equilibrium. The conditions and properties of a system at equilibrium
are summarized below.
1. The system must be closed, meaning no substances can enter or leave the system.
2. Equilibrium is a dynamic process. Even though we don’t observe any changes, both the forward and reverse
reactions are still taking place.
3. The rates of the forward and reverse reactions must be equal.
4. The amounts of reactants and products do not have to be equal. However, after equilibrium is attained, the
amounts of reactants and products will remain constant.
The description of equilibrium in this chapter refers primarily to equilibrium between reactants and products in a
chemical reaction. In previous chapters, we introduced the concepts of phase equilibrium and solution equilibrium.
A phase equilibrium occurs when a substance is in equilibrium between two states. For example, a stoppered flask
of water attains equilibrium when the rate of evaporation is equal to the rate of condensation. A solution equilibrium
occurs when a solid substance is in a saturated solution. At this point, the rate of dissolution is equal to the rate of
recrystallization. Although these are all different types of transformations, most of the rules regarding equilibrium
apply to any situation in which a process occurs reversibly.
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Chapter 1. Equilibrium
The Equilibrium Constant
Consider the hypothetical reversible reaction in which reactants A and B react to form products C and D. This
equilibrium can be shown below, where the lower case letters represent the coefficients of each substance.
aA + bB *
) cC + dD
As we have established, the rates of the forward and reverse reactions are the same at equilibrium, and so the
concentrations of all of the substances are constant. The equilibrium constant (Keq ) is the ratio of the mathematical
product of the concentrations of the products to the mathematical product of the concentrations of the reactants for
a reaction that is at equilibrium. Each concentration is raised to the power of its coefficient in the balanced chemical
equation. For the general reaction above, the equilibrium constant expression is written as follows:
Keq =
[C]c [D]d
[A]a [B]b
The concentrations of each substance, indicated by the square brackets around the formula, are measured in molarity
units (mol/L).
The value of the equilibrium constant for any reaction can be determined by experiment. As detailed in the
above section, the equilibrium position for a given reaction does not depend on the starting concentrations, so
the equilibrium constant has the same value regardless of the initial amounts of each reaction component. It does,
however, depend on the temperature of the reaction. Equilibrium is defined as a condition in which the rates of the
forward and reverse reactions are equal. A change in temperature will change the rates of both the forward and
reverse reactions, but not to the same extent. As a result, the equilibrium constant is altered. For any reaction in
which a Keq is given, the temperature should be specified.
The general value of the equilibrium constant gives us information about whether the reactants or the products are
favored at equilibrium. Since the product concentrations are in the numerator of the equilibrium expression, a Keq
>1 means that the products are favored over the reactants. A Keq <1 means that the reactants are favored over the
products.
Though it would often seem that the Keq value would have various units depending on the values of the exponents
in the expression, the general rule is that any units are dropped. All Keq values will be reported as having no units.
Sample Problem 19.1: Calculating an Equilibrium Constant
Equilibrium occurs when nitrogen monoxide gas reacts with oxygen gas to form nitrogen dioxide gas.
2NO(g)+O2 (g)*
) 2NO2 (g)
At 230°C, the equilibrium concentrations for a certain experiment are measured to be [NO] = 0.0542 M, [O2 ] =
0.127 M, and [NO2 ] = 15.5 M. Calculate the equilibrium constant at this temperature.
Step 1: List the known values and plan the problem.
Known
• [NO] = 0.0542 M
• [O2 ] = 0.127 M
• [NO2 ] = 15.5 M
Unknown
• Keq value
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1.1. The Nature of Equilibrium
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The equilibrium expression is first written according to the general form in the text. The equilibrium values are
substituted into the expression, and the value is calculated.
Step 2: Solve.
Keq =
[NO2 ]2
[NO]2 [O2 ]
Substituting in the concentrations at equilibrium:
Keq =
(15.5)2
= 6.44 × 105
(0.0542)2 (0.127)
Step 3: Think about your result.
The equilibrium concentration of the product NO2 is significantly higher than the concentrations of the reactants NO
and O2 . As a result, the Keq value is much larger than 1, an indication that the product is favored at equilibrium.
Practice Problem
1. The Haber process for the production of ammonia results in the equilibrium represented by the reaction:
N2 (g) + 3H2 (g) *
) 2NH3 (g). At equilibrium at a certain temperature, a 5.0 L flask contains 1.25 mol N2 , 0.75
mol H2 , and 0.50 mol NH3 . Calculate Keq for the reaction at this temperature.
The equilibrium expression only shows those substances whose concentrations are variable during the reaction. A
pure solid or a pure liquid does not have a concentration that will vary during a reaction. Therefore, an equilibrium
expression omits pure solids and liquids and only shows the concentrations of gases and aqueous solutions. The
decomposition of mercury(II) oxide can be shown by the equation below, followed by its equilibrium expression.
2HgO(s) *
) 2Hg(l) + O2 (g) Keq = [O2 ]
The stoichiometry of an equation can also be used in a calculation of an equilibrium constant. At 40°C, solid
ammonium carbamate decomposes to ammonia and carbon dioxide gases.
NH4 CO2 NH2 (s) *
) 2NH3 (g) + CO2 (g)
At equilibrium, [CO2 ] is found to be 4.71 × 10−3 M. Can the Keq value be calculated from just that information?
Because the ammonium carbamate is a solid, it is not present in the equilibrium expression.
Keq = [NH3 ]2 [CO2 ]
The stoichiometry of the chemical equation indicates that, as the ammonium carbamate decomposes, 2 mol of
ammonia gas is produced for every 1 mol of carbon dioxide. If we can assume that all of the ammonia and carbon
dioxide present is from the decomposition of ammonium carbamate, the concentration of ammonia at any point
will be twice the concentration of carbon dioxide. At equilibrium, [NH3 ] = 2 × (4.71 × 10−3 ) = 9.42 × 10−3 M.
