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Chapter 15
Chemical Equilibrium

15-01
Title
A reaction with a fast rate
Caption
In a reaction with a fast rate, the reactants react to form products in a short period of time.
Notes
Rate is defined as the change in concentration of reactants or products over time.
Keywords
rate, reactants, products, time
15-02
Title
A reaction with a slow rate
Caption
In a reaction with a slow rate, the reactants react to form products over a long period of time.
Notes
Rate is defined as the change in concentration of reactants or products over time.
Keywords
rate, reactants, products, time
15-03
Title
Increasing the concentration increases the reaction rate
Caption
Which reaction mixture will have the fastest initial rate? The mixture in (c) is fastest because it has the highest concentration of reactants and therefore the highest rate of collisions.
Notes
Rate is defined as the change in concentration of reactants or products over time. Increasing concentration makes it more likely that reactants will collide to form products.
Keywords
rate, reactants, products, time, collision theory, concentration
15-04
Title
Increasing the temperature increases the reaction rate
Caption
Which reaction mixture will have the fastest initial rate? The mixture in (c) is fastest because it has the highest temperature.
Notes
Rate is defined as the change in concentration of reactants or products over time. Increasing temperature provides more energy that the reactants need to change into products.
Keywords
rate, reactants, products, time, collision theory, temperature
15-05
Title
Equilibrium in a reversible reaction
Caption
When the concentrations of the reactants and products no longer change, equilibrium has been reached.
Notes
Concentrations will stop changing when the forward rate equals the reverse rate. The reaction still occurs in both directions, but the number of atoms changing into products (right side) exactly equals the number of atoms changing back into reactants (left side).
Keywords
rate, reactants, products, time, collision theory, equilibrium
15-06
Title
An equilibrium analogy
Caption
Population analogy of a chemical reaction proceeding to equilibrium.
Notes
Equilibrium is dynamic: In this analogy, population moves from Narnia to Middle Earth, but the same number moves from Middle Earth to Narnia. Different people may move from one land to the other, but the numbers moving in each direction are the same: equilibrium is established.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium
15-06-01un
Title
The equilibrium constant: a mathematical treatment of equilibrium
Caption
The variables in the expression arise from the general reaction, aA + bB <=> cC + dD In the equilibrium constant, A, B, C, D are the species' concentrations, and a, b, c, d are the coefficients from the balanced equation.
Notes
This equation allows us to predict the concentrations of reactants and products for any chemical reaction at equilibrium.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-06-02un
Title
Example of the equilibrium constant for a real chemical reaction
Caption
The variables in the expression arise from the reaction, 2 N2O5 <=> 4 NO2 + O2 In the equilibrium constant, the bracketed formulas represent the respective species' concentrations, and their powers are the coefficients from the balanced equation.
Notes
This equation can be adapted to allow us to predict the concentrations of reactants and products for any chemical reaction at equilibrium.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-06-03un
Title
Example of the equilibrium constant for a real chemical reaction
Caption
The variables in the expression arise from the reaction, CO + 2 H2 <=> CH3OH In the equilibrium constant, the bracketed formulas represent the respective species' concentrations, and their powers are the coefficients from the balanced equation.
Notes
This equation can be adapted to allow us to predict the concentrations of reactants and products for any chemical reaction at equilibrium.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-07
Title
Products predominate when the equilibrium constant is large
Caption
A large equilibrium constant means that, at equilibrium, there will be a high concentration of products and a low concentration of reactants.
Notes
This result follows directly from the Keq expression: The product concentration is in the numerator, and large numerators mean the result of the calculation will be large. But there is more: The denominator will be small, reinforcing the trend to a large Keq.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-08
Title
Reactants predominate when the equilibrium constant is small
Caption
A small equilibrium constant means that, at equilibrium, there will be a high concentration of reactants and a low concentration of products.
Notes
This result follows directly from the Keq expression: The product concentration is in the numerator, and small numerators mean the result of the calculation will be small. But there is more: The denominator will be large, reinforcing the trend to a small Keq.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-08-02un
Title
Solution map for calculating the equilibrium constant
Caption
The stated factors use the balanced equation, H2 + I2 <=> 2 HI and the definition of the equilibrium constant. The concentrations (molarities) of H2, I2, and HI are provided in the problem statement.
