Many Chemical Reactions Occur in Our Atmosphere Chapter 12

Reaction Rates and Chemical Equilibrium

Figure 12.1 1 2

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Introduction Chapter 12 Topics

• What factors effect how fast a reaction 1. Reaction rates goes? 2. Collision theory • How do we describe a reaction that does 3. Conditions that effect reaction rates not go to completion? 4. Chemical equilibrium 5. The Equilibrium constant 6. Le Chatelier’s principle

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Grill 12.1 Reaction Rates Effect of Concentration Mg/HCl

• Reaction rate is a measure of how fast a • Changing the concentration of a reactant reaction occurs. can change the reaction rate: • Some reactions are inherently fast and some are slow:

Figure 12.2 5 6 Figure 12.3

1 Sugar MnO2/H2O2 Effect of Surface Area Lycopodium Catalysis

• The catalyst called catalase in this piece of

liver causes the decomposition of H2O2 to occur faster.

Figure 12.4 Iron Nail Steel wool Figure 12.5 7 8

Collision Theory collisions 12.2 Collision Theory Orientation • In order for a reaction to occur, reactant molecules must collide • Consider the following reaction that occurs in smog: – with proper orientation NO(g) + O (g) Æ O (g) + NO (g) – with enough energy 3 2 2 • Only a small fraction of the collisions that • Which of the following collisions has a proper do occur meet these requirements. orientation?

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Collision Theory Collision Theory Energy diagram Energy Diagrams (Before and Energy Diagram (During) After)

Figure 12.6 Figure 12.7 11 12

2 Collision Theory 12.3 Conditions that Effect Energy Requirements Reaction Rates

• In order for reactants to convert to • Increasing the concentration (or surface products, an energy barrier called the area) of reactants or the reaction activation energy, Ea, must be overcome. temperature increases reaction rate by • Collisions that have the proper orientation increasing the number of effective and have at least the minimum Ea can collisions. convert to products. • The activation energy needed is related to the amount of energy needed to break bonds. 13 14

Conditions that Effect Reaction Conditions that Effect Reactiontemperature Rates Rates

• Increasing the concentration or surface area of • Increasing the one or more reactants increases the number of temperature of the effective collisions by increasing the total number reaction increases of collisions (fraction remains the same). the number of effective collisions by both increasing the total number of collisions and increasing the Figure 12.8 fraction of collisions 15 that are effective. Figure 12.9 16

Effect of temperature on fraction of Conditions that Effect Reaction effective collisions: Rates

• Adding an appropriate catalyst increases the number of effective collisions by lowering the activation energy. This also increases the fraction of collisions that are effective. Figure 12.10 17 18 Figure 12.9

3 Catalysis Catalysis

• A catalyst is not a reactant or product. It • Catalytic converters dramatically speed of the interacts with the reactants, but is not reactions of toxic gases to form harmless permanently changed during the reaction. products: CO anim NO anim • Since catalysts are “recycled,” small – CO to CO2

– NO to N2 and O2 amounts are needed and last a long time.

Figure 12.11 Catalyst is a palladium/platinum metal surface 19 20

The thousands of enzymes in our The enzyme sucrase catalyzes the bodies act to catalyze specific decomposition of sucrose by making biological processes. bond-breaking easier:

Figure 12.13 Figure 12.12 21 22

Destruction of Ozone in the Destruction of Ozone in the Stratosphere Stratosphere

• Chlorine atoms from CF2Cl2 catalyze the • Chlorine atoms from CF2Cl2 catalyze the decomposition of ozone in the stratosphere: decomposition of ozone in the stratosphere:

O3(g) + Cl(g) Æ ClO(g) + O2(g) O3(g) + Cl(g) Æ ClO(g) + O2(g) ClO(g) + O3(g) Æ Cl(g)+ 2O2(g) ClO(g) + O3(g) Æ Cl(g)+ 2O2(g) • The ClO(g) formed in step 1 is an intermediate that is formed temporarily.

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4 Energy Diagram of Catalyzed 12.4 Chemical Equilibrium Reaction

• This catalyzed reaction has a two transition states, and a lowered energy for the intermediate.

Figure 12.10 Figure 12.14 25 N2O4(g) U 2NO2(g) 26

Chemical Equilibrium Chemical Equilibrium

• When a chemical reaction reaches a state where • At equilibrium, the rate of the forward the concentrations of reactants and products reaction is equal to the rate of the reverse remain constant, a chemical equilibrium has reaction: been established.

Figure 12.14 27 Figure 12.15 28

Dynamic equilibrium Chemical Equilibrium Chemical Equilibrium

• At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.

Figure 12.15

Figure 12.15 29 30

5 12.5 The Equilibrium Constant The Equilibrium Constant

• The position of equilibrium is a constant • How can we describe a reaction that reaches for a reaction at a specific temperature. equilibrium? ƒ The relative amount of reactants and products ¾ Some have similar amounts of reactants and products at equilibrium. is the same. ¾ Some are reactant favored. ƒ How do we determine what the “constant” is? ¾ Some are product favored.