Substituting these values into the Keq expression:
Keq = (9.42 × 10−3 )2 (4.71 × 10−3 ) = 4.18 × 10−7
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Chapter 1. Equilibrium
Using Equilibrium Constants
The equilibrium constants are known for a great many reactions. Hydrogen and bromine gases combine to form
hydrogen bromide gas. The equation and Keq for this reaction when it is run at 730°C are given below.
H2 (g) + Br2 (g) *
) 2HBr(g) Keq = 2.18 × 106
A certain reaction is begun with only HBr. When the reaction mixture reaches equilibrium at 730°C, the concentration of bromine gas is measured to be 0.00243 M. What is the concentration of the H2 and the HBr at equilibrium?
Since the reaction begins with only HBr, and the mole ratio of H2 to Br2 is 1:1, the concentration of H2 at equilibrium
is also 0.00243 M. The equilibrium expression can be rearranged to solve for the concentration of HBr at equilibrium.
[HBr]2
[H2 ][Br2 ]
q
[HBr] = Keq [H2 ][Br2 ]
q
= 2.18 × 106 (0.00243)(0.00243) = 3.59 M
Keq =
Since the value of the equilibrium constant is very high, the concentration of HBr is much greater than that of H2
and Br2 at equilibrium.
Lesson Summary
• A reversible reaction is one in which products are converted to reactants as well as reactants being converted
to products. Equilibrium is achieved when the rate of the forward reaction is equal to the rate of the reverse
reaction. Once at equilibrium, the concentrations of all substances remain constant, and no net change occurs
in the system.
• An equilibrium constant can be calculated for any reaction. Once a reaction reaches equilibrium, the ratio
of the mathematical product of all product concentrations to the mathematical product of all reactant concentrations, each raised to the power of its coefficient, will always be equal to its equilibrium constant at
that temperature. Note that solid and liquid reaction components are not included in the calculation of the
equilibrium constant, since their concentrations are not dependent on the amount present and thus do not
change over the course of the reaction.
• An equilibrium constant greater than 1 indicates that the products of the reaction as written are favored. An
equilibrium constant less than 1 indicates that the reactants are favored.
Lesson Review Questions
Reviewing Concepts
1.
2.
3.
4.
How do the amounts of reactants and products change after a reaction has reached equilibrium?
How does the position of equilibrium depend upon the starting concentrations of all the reactants and products?
What types of substances are not included in equilibrium expressions and why?
In general, which reaction is favored (forward, reverse, or neither) if the value of Keq at a specified temperature
is
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1.1. The Nature of Equilibrium
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a. equal to 1?
b. very small?
c. very large?
Problems
5. Write Keq expressions for each of the following equilibrium reactions.
a. 3O2 (g) *
) 2O3 (g)
b. H3 PO4 (aq) *
) 3H+ (aq) + PO3−
4 (aq)
c. 2NO2 (g) + 7H2 (g) *
2NH
(g)
+ 4H2 O(l)
)
3
*
d. 2NaHCO3 (s) ) Na2 CO3 (s) + CO2 (g) + H2 O(g)
* 2Cl2 (g) +
6. Oxygen reacts with hydrogen chloride to form chlorine gas and water vapor: 4HCl(g) + O2 (g) )
2H2 O(g). At a certain temperature, the equilibrium mixture consists of 0.0012 M HCl, 3.8 × 10−4 M O2 ,
0.058 M Cl2 , and 0.058 M H2 O. Calculate the value of the equilibrium constant at this temperature.
7. At equilibrium at 2500 K, [HCl] = 0.0625 M and [H2 ] = [Cl2 ] = 0.00450 M for the reaction: H2 (g) + Cl2 (g) *
)
2HCl(g).
a. Calculate the equilibrium constant for the reaction as written above.
b. Calculate the equilibrium constant for the reaction written instead as:
* H2 (g) + Cl2 (g). What is the relationship of the Keq values in parts a and b?
2HCl(g) )
8. Consider the following reaction: H2 S(aq) *
) H+ (aq) + HS− (aq), Keq =9.5 × 10−8 at 25°C. In a certain
+
−
equilibrium mixture at 25°C, [H ] = [HS ] = 2.7 × 10−4 M. Determine the concentration of H2 S in this
mixture.
9. Phosphorus pentachloride gas decomposes to phosphorus trichloride and chlorine: PCl5 (g)*
) PCl3 (g)+Cl2 (g).
In a certain reaction, 0.500 mol of PCl5 is introduced into a 5.00 L container at 250°C. When the reaction
reaches equilibrium, the mixture is analyzed and found to contain 0.194 mol Cl2 . Determine the value of Keq
at 250°C for this reaction.
10. Nitrogen monoxide combines with chlorine at 400°C to form nitrosyl chloride by the following reaction:
2NO(g)+Cl2 (g)*
) 2NOCl(g), Keq =28.1 at 400°C. In a certain reaction, a quantity of NOCl is allowed to
decompose at 400°C until equilibrium is reached. The [NO] at equilibrium is 9.40 × 10−3 M. Find the
concentrations of Cl2 and of NOCl.
Further Reading / Supplemental Links
• The Equilibrium Constant and the Mass Action Expression, http://www.kentchemistry.com/links/Kinetics/
EquilibriumConstant.htm
• Equilibrium Constants: Kc, http://www.chemguide.co.uk/physical/equilibria/kc.html
Points to Consider
Disruptions to a system at equilibrium can occur in the form of a change in temperature, pressure, or concentrations
of one or more of the substances in the equilibrium.
• How does an equilibrium respond to a change in conditions?
• Is the value of the equilibrium constant affected by such changes?
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Chapter 1. Equilibrium
1.2 Le Châtelier’s Principle
Lesson Objectives
• Explain the factors that stress a system at equilibrium, and use Le Châtelier’s principle to predict how the
system will respond to each stress.