Notes
The approach can be adapted to any equilibrium reaction.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-09
Title
Population analogy of disturbed equilibrium
Caption
When a system at equilibrium is disturbed, it shifts to minimize the disturbance. In this case, adding population to Middle Earth (the disturbance) causes population to move out of Middle Earth (minimizing the disturbance). What would happen if you disturbed the equilibrium by taking population out of Middle Earth? In which direction would population move to minimize the disturbance?
Notes
Equilibrium is dynamic: In this analogy, population moves from Narnia to Middle Earth, but the same number moves from Middle Earth to Narnia. Different people may move from one land to the other, but the numbers moving in each direction are the same: equilibrium is established. When equilibrium is disturbed, the system adjusts in such a way as to reestablish equilibrium.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium
15-10
Title
Changing concentration affects equilibrium
Caption
When a system at equilibrium is disturbed, it shifts to minimize the disturbance. In this case, adding NO2 (the disturbance) causes the reaction to shift left, consuming NO2 (minimizing the disturbance).
Notes
Adding NO2 causes the equilibrium to shift left because the reverse rate increased, relative to the forward rate. As the reaction proceeded, the extra NO2 was used, lowering its concentration, and consequently lowering the rate for the reverse reaction. Eventually, equilibrium was reestablished.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium
15-11
Title
Changing concentration affects equilibrium
Caption
When a system at equilibrium is disturbed, it shifts to minimize the disturbance. In this case, adding N2O4 (the disturbance) causes the reaction to shift right, producing NO2 (minimizing the disturbance).
Notes
Adding N2O4 causes the equilibrium to shift right because the forward rate increased, relative to the reverse rate. As the reaction proceeded, the extra N2O4 was used, lowering its concentration, and consequently lowering the rate for the forward reaction. Eventually, equilibrium was reestablished.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium
15-11-02un
Title
Changing reactant concentration shifts equilibrium to the right
Caption
When a system at equilibrium is disturbed, it shifts to minimize the disturbance. In this case, adding N2O4 (the disturbance) causes the reaction to shift right, producing NO2 (minimizing the disturbance).
Notes
Adding N2O4 causes the equilibrium to shift right because the forward rate increased, relative to the reverse rate. As the reaction proceeded, the extra N2O4 was used, lowering its concentration, and consequently lowering the rate for the forward reaction. Eventually, equilibrium was reestablished.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium
15-11-03un
Title
Equilibrium explains respiration I
Caption
In the lungs, the oxygen concentration is high, so the reaction shifts right. Hemoglobin picks up oxygen for transport.
Notes
For many years, scientists have sought out other compounds that reversibly take up oxygen, as hemoglobin does. Challenge students to suggest applications that would benefit from a hemoglobin-like compound. For example, the Navy would like to have a reliable oxygen storage system for submarines, divers, etc.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium, respiration, oxygen, hemoglobin
15-11-04un
Title
Equilibrium explains respiration II
Caption
In the cells, the oxygen concentration is low, so the reaction shifts left. Hemoglobin releases oxygen to the cells.
Notes
For many years, scientists have sought out other compounds that reversibly take up oxygen, as hemoglobin does. Challenge students to suggest applications that would benefit from a hemoglobin-like compound. For example, the Navy would like to have a reliable oxygen storage system for submarines, divers, etc.
Keywords
reactants, products, time, equilibrium, dynamic equilibrium, respiration, oxygen, hemoglobin
15-12
Title
Increased pressure can affect equilibrium I
Caption
Effect of volume change on equilibrium. When the volume of an equilibrium mixture is decreased, the pressure increases. This system responds (to bring the pressure back down) by shifting to the right, the side of the reaction with the fewest moles of gas particles.
Notes
In this example, the reaction responds to an increase in pressure by shifting right. The shift converts four moles of gas to two moles: the pressure drops. Recall from Chapter 11 that the ideal gas law establishes a direct relationship between moles and pressure.
