EQUILIBRIUM
Course
Chemistry
Facilitator Tara LoPresti
April 4, 2016
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Sync March 2015 SF
Objectives
Equilibrium
Define the term
reversible reaction.
Describe chemical
equilibrium.
Explain the equilibrium
constant.
Calculate an equilibrium
constant expression for
a reaction.
Mastery Map
KEYWORDS
chemical equilibrium
the process by which the reactants form products
and products form reactants at equal rates
equilibrium constant
the ratio between the concentrations of the
products and the concentrations of the reactants
reversible reaction
a reaction in which there is a dynamic equilibrium
between products and reactants
CONTENT
Equilibrium can be discussed in terms of chemical reactions.
• Consider the following reaction:
- H2 (g) + Cl2 (g)  2HCl (g)
• When this reaction occurs, the reaction goes almost all the way to the
right; that is, the reaction favors the products.
• However, a small amount of the HCl product actually breaks back
down into the reactants in a reverse of the reaction.
• Such reactions are called reversible reactions.
CONTENT
Reversible reactions occur when the products of a reaction re-form
into the reactants.
• Consider the following equation:
- 2NO2  N2O4
• Nitrogen dioxide gas (NO2), which is produced in automobile exhaust,
is converted to dinitrogen tetroxide gas (N2O4).
• If you placed NO2 in a closed container, you would expect that at
some point the entire container would turn to N2O4.
• But that doesn't happen.
• The reason is that some of the N2O4 reverts to NO2 in a reversible
reaction.
CONTENT
Chemical equilibrium is achieved when concentrations of reactants
and products remain constant.
• A dynamic interaction occurs in chemical reactions.
• The term chemical equilibrium denotes that a chemical reaction
exists as a dynamic, stable movement between products and
reactants.
- The term does not imply that the ratio of products to reactants is 1 : 1.
- In fact, equilibrium exists in some reactions where the ratio of products to reactants is a
million to one.
• Equilibrium means that there exists a reversible reaction that over
time remains in balance, as shown in the graph of the following
reaction:
- H2 + I2  2HI
CONTENT
A reaction at chemical equilibrium can be graphed.
• One way to visualize equilibrium is to make a graph that compares
the concentrations of reactants and products over time.
• When you consider an equilibrium reaction graphically, keep two
points in mind:
1. A reaction reaches equilibrium when the net concentrations of the reactants and products
stop changing.
2. The concentrations of reactants and products do not need to be equal for equilibrium to
occur.
CONTENT
The concentrations of the products and reactants at equilibrium in a reversible
reaction can be expressed with the equilibrium constant.
•The equilibrium constant, Keq, gives information
about the equilibrium concentrations of reactants
and products.
•Each reaction has a value of Keq for any specific
temperature.
•The equation for determining the equilibrium
constant is shown here, where A and B are the
reactants; C and D are the products; and a, b, c,
and d are the coefficients of the compounds.
EXAMPLE
Consider the general reaction aA + bB ⇋ cC + dD
The equation
• The value of Keq for this reaction is given by this equation:
- 𝐾eq =
[C]c [D]d
[A]a [B]b
The alternative
• It is sometimes useful to think of it as:
- 𝐾eq =
[products]
[reactants]
• Thinking about it this way will help you identify general trends and
get a feel for what Keq means.
CONTENT
Determine the equilibrium expression for a reversible reaction.
• Suppose you are given the chemical reaction shown.
• How would you go about determining its equilibrium constant?
EXAMPLE
You are ready to calculate the equilibrium constant for an equation.
• Start with this equation:
- N2 (g) + 3H2 (g)  2NH3 (g)
• This equation represents the reaction for the production of ammonia,
a reaction known as the Haber process.
- Nitrogen gas and hydrogen gas react to form ammonia gas.
- Ammonia is an important component of fertilizers.
• The reaction reaches equilibrium with the following concentrations:
- [N2] = 0.8 M
- [H2] = 0.6 M
- [NH3] = 1.7 M
• How do you find the equilibrium constant for this reaction at this
temperature?