• Explain how a reaction is able to go to completion.
• Analyze how a change in the concentration of a reaction component can change the equilibrium position but
not the value of the equilibrium constant.
Lesson Vocabulary
• Le Châtelier’s principle
Check Your Understanding
Recalling Prior Knowledge
• How is equilibrium defined?
• What is an equilibrium constant, and how is it calculated?
A chemical system that is at equilibrium can be disrupted by changes in concentrations, temperature, or pressure. In
this lesson, you will learn how to analyze stresses to a system at equilibrium and predict how the system will respond
to that stress.
Factors Affecting an Equilibrium System
Equilibrium represents a balance between the reactants and the products of a chemical reaction. Changes to the
conditions of the system can disturb that equilibrium. When this occurs, the system reacts in such a way as to restore
the equilibrium. However, the position of equilibrium will be changed following the disturbance. In other words,
the response of the system involves a change in the amounts of the reactants and products. Some will increase and
some will decrease until equilibrium is reestablished.
Chemical equilibrium was studied by French chemist Henri Le Châtelier (1850-1936), and his description of how a
system at equilibrium responds to a change in conditions has become known as Le Châtelier’s principle: When a
chemical system that is at equilibrium is disturbed by a stress, the system will respond by attempting to counteract
that stress until a new equilibrium is established ( Table 1.1). Stresses to a chemical system include changes in
the concentrations of reactants or products, changes in the temperature of the system, or changes in the pressure of
the system. We will discuss each of these stresses separately. In each case, the change to the equilibrium position
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1.2. Le Châtelier’s Principle
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will cause either the forward or reverse reaction to be favored over the opposite process. When the forward reaction
is favored, the concentrations of the products increase, and the concentrations of the reactants decrease. When
the reverse reaction is favored, the concentrations of the products decrease, and the concentrations of the reactants
increase.
TABLE 1.1: Reaction to Stresses
Original Equilibrium
A*
)B
A*
)B
Favored Reaction
forward: A → B
reverse: A ← B
Result
[A] decreases; [B] increases
[A] increases; [B] decreases
Concentration
A change in the concentration of one of the substances in an equilibrium system typically involves either the addition
or the removal of one of the reactants or products. Consider the Haber-Bosch process for the industrial production
of ammonia from nitrogen and hydrogen gases:
N2 (g)+3H2 (g)*
) 2NH3 (g)
If the concentration of one substance in a system is increased, the system will respond by favoring the reaction that
removes that substance. When more N2 is added, the forward reaction will be favored because the forward reaction
uses up N2 and converts it to NH3 . Initially, the forward reaction speeds up because one of the reactants is present at
a higher concentration, but the rate of the reverse reaction is unaffected. Since the two rates are no longer equal, the
system is no longer at equilibrium, and there will be a net shift to the right (producing more NH3 ) until the two rates
are once again balanced. The concentration of NH3 increases, while the concentrations of N2 and H2 decrease. After
some time passes, equilibrium is reestablished with new concentrations of all three substances. As illustrated in
the Figure 1.3, the new concentration of NH3 is higher than it was originally, because the forward reaction became
temporarily favored due to the stress. The new concentration of H2 is lower. The final concentration of N2 is higher
than it was in the original equilibrium, but lower than it was immediately after the addition of N2 that disturbed the
original equilibrium. By responding in this way, the value of the equilibrium constant for the reaction, Keq , does not
change as a result of the stress to the system.
FIGURE 1.3
The Haber-Bosch process is an equilibrium between the reactants (N2 and H2 )
and the product (NH3 ). When more N2 is
added, the system favors the forward reaction until equilibrium is reestablished.
Conversely, if more NH3 were added, the reverse reaction would be favored. This “favoring” of a reaction means
temporarily speeding up the reaction in that direction until equilibrium is reestablished. Recall that once equilibrium
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Chapter 1. Equilibrium
is reestablished, the rates of the forward and reverse reactions are again equal. The addition of NH3 would result in
a net increase in the formation of the reactants, N2 and H2 .
An equilibrium can also be disrupted by the removal of one of the substances. If the concentration of a substance
is decreased, the system will respond by favoring the reaction that replaces that substance. In the industrial HaberBosch process, NH3 is removed from the equilibrium system as the reaction proceeds. As a result, the forward
reaction is favored so that more NH3 will be produced. The concentrations of N2 and H2 decrease. Continued
removal of NH3 will eventually force the reaction to go to completion until all of the reactants are used up. If either
N2 or H2 were removed from the equilibrium system, the reverse reaction would be favored, and the concentration
of NH3 would decrease.
The effects of changes in concentration on a system at equilibrium are summarized below ( Table 1.2).
TABLE 1.2: Stresses and Responses
Stress
addition of reactant
addition of product
removal of reactant
removal of product
Response
forward reaction favored
reverse reaction favored
reverse reaction favored
forward reaction favored
Temperature
Increasing or decreasing the temperature of a system at equilibrium is also a stress to the system. The equation for
the Haber-Bosch process is written again below, this time as a thermochemical equation:
N2 (g) + 3H2 (g) *
) 2NH3 (g) + 91 kJ
The forward reaction is the exothermic direction: the formation of NH3 releases heat. The reverse reaction is the
endothermic direction: as NH3 decomposes to N2 and H2 , heat is absorbed. An increase in the temperature of a
system favors the direction of the reaction that absorbs heat, the endothermic direction. Absorption of heat in this
case is a relief of the stress provided by the temperature increase. For the Haber-Bosch process, an increase in
temperature favors the reverse reaction. The concentration of NH3 in the system decreases, while the concentrations
of N2 and H2 increase.
Conversely, a decrease in the temperature of a system favors the direction of the reaction that releases heat: the
exothermic direction. For the Haber-Bosch process, a decrease in temperature favors the forward reaction. The
concentration of NH3 in the system increases, while the concentrations of N2 and H2 decrease.