Keywords
reactants, products, equilibrium, dynamic equilibrium, pressure, volume
15-13
Title
Increased pressure can affect equilibrium II
Caption
Effect of volume change on equilibrium. When the volume of an equilibrium mixture is increased, the pressure decreases. This system responds (to raise the pressure) by shifting to the left, the side of the reaction with the most moles of gas particles.
Notes
In this example, the reaction responds to an decrease in pressure by shifting left. The shift converts two moles of gas to four moles: the pressure rises. Recall from Chapter 11 that the ideal gas law establishes a direct relationship between moles and pressure.
Keywords
reactants, products, equilibrium, dynamic equilibrium, pressure, volume
15-13-01un
Title
Heat affects equilibrium I
Caption
Effect of heat on equilibrium. If the reaction is exothermic, heat can be considered a product; adding heat to the system would be like increasing the concentration of a product species: the equilibrium would shift left.
Notes
In this example, the reaction responds to an addition of heat by shifting left. If the reaction is exothermic in the forward direction, it will be endothermic in the reverse direction.
Keywords
reactants, products, equilibrium, dynamic equilibrium, exothermic
15-13-02un
Title
Heat affects equilibrium II
Caption
Effect of heat on equilibrium. If the reaction is exothermic, heat can be considered a product; removing heat from the system would be like decreasing the concentration of a product species: the equilibrium would shift right.
Notes
In this example, the reaction responds to cooling by shifting right. If the reaction is exothermic in the forward direction, it will be endothermic in the reverse direction.
Keywords
reactants, products, equilibrium, dynamic equilibrium, exothermic
15-14
Title
Sometimes equilibrium shifts are visible
Caption
N2O4(g) <=> 2 NO2(g) equilibrium as a function of temperature. Since the reaction is endothermic, cool temperatures cause a shift to the left to colorless N2O4. Warm temperatures cause a shift to the right to brown NO2.
Notes
The color change is reversible, with change in temperature.
Keywords
reactants, products, equilibrium, dynamic equilibrium, exothermic, endothermic
15-14-01un
Title
Equilibrium shifts are visible, if reactants and products have different colors I
Caption
N2O4(g) <=> 2 NO2(g) equilibrium as a function of temperature. Since the reaction is endothermic, cool temperatures cause a shift to the left to colorless N2O4. Warm temperatures cause a shift to the right to brown NO2.
Notes
The color change is reversible, with change in temperature.
Keywords
reactants, products, equilibrium, dynamic equilibrium, exothermic, endothermic
15-14-02un
Title
Equilibrium shifts are visible, if reactants and products have different colors II
Caption
N2O4(g) <=> 2 NO2(g) equilibrium as a function of temperature. Since the reaction is endothermic, cool temperatures cause a shift to the left to colorless N2O4. Warm temperatures cause a shift to the right to brown NO2.
Notes
The color change is reversible, with change in temperature.
Keywords
reactants, products, equilibrium, dynamic equilibrium, exothermic, endothermic
15-15
Title
Activation energy and rate
Caption
This plot represents the energy of the reactants/products along the reaction pathway (as the reaction occurs). Notice that the energy of the products is lower than the energy of the reactants, so this is an exothermic reaction. However, notice that the reactants must get over an energy hump—called the activation energy—to proceed from reactants to products.
Notes
One explanation for the reaction energy arises from the octet rule: In order for the reactants' atoms to rearrange to form products, the octets of the reactants must be broken. This is tantamount to saying that the reactants go from low energy to high energy. The energy needed to do this is the activation energy.
Keywords
rate, activation energy, reactant, product, energy, reaction pathway
15-16
Title
A catalyst finds a lower energy pathway from reactants to products: the hill analogy
Caption
There are several ways to get these boulders over the hill as fast as possible. One way is simply to push them hard—this is analogous to an increase in temperature for a chemical reaction. The other way is find a path that goes around the hill—this is analogous to the role of a catalyst for a chemical reaction.
