Chemical Equilibrium

Chemical Equilibrium

Chemical Equilibrium The Concept of Equilibrium (15.1) Ways of Expressing Equilibrium Constants (15.2) What Does the Equilibrium Constant Tell Us? (15.3) Factors that Affect Chemical Equilibrium (15.4)

General Chemistry I Concepts Representations of matter (1.3 and1.4) Formula calculations and stoichiometry (3.6 through 3.9) Weak acids, molar concentrations of solutions and solution stoichiometry (4.4 through 4.6) Pressure of a gas, the ideal gas law, and partial pressures (5.2 through 5.5) Enthalpy and energy diagrams (6.4 and 6.6)

15.1 The Concept of Equilibrium What is equilibrium? Discussed this previously chs 4 and 12 Do reactions occur in both directions? What does this mean in terms of reversibility? Review: what is an elementary step? p. 64-65 of LA book What does reversibility mean in terms of rate? Margin Figure p. 514

p. 65-66 of LA book 15.1 The Concept of Equilibrium Let s consider a reaction from Chapter 14: 2 2 k1 2 2 3 k2 Overall reaction: NO g CO g NO g CO g Elementary step 1: NO g NO g NO g NO g Elementary step 2: NO g CO g NO g CO g Looking at this: 3 2 2 just for the rate determining step and using a possible activated complex in an energy diagram.

p. 66 of LA book

p. 66 of LA book

Or more general p. 66 of LA book

Or more general p. 66 of LA book

p. 66 of LA book 15.1 The Concept of Equilibrium What does reversibility mean in terms of rate? What is the rate law for the forward reaction? What is the rate law for the reverse reaction? What happens when these rates are the same?

p. 67 of LA book 15.1 The Concept of Equilibrium The equilibrium constant: Definition: Relates the concentrations of reactants and products at equilibrium Now consider the derivation where these rates are equal What do we notice about the equilibrium constant? Amounts? Direction of reaction? Mechanism?

p. 68 of LA book 15.1 The Concept of Equilibrium The equilibrium constant: What does this mean what does this number tell us? Consider the example in the textbook: NO g 2NO g 2 4 2 Does it matter if we start with N 2 O 4 only, NO 2 only or a mixture? Margin Figure p. 514

p. 69 of LA book 15.1 The Concept of Equilibrium N O g 2NO g 0.00463 at 298 K K 2 4 2 How did you know which direction the first two reactions will go? Figure 15.1 p. 515

p. 69 of LA book 15.1 The Concept of Equilibrium NO g 2NO g 2 4 2

p. 70 of LA book 15.1 The Concept of Equilibrium The equilibrium constant: What does this mean what does this number tell us? Consider a simpler example: X g Y g

p. 70 of LA book 15.1 The Concept of Equilibrium The equilibrium constant: What does this mean what does this number tell us? K is large Figure 15.2 p. 516

p. 70 of LA book 15.1 The Concept of Equilibrium The equilibrium constant: What does this mean what does this number tell us? K is small Figure 15.2 p. 516

p. 71 of LA book 15.1 The Concept of Equilibrium N O g 2NO g 0.00463 at 298 K K 2 4 2 So, how did you know which direction this reaction went? Figure 15.1 p. 515

p. 71-72 of LA book 15.2 Ways of Expressing Equilibrium Constants Homogeneous equilibrium Homogeneous equilibrium What does this mean? How do we identify this in a balanced equation? Can we use other concentration units than molar concentration? Review: What is partial pressure? How does K P relate to K c? Can K P = K c?

p. 73 of LA book 15.2 Ways of Expressing Equilibrium Constants Homogeneous equilibrium Practice Suppose start with 1.00 atm N 2 O 4 (g) and no NO 2 (g). At 100 o C and at equilibrium, the partial pressure of N 2 O 4 (g) is 0.22 atm and the partial pressure of NO 2 is 1.56 atm. What is K P and what is K c? What does this look like?

p. 73 of LA book 15.2 Ways of Expressing Equilibrium Constants Homogeneous equilibrium Solution phase equilibrium how do we identify this in a balanced equation? Practice: What is the equilibrium expression for K c for the reaction of hydrofluoric acid with water? What is the equilibrium expression for K c for the reaction of hydrofluoric acid with sodium hydroxide? How do we deal with [H 2 O]?

p. 74 of LA book 15.2 Ways of Expressing Equilibrium Constants Heterogeneous equilibrium Heterogeneous equilibrium What does this mean? How do we identify this in a balanced equation? What does this mean in terms of writing the equilibrium expression? In general, what is not included in an equilibrium expression?