For changes in concentration, the system responds in such a way that the value of the equilibrium constant, Keq , is
unchanged. However, a change in temperature shifts the equilibrium and changes the value of Keq . As discussed in
the previous section, values of Keq are dependent on the temperature. When the temperature of the system for the
Haber-Bosch process is increased, the resultant shift in equilibrium toward the reactants means that the Keq value
decreases. When the temperature is decreased, the shift in equilibrium toward the products means that the Keq value
increases.
Le Châtelier’s principle, as related to temperature changes, can be illustrated easily by the equilibrium between
dinitrogen tetroxide and nitrogen dioxide.
N2 O4 (g) + heat *
) 2NO2 (g)
Dinitrogen tetroxide (N2 O4 ) is colorless, while nitrogen dioxide (NO2 ) is dark brown in color. When N2 O4 breaks
down into NO2 , heat is absorbed according to the forward reaction above. Therefore, an increase in the temperature
of the system will favor the forward reaction, while a decrease in temperature will favor the reverse reaction. By
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1.2. Le Châtelier’s Principle
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changing the temperature, the equilibrium between colorless N2 O4 and brown NO2 can be manipulated, resulting in
a visible color change.
The video below shows three sealed glass tubes containing N2 O4 and NO2 . When one tube is placed in hot water,
the equilibrium favors the brown NO2 . When another tube is placed in ice cold water, the equilibrium favors the
colorless N2 O4 .
http://www.youtube.com/watch?v=tlGrBcgANSY (0:55)
MEDIA
Click image to the left for more content.
Pressure
Changing the pressure of an equilibrium system in which gases are involved is also a stress to the system. A change
in the pressure on a liquid or a solid has a negligible effect. We will return again to the equilibrium for the HaberBosch process. Imagine the gases are contained in a closed system in which the volume of the system is controlled
by an adjustable piston as shown in the Figure 1.4.
FIGURE 1.4
(A) A mixture of nitrogen, hydrogen, and
ammonia in equilibrium. (B) When the
pressure is increased on the equilibrium
mixture, the forward reaction is favored
because that results in a reduction of the
total moles of gas present.
(C) Fewer
moles of gas will exert a lower total pressure, so the stress is partially relieved by
such a shift.
On the far left, the reaction system contains primarily N2 and H2 , with only one molecule of NH3 present. As
the piston is pushed inward, the pressure of the system increases according to Boyle’s Law. This is a stress to the
equilibrium. In the middle image, the same number of molecules are now confined to a smaller space, so the pressure
has increased. According to Le Châtelier’s principle, the system responds in order to relieve the stress. In the image
on the right, the forward reaction has been favored, in which three molecules of N2 combine with nine molecules
of H2 to form six molecules of NH3 . The overall result is a decrease in the number of gas molecules in the entire
system. This decreases the pressure and counteracts the original stress of a pressure increase. When the pressure is
increased by decreasing the available volume, the reaction that produces fewer total moles of gas becomes favored.
In this case, it is the forward reaction that is favored.
A decrease in pressure on the above system could be achieved by pulling the piston outward, increasing the container
volume. The equilibrium would respond by favoring the reverse reaction, in which NH3 decomposes to N2 and H2 .
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Chapter 1. Equilibrium
This is because the overall number of gas molecules would increase and so would the pressure. When the pressure
of a system at equilibrium is decreased by providing more total volume, the reaction that produces more total moles
of gas becomes favored. This is summarized in the Table 1.3.
TABLE 1.3: Stresses and Responses to Pressure Changes
Stress
pressure increase
pressure decrease
Response
reaction produces fewer gas molecules
reaction produces more gas molecules
Like changes in concentration, the Keq value for a given reaction is unchanged by a change in pressure.
It is important to remember when analyzing the effect of a pressure change on equilibrium that only gases are
affected. If a certain reaction involves liquids or solids, they should be ignored. Calcium carbonate decomposes
according to the equilibrium reaction:
CaCO3 (s)*
) CaO(s)+O2 (g)
Oxygen is the only gas in the system. An increase in the pressure of the system has no effect on the rate of
decomposition of CaCO3 , but it speeds the reverse reaction by forcing the oxygen molecules closer together, causing
a net shift to the left. When a system contains equal moles of gas on both sides of the equation, pressure has no
effect on the equilibrium position, as in the formation of HCl from H2 and Cl2 .
H2 (g)+Cl2 (g)*
) 2HCl(g)
Use of a Catalyst
Since a catalyst speeds up the rate of a reaction, you might think that it would have an effect on the equilibrium
position. However, catalysts have equal effects on the forward and reverse rates, so for a system at equilibrium,
these two rates remain equal. A system will reach equilibrium more quickly in the presence of a catalyst, but the
equilibrium position itself is unaffected.
Going to Completion
When one of the products of a reaction is removed from the chemical equilibrium system as soon as it is produced,
the reverse reaction cannot establish itself, and equilibrium is never reached. Reactions such as these are said to
go to completion. Reactions that go to completion tend to produce one of three types of products: (1) an insoluble
precipitate, (2) a gas, or (3) a molecular compound such as water. Examples of these reactions are shown below.