Notes
This plot represents the energy of the reactants/products along the reaction pathway (as the reaction occurs). Notice that the energy of the products is lower. One explanation for the reaction energy arises from the octet rule: In order for the reactants' atoms to rearrange to form products, the octets of the reactants must be broken. This is tantamount to saying that the reactants go from low energy to high energy. The energy needed to do this is the activation energy. A catalyst allows the atoms to shorten the time or conditions in which their octets are unsatisfied, resulting in lower activation energy.
Keywords
rate, activation energy, reactant, product, energy, reaction pathway, catalyst
15-17
Title
A catalyst finds a lower energy pathway from reactants to products
Caption
A catalyst provides an alternate pathway with a lower activation energy for the reaction.
Notes
One explanation for the reaction energy arises from the octet rule: In order for the reactants' atoms to rearrange to form products, the octets of the reactants must be broken. This is tantamount to saying that the reactants go from low energy to high energy. The energy needed to do this is the activation energy. A catalyst allows the atoms to shorten the time or conditions in which their octets are unsatisfied, resulting in lower activation energy.
Keywords
rate, activation energy, reactant, product, energy, reaction pathway, catalyst
15-17-01e
Title
An enzyme is catalyst that acts in biological systems
Caption
An enzyme catalyzes the cleavage of sucrose into glucose and fructose.
Notes
The equilibrium constant is large, but the rate of cleavage is low, due to the reaction's large activation energy. The enzyme reduces the activation energy, speeding up the cleavage.
Keywords
rate, activation energy, reactant, product, energy, reaction pathway, catalyst, sucrose, glucose, fructose, sucrase, enzyme
15-18
Title
An enzyme catalyzes the cleavage of sucrose into glucose and sucrose
Caption
The enzyme sucrase creates a pathway with a lower activation barrier for the conversion of sucrose to glucose and fructose.
Notes
The equilibrium constant is large, but the rate of cleavage is low, due to the reaction's large activation energy. The enzyme reduces the activation energy, speeding up the cleavage.
Keywords
rate, activation energy, reactant, product, energy, reaction pathway, catalyst, sucrose, glucose, fructose, sucrase, enzyme
15-18-01f
Title
An enzyme's active site: sucrase cleaves sucrose into glucose and fructose
Caption
Sucrase has a pocket called the active site where sucrose binds. Once sucrose is on the active site, the bond between glucose and fructose weakens, lowering the activation energy for the reaction.
Notes
The enzyme sucrase creates a pathway with a lower activation barrier for the conversion of sucrose to glucose and fructose. The equilibrium constant is large, but the rate of cleavage is low, due to the reaction's large activation energy. The enzyme reduces the activation energy, speeding up the cleavage. The active site provides a "pocket" in which the sucrose can rest while the sucrose is cleaved.
Keywords
rate, activation energy, reactant, product, energy, reaction pathway, catalyst, sucrose, glucose, fructose, sucrase, enzyme
15-18-02un
Title
Solution map for calculating the equilibrium constant: Example 15.12
Caption
The stated factors use the balanced equation, N2 + 3 H2 <=> 2 NH3 and the definition of the equilibrium constant. The concentrations (molarities) of N2, H2, and NH3 are provided in the problem statement.
Notes
The approach can be adapted to any equilibrium reaction.
Keywords
reactants, products, equilibrium, equilibrium constant, stoichiometry, concentration
15-18-03g
Title
At what point is equilibrium reached?
Caption
For the reaction, H2 + I2 <=> 2 HI, when is equilibrium reached? Think of (a) through (f) as stop-action photos of the reaction.
Notes
Equilibrium is reached by panel (e): There is no change from panel (e) to panel (f).
Keywords
equilibrium, reactant, product, concentration
15-18-04h
Title
Which reaction has the largest equilibrium constant?
Caption
For the reactions, C2H4 + Cl2 <=> C2H4Cl2, C2H4 + Br2 <=> C2H4Br2, and C2H4 + I2 <=> C2H4I2, the largest equilibrium constant corresponds to the panel with the highest product concentration and the lowest reactant concentration.
Notes
C2H4 + Cl2 <=> C2H4Cl2 has the largest equilibrium constant.
Keywords
equilibrium, reactant, product, concentration

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