Figure 12.15 31 32

The Equilibrium Constant The Equilibrium Constant

• Consider the following reaction run at a specific • Which expression gives the same value for all temperature: three experiments?

2HI(g) U H2(g) + I2(g) ¾ How can we generalize this expression?

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N (g) + 3H (g) U 2NH (g) The Equilibrium Constant (Keq) 2 2 3 • What is the equilibrium constant • In general, for a reaction with the general form expression? aA + bB U cC + dD • What is the value of the equilibrium constant? the equilibrium constant expression is

[CD]cd[ ] Keq = [][]ABab

ƒ The brackets [C] mean “the concentration of” C. Figure 12.16

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6 N (g) + 3H (g) U 2NH (g) Keq and the Position of 2 2 3 Equilibrium • Is this reaction reactant favored or product favored?

Figure 12.16

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Predicting the Direction of Equilibrium Example 12.07a Figure 12.17 • Is this mixture of reactants and products at equilibrium? • If not which direction will the reaction proceed?

– In this example we can substitute number of molecules for concentration because the number of reactants and products in the balanced equation are the same – volume units of molarity would cancel out. 39 40

Heterogeneous Physical Heterogeneous Equilibrium Equilibria

• Homogeneous equilibria – • Consider the − reactants and products are in the same evaporation of bromine physical state in a closed container:

Br2(l) U Br2(g) • Heterogeneous equilibria – • The concentration of − reactants and products are not all in the same bromine vapor, [Br2], at physical state equilibrium is a constant, and is independent of the amount of bromine 41 42 liquid. Figure 12.18

7 Heterogeneous Physical Heterogeneous Physical Equilibria Equilibria • Because the concentrations of liquids and • Consider the solids are constant, they are left out of the evaporation of bromine [Br (g )] K = 2 equilibrium constant expression. in a closed container: eq constant • Only gases and aqueous phase Br2(l) U Br2(g) substances are included. 1 Keq=× []Br( 2 g ) constant

KK' =× constant = Br ( g ) eq eq [ 2 ]

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Heterogeneous Equilibria 12.6 Le Chatelier’s Principle

• What is the equilibrium constant • If a reactant or product is added to the expression for the decomposition of system at equilibrium, the system is no calcium carbonate? longer at equilibrium. ƒ We say that the equilibrium is disrupted or stressed. • Le Chatelier’s principle helps us predict in which direction the reaction will proceed to reestablish equilibrium.

Figure 12.19

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Reactant or Product Le Chatelier’s Principle Concentration • Ways to disrupt a chemical equilibrium: •Fe3+(aq) + NCS−(aq) U FeNCS2+(aq) ƒ Adding or removing a reactant or product ƒ Changing the volume of the reaction container

ƒ Changing the temperature (changes Keq value)

Figure 12.20

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8 Reactant or Product Reactant or Product Concentration Concentration • When the concentration of a reactant or product concentration is increased, the equilibrium will shift •Fe3+(aq) + NCS−(aq) U FeNCS2+(aq) away from it to consume most of the added • What happens when we add more Fe(NO3)3 or substance. KNCS? • When the concentration of a reactant or product concentration is decreased, the equilibrium will shift toward it to produce more of the removed substance.

Figure 12.20

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Volume of Reaction Container Volume of Reaction Container

• Which direction does the reaction • Reducing the volume proceed? of the container makes the concentration of all gaseous substances to increase. • The system shifts to reestablish equilibrium concentrations. Figure 12. 21 Figure 12.21 51 52

N2O4(g) U 2NO2(g) N2O4(g) U 2NO2(g) Colorless Brown Colorless Brown • Which direction does the reaction • The reaction proceeds in the direction that proceed? will make fewer gas particles.

Figure 12.22 Figure 12.22

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9 Effect of Volume Changes Temperature

•N2O4(g) U 2NO2(g) Colorless Brown

• Which direction does the equilibrium when the temperature is ¾ increased? ¾ decreased?

55 temperature Figure 12.23 56

Temperature Temperature

• To predict the effect of temperature on the position of equilibrium, we must know whether a reaction is endothermic or exothermic. • Endothermic: heat + N2O4(g) U 2NO2(g) • Exothermic: 2SO2(g) + O2(g) U 2SO2(g) + heat

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Catalysts Increasing Product Yield

• A catalyst does not effect the position of N2(g) + 3H2(g) U 2NH3(g) (exothermic) equilibrium. – (It speeds up both the forward and the Under which conditions of temperature reverse reaction.) and volume can the yield of NH3 be • A catalyst only increases the rate at which maximized? equilibrium is reached. a) High or low temperatures? b) Large or small volumes?

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10 Applying Le Chatelier’s Principle Applying Le Chatelier’s Principle

• CO(g) + H O(g) U CO (g) + H (g) exothermic 2 2 2 •N2(g) + O2(g) U 2NO(g) −6 Predict the direction the equilibrium will shift after each The equilibrium constant is 1.0×10 at stress is applied: 1500 K and 6.2×10−4 at 2000 K. a) Add CO (constant V) b) Remove H2O (constant V) ƒ Is this reaction endothermic or exothermic? c) Increase volume d) Increase temperature e) Add a catalyst FeNCS2+ 61 62

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