1. Formation of a precipitate: AgNO3 (aq)+NaCl(aq)→ NaNO3 (aq)+AgCl(s)
2. Formation of a gas: Mg(s)+2HCl(aq)→ MgCl2 (aq)+H2 (g)
3. Formation of water: HCl(aq)+NaOH(aq)→ NaCl(aq)+H2 O(l)
Response of K
Occasionally, when students apply Le Chatelier’s principle to an equilibrium problem involving a change in concentration, they assume that Keq must change. This seems logical, since we talk about “shifting” the equilibrium in
one direction or the other. However, Keq is a constant for a given equilibrium at a given temperature, so it must not
change. Here is an example of how this works. Consider the simplified equilibrium below:
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1.2. Le Châtelier’s Principle
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A*
)B
Let’s say we have a 1.0 liter container. At equilibrium, the following amounts are measured:
A = 0.50 mol
B = 1.0 mol
The value of Keq is given by:
Keq =
[B]
1.0 M
=
= 2.0
[A] 0.50 M
Now we will disturb the equilibrium by adding 0.50 mole of A to the mixture. The equilibrium will shift toward
the right, forming more B. Immediately after the addition of A, and before any response, we now have 1.0 mol of A
and 1.0 mol of B. The equilibrium then shifts in the forward direction. We will introduce a variable (x), which will
represent the change in concentrations as the reaction proceeds. Since the mole ratio of A:B is 1:1, if [A] decreases
by the x moles, [B] increases by the same amount. We can now set up an analysis called ICE, which stands for
Initial, Change, and Equilibrium. The values listed in the Table 1.4 represent molar concentrations.
TABLE 1.4: Sample ICE table
A
1.0
-x
1.0-x
Initial
Change
Equilibrium
B
1.0
+x
1.0+x
At the new equilibrium position, the values for A and B as a function of x can be set equal to the value of the Keq .
Then, one can solve for x.
Keq = 2.0 =
[B] 1.0 + x
=
[A] 1.0 − x
Solving for x:
2.0(1.0 - x) = 1.0 + x
2.0 - 2.0x = 1.0 + x
3.0x = 1.0
x = 0.33
This value for x is now plugged back in to the Equilibrium line of the table, and the final concentrations of A and B
after the reaction are calculated.
A = 1.0 –x = 0.67 M
B = 1.0 + x = 1.33 M
The value of Keq has been maintained since 1.33/0.67 = 2.0. This shows that even though a change in concentration
of one of the substances in an equilibrium causes a shift in the equilibrium position, the value of the equilibrium
constant does not change.
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Chapter 1. Equilibrium
Lesson Summary
• A system at equilibrium can be disrupted by a change in concentration of one of the substances or by a change
in temperature or pressure. Le Châtelier’s principle states that such a system will respond by attempting to
counteract the stress. Either the forward or reverse reaction will temporarily be favored until equilibrium is
reestablished.
• A catalyst increases the rate of both the forward and reverse reactions equally and does not change the
equilibrium position.
• Reactions in which a product is continually removed from the system do not attain equilibrium and, instead,
proceed to completion.
• A change in concentration or pressure does not change the value of the equilibrium constant for a reaction.
Only a temperature change alters the equilibrium constant.
Lesson Review Questions
Reviewing Concepts
1.
2.
3.
4.
5.
What are three stresses that can upset the equilibrium of a chemical system?
Which stress or stresses change the value of the equilibrium constant?
What conditions can drive a reaction to completion?
What must be true of the reaction in order for pressure to have an effect on the equilibrium position?
Does the use of a catalyst influence the position of an equilibrium? Explain.
Problems
6. Given the following equilibrium equation: N2 (g)+2O2 (g)+66.2 kJ *
) 2NO2 (g). Predict the direction of
equilibrium that will be favored (forward, reverse, or neither) for each of the following changes.
a.
b.
c.
d.
e.
f.
g.
h.
N2 is added.
O2 is removed.
The temperature is increased.
The pressure is increased.
A catalyst is used.
NO2 is removed.
The temperature is decreased.
The system volume is increased.
7. For the system in question 6, how would the concentration of NO2 at equilibrium be affected by each change?
8. For the system in question 6, how would the value of Keq be affected by each change?
9. Given the following reaction for the formation of sulfur trioxide from sulfur dioxide and oxygen: 2SO2 (g)+O2 (g)*
) 2SO3 (
What conditions of temperature and pressure would maximize the concentration of SO3 at equilibrium?
Further Reading / Supplemental Links
• Le Châtelier Principle, http://www.kentchemistry.com/links/LeChatelier.htm
• Le Châtelier’s Principle, http://www.chemguide.co.uk/physical/equilibria/lechatelier.html
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Points to Consider
In a saturated solution with excess solute present, an equilibrium exists between dissolved and undissolved solute.
• How is the equilibrium constant for a solution equilibrium determined?
• What is the relationship between the equilibrium constant and the solubility of various ionic compounds?
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Chapter 1. Equilibrium
1.3 Solubility Equilibrium
Lesson Objectives
• Write solubility product constant expressions for nearly insoluble ionic compounds.
• Calculate the value of Ksp for a compound from its solubility and determine the solubility of a compound with
a known Ksp .
• Use the Ksp of a compound to predict whether a precipitate will form when two solutions are mixed together.
• Describe the common ion effect and its relationship to solubility.
Lesson Vocabulary
•
•
•
•
common ion
common ion effect
molar solubility
solubility product constant
Check Your Understanding
Recalling Prior Knowledge
• What is a saturated solution?
• What is the equilibrium that occurs in a saturated solution?
A saturated aqueous solution is one in which the maximum amount of a solute has been dissolved in a given amount
of water. A saturated solution may also have undissolved solute present, in which case an equilibrium exists between
the dissolved and undissolved solute. In this lesson, you will learn about that equilibrium and how to calculate and
use the solubility product constant.
The Solubility Product Constant
Ionic compounds have widely differing solubilities. Sodium chloride has a solubility of about 360 g per liter of
water at 25°C. Salts of alkali metals tend to be quite soluble. On the other end of the spectrum, the solubility of zinc
hydroxide is only 4.2 × 10−4 g/L of water at the same temperature. Many ionic compounds containing hydroxide
are relatively insoluble. The chapter Solutions, summarized a set of rules for predicting the relative solubilities of
various ionic compounds in water.
Most ionic compounds that are considered to be insoluble will still dissolve, to a small extent, in water. These
“mostly insoluble” compounds are still considered to be strong electrolytes because, essentially, any portion of the
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1.3. Solubility Equilibrium
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compound that dissolves will also dissociate into ions. As an example, silver chloride dissociate, to a small extent,
into silver ions and chloride ions upon being added to water.