EXAMPLE
Gather Information
• The general form of Keq for the reaction aA + bB ⇋ cC + dD is as follows:
- 𝐾eq =
[C]c [D]d
[A]a [B]b
Apply the equation
• So the Keq for the equation for the reaction N2 (g) + 3H2 (g) ⇋ 2NH3 (g)
- 𝐾eq =
[NH3 ]2
[N2 ] [H2 ]3
• Remember, the exponents come from the coefficients in the balanced
chemical equation.
Calculate
• To find Keq, plug in the equilibrium concentrations:
- 𝐾eq =
[NH3 ]2
[N2 ] [H2 ]3
=
[1.7]2
[0.8] [0.6]3
= 16.7
• The equilibrium constant for this reaction at this temperature is 16.7
CONTENT
The equilibrium constant can reveal the nature of the chemical
reaction.
• The equilibrium constant is always a positive number.
- It can range from extremely small to extremely large.
• What does the value of Keq tell you about a reaction?
- If a reaction creates far more products than reactants, the numerator is large and the
denominator is small.
» That means Keq will be relatively large, and equilibrium exists when the far right of
the reaction is favored by nature.
- By contrast, if few products form, and many reactants remain, the numerator is small and the
denominator is large.
» So a relatively small Keq means that equilibrium is far to the left.
hydrogen iodide gas (HI)
RECAP
The equilibrium constant for a reaction describes the balance of
reactants and products in a reaction system.
• Many chemical reactions, such as the dissociation of a small amount of HCl in water, are
irreversible.
- In other words, the reaction proceeds only from left to right, and all the reactants are
converted to products.
- Chemical equations for irreversible reactions like these include only a single arrow that
points from reactants to products.
• However, many chemical reactions are not irreversible.
- In these reactions, at the same time that reactants are being converted to products, some
-
of the products are themselves reacting together and turning back into reactants.
This type of reaction is a reversible reaction.
Chemical equations for reversible reactions are indicated by a double arrow.
The reaction that proceeds from left to right is called the forward reaction.
The reaction that proceeds from right to left is called the reverse reaction.
• A reaction is said to be at equilibrium when the rates of forward and reverse reactions are
equal.
- At equilibrium, the amounts of reactants and products stop changing with time.
- The equilibrium constant for a reaction represents the ratio of products to reactants at
equilibrium.
REMEMBER…
The Equilibrium Constant
• The equilibrium constant K for a reaction has no units.
• The concentrations used in the equilibrium constant calculation are
concentrations in moles per liter.
• The concentration of a solid does not appear in an equilibrium
expression for a reaction.
• The equilibrium constant for a forward reaction is the reciprocal of
the equilibrium constant for the reverse reaction
EXAMPLE 1
Write the Equilibrium Expression for a Reaction
• Methanol (CH3OH) is a simple alcohol that can be used as a fuel or
as a reactant in the production of some important industrial chemicals.
Methanol can form from carbon monoxide (CO) and hydrogen (H2) in
a reversible reaction, as shown below:
- CO(g) + 2H2 (g) ⇋ CH3OH(g)
• Write the equilibrium expression for this reaction.
EXAMPLE 1
Solution
Strategize
• The equilibrium expression for a reaction is equal to the ratio of products to
reactants. Each species in the reaction is raised to a power equal to its
stoichiometric coefficient in the balanced chemical equation. The expression
includes the concentrations of products and reactants, not just their chemical
formulas. For this reaction, the equilibrium expression will include the
following terms: [CO], [H2]2, and [CH3OH]. The square brackets indicate
concentration in moles per liter.