p. 74 of LA book 15.2 Ways of Expressing Equilibrium Constants Heterogeneous equilibrium Figure 15.3 p. 522

15.2 Ways of Expressing Equilibrium Constants Practice: What are the equilibrium expressions for K c for the reactions: a. HCl g NH3 g NH4Cl s b. FeS2 s 2H2 g Fe s 2H2S g c. PbCl s Pb 2 aq 2Cl aq 2 Is K c = K P for a and b? p. 74 of LA book

p. 75 of LA book Rules and forms for the equilibrium expression 15.2 Ways of Expressing Equilibrium Constants What are the rules and forms for the equilibrium constant and writing the equilibrium expression? 1. How do we express concentration? How does K P relate to K c? 2. What is included in the equilibrium expression? What is excluded? 3. What are the units on K P or K c? 4. Why do we have to give a balanced equation with the equilibrium constant?

p. 75 of LA book Rules and forms for the equilibrium expression 15.2 Ways of Expressing Equilibrium Constants What are the rules and forms for the equilibrium constant and writing the equilibrium expression? 5. How is K affected by the direction of the reaction? 6. How is K affected by the stoichiometric amounts of products/reactants? 7. What is K for multiple steps for an overall reaction?

15.3 What Does the Equilibrium Constant Tell Us? Review: K>>1 means? K<<1 means? K<0? Fundamentally important: p. 78 of LA book K is a measure of equilibrium concentrations for a reaction written in specific stoichiometric amounts and for a certain direction.

15.3 What Does the Equilibrium Constant Tell Us? p. 79-80 of LA book K is a measure of equilibrium concentrations for a reaction written in specific stoichiometric amounts and for a certain direction. Calculations at equilibrium include: Calculating K given equilibrium concentrations Calculating equilibrium concentrations given K Calculating both given other measures for the system

15.3 What Does the Equilibrium Constant Tell Us? p. 80 of LA book K is a measure of equilibrium concentrations for a reaction written in specific stoichiometric amounts and for a certain direction. Calculations at equilibrium must include: Balanced equation (for which equilibrium is represented) Equilibrium concentrations Sometimes these concentrations are given and sometimes these are calculated.

Calculating K given equilibrium concentrations 15.3 What Does the Equilibrium Constant Tell Us? Practice: p. 81 of LA book Under certain conditions, begin with 1.00 atm of each N 2 and O 2 and at equilibrium obtain partial pressure of NO to be 0.20 atm. What is K P?

Calculating equilibrium concentrations given K 15.3 What Does the Equilibrium Constant Tell Us? Practice: p. 82 of LA book If the equilibrium constant of the previous reaction changes to 2.5 10 5 as the temperature changes, what is the concentration of all substances if the initial concentration of nitrogen and oxygen are still 1.00 atm?

Calculating equilibrium concentrations given K 15.3 What Does the Equilibrium Constant Tell Us? Practice p. 83 of LA book At 100 o C, K P for the reaction of N 2 O 4 reacting to form NO 2 is 11. If the reaction begins with 1.5 atm of N 2 O 4, what is the equilibrium pressures of both gases?

15.3 What Does the Equilibrium Constant Tell Us? Calculating equilibrium concentrations given K sometimes the calculations can get more difficult. But, are there approximations we make if we know about the value of K? Review: K>>1 means? K<<1 means? p. 84 of LA book

Calculating equilibrium concentrations given K 15.3 What Does the Equilibrium Constant Tell Us? Sometimes the calculations be simplified through an approximation: p. 85 of LA book Practice: At 25 o C, K P for the reaction of nitrogen and oxygen to form NO is 1.0x10-30. If the reaction begins with 1.0 atm of nitrogen and 0.750 of oxygen, what is the equilibrium pressures of all gases?

Calculating both given other measures 15.3 What Does the Equilibrium Constant Tell Us? p. 86 of LA book Calculating both given other measures for the system Practice: At 150 o C, 15.5 g of N 2 O 4 is placed in a 7.5 L container without any NO 2. The reaction proceeds to equilibrium with a final pressure in the container of 1.49 atm. What is K P at this temperature?

15.3 What Does the Equilibrium Constant Tell Us? p. 87 of LA book What if the system is not yet at equilibrium or the equilibrium is perturbed in some way? What is the Reaction Quotient (Q) The quantity obtained by substituting the initial concentrations into the equilibrium constant expression How is this the same as the equilibrium constant? How is this different from the equilibrium constant?