AgCl(s)*
) Ag+ (aq)+Cl− (aq)
The process is written as an equilibrium because the dissociation occurs only to a small extent. Therefore, an
equilibrium expression can be written for the process. Keep in mind that the solid silver chloride does not have a
variable concentration, so it is not included in the equilibrium expression.
Ksp = [Ag+ ][Cl− ]
This equilibrium constant is called the solubility product constant, (Ksp ) and is equal to the mathematical product
of the ions, each raised to the power of the coefficient of the ion in the dissociation equation.
The formula of the ionic compound dictates the form of the Ksp expression. For example, the formula of calcium
phosphate is Ca3 (PO4 )2 . The dissociation equation and Ksp expression are shown below:
Ca3 (PO4 )2 (s)*
) 3Ca2+ (aq)+2PO3−
4 (aq)
2+
3
2
Ksp = [Ca ] [PO3−
]
4
Listed below ( Table 1.5) are the solubility product constants for some common nearly insoluble ionic compounds.
TABLE 1.5: Solubility Product Constants (25°C)
Compound
AgBr
AgCl
Al(OH)3
BaCO3
BaSO4
CaCO3
Ca(OH)2
Ca3 (PO4 )2
CaSO4
Ksp
5.0 × 10−13
1.8 × 10−10
3.0 × 10−34
5.0 × 10−9
1.1 × 10−10
4.5 × 10−9
6.5 × 10−6
1.2 × 10−26
2.4 × 10−5
Compound
CuS
Fe(OH)2
Mg(OH)2
PbCl2
PbCO3
PbI2
PbSO4
Zn(OH)2
ZnS
Ksp
8.0 × 10−37
7.9 × 10−16
7.1 × 10−12
1.7 × 10−5
7.4 × 10−14
7.1 × 10−9
6.3 × 10−7
3.0 × 10−16
3.0 × 10−23
Solubility and K
Solubility is normally expressed in grams of solute per liter of saturated solution. However, solubility can also be
expressed as moles per liter. Molar solubility is the number of moles of solute in one liter of a saturated solution.
In other words, the molar solubility of a given compound represents the highest molarity solution that is possible
for that compound. The molar mass of a compound is the conversion factor between solubility and molar solubility.
Given that the solubility of Zn(OH)2 is 4.2 × 10−4 g/L, the molar solubility can be calculated as shown below:
4.2 × 10−4 g
1 mol
×
= 4.2 × 10−6 mol/L (M)
1L
99.41 g
Solubility data can be used to calculate the Ksp for a given compound. The following steps need to be taken.
1. Convert from solubility to molar solubility.
2. Use the dissociation equation to determine the concentration of each of the ions in mol/L.
3. Apply the Ksp equation.
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Chapter 1. Equilibrium
Sample Problem 19.2: Calculating Ksp from Solubility
The solubility of lead(II) fluoride is found experimentally to be 0.533 g/L. Calculate the Ksp for lead(II) fluoride.
Step 1: List the known quantities and plan the problem.
Known
• solubility of PbF2 = 0.533 g/L
• molar mass of PbF2 = 245.20 g/mol
Unknown
• Ksp of PbF2 = ?
The dissociation equation for PbF2 and the corresponding Ksp expression can be constructed as follows:
PbF2 (s)*
) Pb2+ (aq)+2F− (aq) Ksp = [Pb2+ ][F− ]2
The steps above will be followed to calculate Ksp for PbF2 .
Step 2: Solve.
molar solubility:
1 mol
0.533 g
×
= 2.17 × 10−3 M
1L
245.20 g
The dissociation equation shows that, for every mole of PbF2 that dissociates, 1 mol of Pb2+ and 2 mol of F− are
produced. Therefore, at equilibrium, the concentrations of the ions are:
Pb2+ = 2.17 × 10−3 M and [F− ] = 2 × 2.17 × 10−3 = 4.35 × 10−3 M
Substitute into the equilibrium expression, and solve for Ksp .
Ksp = (2.17 × 10−3 )(4.35 × 10−3 )2 = 4.11 × 10−8
Step 3: Think about your result.
The solubility product constant is significantly less than 1 for a nearly insoluble compound such as PbF2 .
Practice Problem
1. From the given solubility data, calculate Ksp for each of the following compounds.
a. copper(II) iodide, CuI = 4.30 × 10−4 g/L
b. silver sulfide, Ag2 S = 2.84 × 10−15 g/L
The known Ksp values from the table above ( Table 1.5) can be used to calculate the solubility of a given compound
by following the steps listed below.
1. Set up an ICE problem (Initial, Change, Equilibrium) in order to use the Ksp value to calculate the concentration of each of the ions. Assume that no ions are initially present in the solution.
2. The concentrations of the ions can be used to calculate the molar solubility of the compound.
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1.3. Solubility Equilibrium
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3. Use the molar mass to convert from molar solubility to solubility in g/L.
The Ksp value of calcium carbonate is 4.5 × 10−9 . We begin by setting up an ICE table showing the dissociation of
CaCO3 into calcium ions and carbonate ions. The variable s will be used to represent the molar solubility of CaCO3 .
In this case, each formula unit of CaCO3 yields one Ca2+ ion and one CO3 2− ion. Therefore, the equilibrium
concentrations of each ion are equal to s.
TABLE 1.6: ICE table
CO2−
3 (aq)
0.00
+s
s
Ca2+ (aq)+
0.00
+s
s
CaCO3 (s)*
)
Initial (M)
Change (M)
Equilibrium (M)
The Ksp expression can be written in terms of s and then used to solve for s.
2−
2
Ksp = [Ca2+ ][CO
p 3 ] = (s)(s) = s
p
s = Ksp = 4.5 × 10−9 = 6.7 × 10−5 M
The concentration of each of the ions at equilibrium is 6.7 × 10−5 M. We can now use the molar mass to convert
from molar solubility to solubility in g/L.