Solve
• Methanol is the product, and CO and H2 are the reactants. Therefore, the
correct equilibrium expression for this reaction is the one shown below:
-
[CH3 OH]
[CO] [H2 ]2
EXAMPLE 2
Calculate the Equilibrium Constant for a Reaction
• A scientist combines carbon dioxide and hydrogen in a reaction at
210ºC to produce methanol according to the equation in the prior
example. At equilibrium, the concentrations of methanol, carbon
monoxide, and hydrogen for the reaction are as follows:
- [CH3OH] = 0.0494 mol/L
- [CO] = 0.151 mol/L
- [H2] = 0.151 mol/L
• What is the equilibrium constant, K, for this reaction at 210ºC
EXAMPLE 2
Solution
Collect and Strategize
• The equilibrium expression for the reaction is given below:
[CH3 OH]
- [CO]
[H ]2
2
• To calculate the equilibrium constant for this reaction, substitute the
equilibrium concentrations given in the problem into the equilibrium
expression.
Solve
• Substituting the given values and solving yields the following:
[CH3 OH]
0.0494
0.0494
- 𝐾 = [CO]
=
=
= 14.4
[H ]2
(0.151)(0.151)2
3.44 ×10−3
2
EXAMPLE 3
Interpret an Equilibrium Constant for a Reaction
• Does the reaction for the formation of methanol in the last two
examples favor the products or the reactants?
EXAMPLE 3
Solution
Recall Relevant Information
• Remember that the equilibrium constant, K, for the reaction is the ratio
of products to reactants at equilibrium. The product of the reaction
(methanol) is in the numerator, and the reactants (CO and H2) are in
the denominator.
Solve
• The equilibrium constant for the reaction is greater than 1. Therefore,
at equilibrium, the products are favored over the reactants.
EXAMPLE 4
Write an Equilibrium Expression for a Reverse Reaction and Calculate
its K’
• A scientist places some methanol in a closed container. The methanol
begins to decompose into carbon monoxide and hydrogen gas
according to the equation below:
- CH3OH(g) ⇋ CO(g) + 2H2 (g)
• Write the equilibrium expression for this reaction, and calculate the
equilibrium constant, K’, for the reaction at 210ºC. (This reaction is the
reverse of the reaction in Example 1. Chemists often use K’ to
represent the equilibrium constant for a reverse reaction.
EXAMPLE 4
Solution
Recall Relevant Information
• For any reaction, the equilibrium expression is the ratio of products to reactants, with
each species raised to a power equal to its stoichiometric coefficient in the balanced
equation. In the reaction given, the products are CO and H2, and the reactant is
methanol. Therefore, this reaction is the reverse of the reaction in Example 1. The
equilibrium constant K for the reaction in Example 1 at 210ºC is 14.4.
Solve
• The equilibrium expression is the ratio of products to reactants for the reaction as
written, as shown below:
-
[CO][H2 ]2
[CH3 OH]
• Notice that this is the reciprocal of the equilibrium expression for the reaction in
Example 1. Therefore, the equilibrium constant K’ for this reaction must be the reciprocal
of the equilibrium constant K calculated in Example 2, as shown below:
1
- 𝐾 = 𝐾1 = 14.4
= 0.0694
EXAMPLE 5
Write an Equilibrium Expression for a Reaction Involving a Solid
• Only a tiny amount of silver chloride (AgCl) can dissolve in water. The
equation below shows what happens when silver chloride dissolves in
water:
- AgCl (s) ⇋ Ag+ (aq) + Cl− (aq)
• Write an equation for the equilibrium constant, K, for this dissolution
reaction
EXAMPLE 5
Solution
Recall Relevant Information
• The equilibrium expression for a reaction is the ratio of products to reactants in the reaction. In this
reaction, solid AgCl is the reactant, and Ag+ ions and Cl− ions are the products. The equilibrium
expression includes the concentrations of the reactants and products. The concentration of a solid is
a constant, unlike the concentrations of aqueous ions.