15.3 What Does the Equilibrium Constant Tell Us? What is the Reaction Quotient (Q) p. 87 of LA book Figure 15.4 p. 525

15.3 What Does the Equilibrium Constant Tell Us? What is the Reaction Quotient (Q) Practice: p. 87 of LA book If for the preceding reaction (K P = 11), it is determined that at some point, the partial pressure of NO 2 is 2.5 atm and N 2 O 4 is 0.5 atm, is the reaction at equilibrium? If not, which way will the reaction shift in order to achieve to equilibrium?

15.4 Factors That Affect Chemical Equilibrium Practice: p. 88 of LA book Hydrogen reacts with ethylene to produce ethane (all gases). Calculate the equilibrium constant if initially begin with 1.00 atm of each reactant (no product) and at equilibrium have 0.6 atm of product.

15.4 Factors That Affect Chemical Equilibrium p. 88 of LA book Practice: Calculate Q if have 0.5 atm of all reactants and products. Indicate the direction the reaction will proceed to reach equilibrium. If the reaction is begun with 2.00 atm of ethane (and nothing else), what will the partial pressures of all substance be at equilibrium and what is the total pressure at equilibrium?

15.4 Factors That Affect Chemical Equilibrium p. 88 of LA book What is Le Châtelier s Principle? If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as it tries to reestablish equilibrium What stresses will we consider? Changing concentrations Changing volumes Changing pressures Changing temperatures

15.4 Factors That Affect Chemical Equilibrium Changing concentrations This includes: Adding more reactant Adding more product Removing reactant Removing product p. 89 of LA book

15.4 Factors That Affect Chemical Equilibrium p. 89 of LA book Changing concentrations Consider the reaction of N 2 O 4 and NO 2 : NO g 2NO g 2 4 2 If the partial pressure of N 2 O 4 is increased, what is Q? What will happen to the reaction?

15.4 Factors That Affect Chemical Equilibrium p. 89 of LA book Changing concentrations Consider the reaction of N 2 O 4 and NO 2 : NO g 2NO g 2 4 2 If the partial pressure of N 2 O 4 is decreased, what is Q? What will happen to the reaction?

15.4 Factors That Affect Chemical Equilibrium p. 89 of LA book Changing concentrations Consider the reaction of N 2 O 4 and NO 2 : NO g 2NO g 2 4 2 If the partial pressure of NO 2 is increased, what is Q? What will happen to the reaction?

15.4 Factors That Affect Chemical Equilibrium p. 89 of LA book Changing concentrations Consider the reaction of N 2 O 4 and NO 2 : NO g 2NO g 2 4 2 If the partial pressure of NO 2 is decreased, what is Q? What will happen to the reaction?

15.4 Factors That Affect Chemical Equilibrium p. 90 of LA book 3H g N g 2NH g 2 2 3 Figure 15.7 p. 531

15.4 Factors That Affect Chemical Equilibrium p. 90 of LA book Changing concentrations Consider an equilibrium of an insoluble salt in water: AgCl s Ag aq Cl aq If sodium bromide is added and the silver ion reacts with the bromide ion, what is Q and what will happen?

15.4 Factors That Affect Chemical Equilibrium Changing volumes Will changing volume of a container affect: Gases Solutions p. 90 of LA book Consider the concentration of a gas and how this will be affected by a change in volume

15.4 Factors That Affect Chemical Equilibrium Changing pressure Will changing the pressure affect: Gases Solutions p. 91 of LA book Consider the concentration of a gas and how this will be affected by a change in pressure

15.4 Factors That Affect Chemical Equilibrium p. 91 of LA book Consider the concentration of a gas and how this will be affected by a change in pressure Adding more gases or removing gases Increasing the volume or decreasing the volume Adding an inert gas to the container where the volume of the container is unchanged

15.4 Factors That Affect Chemical Equilibrium Changing temperatures p. 91 of LA book Think again in terms of the energy diagram:

15.4 Factors That Affect Chemical Equilibrium p. 91 of LA book

15.4 Factors That Affect Chemical Equilibrium Changing temperatures p. 91-92 of LA book Think again in terms of the energy diagram: Review: What is the rate of the forward and reverse reactions in terms of activation energy?

15.4 Factors That Affect Chemical Equilibrium Two possibilities: Endothermic system: p. 92 of LA book

15.4 Factors That Affect Chemical Equilibrium Two possibilities: Exothermic system: p. 93 of LA book

15.4 Factors That Affect Chemical Equilibrium Practice: Give the expression for K c What direction will reaction go if: Add hydrogen Add ammonia Remove nitrogen Remove ammonia Increase volume Decrease volume Increase temperature Decrease temperature p. 94 of LA book 5 3 2 92.9 kj; 6.0 10 N2 g H2 g NH3 g H K25 C