100.09 g
mol
6.7 × 10−5 −3
×
= 6.7 × 10 g/L
mol
1L
1
So the maximum amount of calcium carbonate that is capable of dissolving in 1 liter of water at 25°C is 6.7 × 10−3
grams. Note that in the case above, the 1:1 ratio of the ions upon dissociation led to the Ksp being equal to s2 . This
is referred to as a formula of the type AB, where A is the cation and B is the anion. Now let’s consider a formula of
the type AB2 , such as Fe(OH)2 . In this case, the setup of the ICE table would look like the following:
TABLE 1.7: ICE table
Fe(OH)2 (s)*
)
Initial (M)
Change (M)
Equilibrium (M)
2OH− (aq)
0.00
+2s
2s
Fe2+ (aq)+
0.00
+s
s
− 2
2
3
Ksp = [Fe2+ ][OH
r ] = (s)(2s) = 4s
r
−16
3 7.9 × 10
3 Ksp
s=
=
= 5.8 × 10−6 M
4
4
The table below ( Table 1.8) shows the relationship between Ksp and molar solubility based on the formula.
TABLE 1.8: Ksps and Molar Solubility
Compound
Type
AB
AB2 or A2 B
AB3 or A3 B
A2 B3 or A3 B2
20
Example
Ksp Expression
Cation
Anion
CuS
Ag2 CrO4
Al(OH)3
Ba3 (PO4 )2
[Cu2+ ][S2− ]
[Ag+ ]2 [CrO4 2− ]
[Al3+ ][OH− ]3
[Ba2+ ]3 [PO4 3− ]2
s
2s
s
3s
s
s
3s
2s
Ksp in Terms of
s
s2
4s3
27s4
108s5
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Chapter 1. Equilibrium
The Ksp expressions in terms of s can be used to solve problems in which the Ksp is used to calculate the molar
solubility as in the examples above. Molar solubility can then be converted to solubility in g/L.
Predicting Precipitates
Knowledge of Ksp values will allow you to be able to predict whether or not a precipitate will form when two
solutions are mixed together. For example, suppose that a known solution of barium chloride is mixed with a known
solution of sodium sulfate. Barium sulfate ( Figure 1.5) is a mostly insoluble compound that could potentially
precipitate from the mixture. However, it is first necessary to calculate the ion product, [Ba2+ ][SO4 2− ], for the
solution. If the value of the ion product is less than the value of the Ksp , then the solution will remain unsaturated.
No precipitate will form because the concentrations are not high enough to begin the precipitation process. If the
value of the ion product is greater than the value of Ksp , then a precipitate will form. The formation of the precipitate
lowers the concentration of each of the ions until the ion product is exactly equal to Ksp , at which point equilibrium
is attained.
FIGURE 1.5
Barium sulfate is used as a component of
white pigments in paints and as an agent
in certain x-ray imaging processes.
Sample Problem 19.3: Predicting Precipitates
Will a precipitate of barium sulfate form when 10.0 mL of 0.0050 M BaCl2 is mixed with 20.0 mL of 0.0020 M
Na2 SO4 ?
Step 1: List the known quantities and plan the problem.
Known
•
•
•
•
•
concentration of BaCl2 solution = 0.0050 M
volume of BaCl2 solution = 10.0 mL
concentration of Na2 SO4 solution = 0.0020 M
volume of Na2 SO4 solution = 20.0 mL
Ksp of BaSO4 = 1.1 × 10−10 ( Table 1.5)
Unknown
• value of [Ba2+ ][SO4 2− ]
• if a precipitate forms
The concentration and volume of each solution that is mixed together must be used to calculate the values of [Ba2+ ]
and [SO4 2− ]. Each individual solution is diluted when they are mixed together. The ion product is calculated and
compared to the Ksp to determine whether a precipitate forms.
Step 2: Solve.
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The moles of each ion from the original solutions are calculated by multiplying the molarity by the volume in liters.
mol Ba2+ = 0.0050 M × 0.010 L = 5.0 × 10−5 mol Ba2+
mol SO4 2− = 0.0020 M × 0.020 L = 4.0 × 10−5 mol SO4 2−
The concentration of each ion after dilution is then calculated by dividing the moles by the final solution volume of
0.030 L.
5.0 × 10−5 mol
= 1.7 × 10−3 M
0.030 L
4.0 × 10−5 mol
[SO2−
]
=
= 1.3 × 10−3 M
4
0.030 L
[Ba2+ ] =
Now, the ion product is calculated.
Ba2+ [SO4 2− ] = (1.7 × 10−3 )(1.3 × 10−3 ) = 2.2 × 10−6
Since the ion product is greater than the Ksp , a precipitate of barium sulfate will form.
Step 3: Think about your result.
Two significant figures are appropriate for the calculated value of the ion product.
Practice Problem
2. Calculate the ion product for calcium hydroxide when 20.0 mL of 0.010 M CaCl2 is mixed with 30.0 mL of
0.0040 M KOH. Decide if a precipitate will form.
The Common Ion Effect
In a saturated solution of calcium sulfate, an equilibrium exists between the solid calcium sulfate and its ions in
solution.
−5
CaSO4 (s)*
) Ca2+ (aq)+SO2−
4 (aq) Ksp = 2.4 × 10
Suppose that some calcium nitrate were added to this saturated solution. Immediately, the concentration of the
calcium ion in the solution would increase. As a result, the ion product [Ca2+ ][SO4 2− ] would increase to a value
that is greater than the Ksp . According to Le Châtelier’s principle, the equilibrium above would shift to the left in
order to relieve the stress of the added calcium ion. Additional calcium sulfate would precipitate out of the solution
until the ion product, once again, becomes equal to Ksp . Note that in the new equilibrium, the concentrations of the
calcium ion and the sulfate ion would no longer be equal to each other. The calcium ion concentration would be
larger than the sulfate ion concentration.
