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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 + dDIn 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 + O2In 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 <=> CH3OHIn 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 HIand 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 humpÑcalled the activation energyÑto 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 hardÑthis is analogous to an increase in temperature for a chemical reaction. The other way is find a path that goes around the hillÑthis 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 NH3and 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

Chapter 15Chemical Equilibrium

Chapter 15
Chemical Equilibrium