Collect
• First, write the equilibrium expression in the usual way, as shown below:
- 𝐾=
[Ag+ ][Cl− ]
[AgCl]
Solve
• K is a constant, and so is [AgCl]. Therefore, their product must be a constant, and they can be
combined to eliminate [AgCl] from the expression, as shown below:
- K × [AgCl] = [ Ag+][Cl−]
- K = [ Ag+][Cl−]
• It is conventional in chemistry to eliminate the concentrations of all solid species from equilibrium
expressions in this way. The equilibrium constant for the dissolution of a salt is called the solubility
product constant, or Ksp. Therefore, the Ksp for silver chloride is given by the equation Ksp =
[Ag+][Cl−]
PROBLEM SET 85
1.
The equilibrium constant for the following reaction at 2,000 K is 0.154:
2CH4 (g) ⇋ C2H2 (g) + 3H2 (g)
Are the products or reactants favored in this reaction at 2,000 K? Explain your answer.
2.
Write the equilibrium expression for the equation below:
N2 (g) + 3H2 (g) ⇋ 2NH3 (g)
3.
The reaction between hydrogen and iodine at 700 K is described by the equation below:
H2 (g) + I2 (g) ⇋ 2HI (g)
For this reaction, 𝐾 =
[HI]2
.
[H2 ][I2 ]
At equilibrium, the concentration of H2 is 0.035 mol/L, the
concentration of I2 is 0.045 mol/L, and the concentration of HI is 0.29 mol/L. Calculate the
equilibrium constant for this reaction at 700 K. Are the products or reactants favored at equilibrium?
4.
Explain why a chemical equilibrium is called a dynamic equilibrium.
5.
H2 and Br2 are heated to give HBr according to the equation below:
H2 (g) + Br2 (g) ⇋ 2HBr (g)
At equilibrium, at a specific temperature, the concentration of H2 is 0.123 mol/L, the concentration of
Br2 is 0.121 mol/L, and the concentration of HBr is 0.712 mol/L. Calculate the equilibrium constant
for this reaction at that specific temperature.
PROBLEM SET 85
6. At 298 K, the equilibrium constant for the reaction 2SO2 (g) + O2 (g) ⇋ 2SO3 (g) is 7.2 × 1023.
Are the products or the reactants favored?
7. Challenge The solubility product constant, Ksp, for silver chloride (AgCl) is 1.8 × 10–10 at 25°C.
Use a scientific calculator to calculate the concentrations of Ag+ ions and Cl– ions in a saturated
AgCl solution. (Hint: Use the balanced chemical equation for the dissolution of AgCl to find a
relationship between [Ag+] and [Cl–].)
8. Challenge Nitrogen dioxide (NO2) can form from N2O4 according to the following reaction:
N2O4 (g) ⇋ 2NO2 (g)
At equilibrium at a certain temperature, the concentration of N2O4 is 0.029 mol/L, and the
concentration of NO2 is 0.112 mol/L. Is the reactant or product favored at equilibrium?
9. Challenge Ethanol (C2H5OH) is manufactured by reacting ethane (C2H4) with steam according to
the equation C2H4 (g) + H2O (g) ⇋ C2H5OH (g). The equilibrium constant for this reaction at a
specific temperature is 448.6. In a particular reaction system, the equilibrium concentrations of
C2H4 and C2H5OH are 0.019 mol/L and 0.179 mol/L, respectively. What is the equilibrium
concentration of H2O (g)?
PROBLEM SET 85 ANSWERS
PROBLEM SET 85 ANSWERS
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PROBLEM SET 85 ANSWERS
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PROBLEM SET 85 ANSWERS
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PROBLEM SET 85 ANSWERS
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LESSON SUMMARY
In this lesson, you learned:
In chemical equilibrium, the products and reactants of a reversible
reaction remain constant over time.
• Reversible reactions reach chemical equilibrium.
• If the rate at which the products and reactants are produced remains
in a dynamic constancy, the reaction is at equilibrium.
• Equations at equilibrium can be described quantitatively in terms of
their equilibrium constant.
STUDENT SURVEY
Poll
Decide which statement best describes your understanding of this lesson.
a) I totally get this!
b) I think I’ll be okay.
c) I’m a little lost and will contact my course instructor if I have
questions.
d) I’m completely lost and will schedule a meeting with my course
instructor.