This situation describes the common ion effect. A common ion is an ion that is common to more than one salt in
a solution. In the above example, the common ion is Ca2+ . The common ion effect is a decrease in the solubility
of an ionic compound as a result of the addition of a common ion. Adding calcium ions to a saturated solution of
calcium sulfate causes additional CaSO4 to precipitate from the solution, lowering its solubility. The addition of a
solution containing sulfate ion, such as potassium sulfate, would result in the same common ion effect.
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Chapter 1. Equilibrium
Sample Problem 19.4: The Common Ion Effect
What is the concentration of zinc ion in 1.00 L of a saturated solution of zinc hydroxide to which 0.040 mol of NaOH
has been added?
Step 1: List the known quantities and plan the problem.
Known
• Ksp = 3.0 × 10−16 ( Table 1.5)
• moles of added NaOH = 0.040 mol
• volume of solution = 1.00 L
Unknown
• [Zn2+ ] = ? M
Express the concentrations of the two ions relative to the variable s. The concentration of the zinc ion will be equal
to s, while the concentration of the hydroxide ion will be equal to 0.040 + 2s. Note that the value of s in this case is
not equal to the value of s when zinc hydroxide is dissolved in pure water.
Step 2: Solve.
The Ksp expression can be written in terms of the variable s.
Ksp = [Zn2+ ][OH− ]2 = (s)(0.040+2s)2
Because the value of the Ksp is so small, we can make the assumption that the value of s will be very small compared
to 0.040. This simplifies the mathematics involved in solving for s.
Ksp = (s)(0.040)2 = 0.0016s = 3.0 × 10−16
Ksp
3.0 × 10−16
s=
=
= 1.9 × 10−13 M
[OH− ]2
0.0016
The concentration of the zinc ion is equal to s, so [Zn2+ ] = 1.9 × 10−13 M.
Step 3: Think about your result.
The relatively high concentration of the common ion, OH− , results in a very low concentration of the zinc ion. The
molar solubility of the zinc hydroxide is less in the presence of the common ion than it would be in pure water.
Practice Problem
3. Determine the concentration of silver ions in 1.00 L of a saturated solution of silver chloride to which 0.0020
mol of sodium chloride has been added.
Lesson Summary
• In a saturated solution, an equilibrium exists between the dissolved and undissolved solute, at which point the
rate of dissolution is equal to the rate of recrystallization. The equilibrium constant expression for this type of
equilibrium is called a solubility product constant (Ksp ).
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1.3. Solubility Equilibrium
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• The Ksp of a compound can be calculated from its solubility (g/L) or molar solubility (mol/L). Known Ksp
values can be used to calculate the solubility of a compound.
• When two solutions are mixed, a precipitate may be produced. The starting ion concentrations are used to
calculate the ion product, which is then compared to the Ksp . If the ion product is greater than the value of
Ksp , a precipitate will form.
• The common ion effect describes a reduction in the solubility of a salt that results from the addition of an ion
that is common to both the original solution and the salt being added.
Lesson Review Questions
Reviewing Concepts
1. Explain what the following statement means: All ionic compounds are strong electrolytes, no matter how
soluble.
2. What is the relationship between a compound’s solubility and its solubility product constant?
3. Write the solubility product constant (Ksp ) expression for the following compounds.
a. NiS
b. Ag2 CO3
c. Fe3 (PO4 )2
4. Use the table above ( Table 1.5) to rank the following salts from most to least soluble.
a.
b.
c.
d.
AgBr
BaSO4
ZnS
PbCO3
5. How does the addition of lead(II) ions affect the solubility of lead(II) chloride in water?
Problems
6. The molar solubility of copper(I) bromide, CuBr, is 2.0 × 10−4 M. Calculate the solubility of CuBr in g/L.
7. Calculate Ksp for the following compounds from the given solubilities at 25°C.
a. SrCO3 , 5.9 × 10−3 g/L
b. Ag2 SO4 , 4.74 g/L
c. Cr(OH)3 , 3.4 × 10−6 g/L
8. What is the concentration of lead(II) ions and iodide ions in a saturated solution of lead(II) iodide at 25°C?
Refer to the table above ( Table 1.5) for the Ksp .
9. Use the Ksp values listed above ( Table 1.5) to calculate the solubility in g/L of the following compounds.
a. Ca3 (PO4 )2
b. PbSO4
c. Ca(OH)2
10. Calculate the ion product of silver bromide when 100.0 mL of 0.0020 M AgNO3 is mixed with 100.0 mL of
1.0 × 10−4 M KBr. Will a precipitate of AgBr form when the solutions are mixed?
11. Determine the concentration of lead(II) ions in 1.00 L of a saturated solution of PbF2 to which 0.025 mol of
fluoride ions has been added. The Ksp of PbF2 is 4.1 × 10−8 .
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Chapter 1. Equilibrium
Further Reading / Supplemental Links
• Solubility Product (Ksp), http://www.kentchemistry.com/links/Kinetics/Ksp.htm
• Understanding Chemistry: Solubility Products, http://www.chemguide.co.uk/physical/kspmenu.html#top
Points to Consider
In the course of an exothermic reaction, heat is released from the system into the surroundings, resulting in a decrease
in the enthalpy of the system. This is a favorable reaction because nature prefers a state of lower energy.
• What is meant by the term "driving force" as it relates to chemical reactions?
• What other force is responsible for the occurrence of endothermic reactions, which absorb heat into the
system?
25
1.4. References
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1.4 References
1.
2.
3.
4.
5.
26
Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.0
Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.0
Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.0
Zachary Wilson. CK-12 Foundation . CC BY-NC 3.0
Ondřej Mangl. http://commons.wikimedia.org/wiki/File:S%C3%ADran_barnat%C3%BD.PNG . Public Domain