Chemical Equilibrium and Equilibrium Calculations PDF

CHEM 326A – ANALYTICAL CHEMISTRY MODULE 2 EFFECT OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM EQUILIBRIUM CALCULATION IN COMPLEX SYSTEMS  CONTENTS Introduction to Chemical Equilibrium Solubility Product Constants Effects of Electrolytes on Chem...

CHEM 326A – ANALYTICAL CHEMISTRY MODULE 2 EFFECT OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM EQUILIBRIUM CALCULATION IN COMPLEX SYSTEMS  CONTENTS Introduction to Chemical Equilibrium Solubility Product Constants Effects of Electrolytes on Chemical Equilibrium Ionic Strength Properties of Activity Coefficients Equilibrium Calculation in Complex Systems.  INTRODUCTION TO CHEMICAL EQUILIBRIUM Chemical Equilibrium The state where the concentrations of all reactants and products remain constant with time. Concentrations reach levels where the rate of the forward reaction equals the rate of the reverse reaction. Equilibrium-constant expressions are algebraic equations that describe the concentration relationships that exist among reactants and products at equilibrium. equilibrium - is to be in a state of balance INTRODUCTION TO CHEMICAL EQUILIBRIUM equilibrium - is to be in a state of balance  INTRODUCTION TO CHEMICAL EQUILIBRIUM Law of Mass Action The value of the equilibrium constant expression, K, is constant for a given reaction at equilibrium and at a constant temperature. The equilibrium concentrations of reactants and products may vary, but the value for K remains the same. K/Keq involves concentrations. Kp involves pressures. Sample Problem 1 Write the equilibrium constants (K eq and K p ) of the following reaction: ↔  INTRODUCTION TO CHEMICAL EQUILIBRIUM Equilibrium pressures at a certain temperature:  INTRODUCTION TO CHEMICAL EQUILIBRIUM Equilibrium pressures at a certain temperature:  INTRODUCTION TO CHEMICAL EQUILIBRIUM How about for solids and liquids? The position of a chemical equilibrium does NOT depend on the amounts of pure solids or liquids present. The concentrations of pure liquids and solids are constant. Sample Problem 2 Write the equilibrium constants for the following: Sample Problem 3 Sample Problem 4 Sample Problem 5 Calculate the equilibrium constants for the following: Sample Problem 6 Calculate the equilibrium constants for the following:  INTRODUCTION TO CHEMICAL EQUILIBRIUM Relationship of K and Kp Sample Problem 7 Sample Problem 8 Sample Problem 9 Sample Problem 10 APPLICATIONS OF THE EQUILIBRIUM CONSTANT  INTRODUCTION TO CHEMICAL EQUILIBRIUM Reaction Quotient  INTRODUCTION TO CHEMICAL EQUILIBRIUM Reaction Quotient Sample Problem 11 At 448°C the Kc for the reaction H2(g) + I2(g) ⇔ 2HI (g) is 51. Predict how the reaction will proceed to reach equilibrium if we start with 1.0 x 10-2 M HI, 5.0 x 10-3 M H2, and 1.5 x 10-2 M I2 SOLUBILITY PRODUCT CONSTANTS SOLUBILITY PRODUCT CONSTANTS SOLUBILITY PRODUCT CONSTANTS SOLUBILITY PRODUCT CONSTANTS Sample Problem 12 Sample Problem 13 Sample Problem 14 Sample Problem 15  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Ionic Strength The effect of added electrolyte on equilibria is independent of the chemical nature of the electrolyte but depends on a property of the solution called the ionic strength, which is defined as: where [A], [B], [C],... represent the species molar concentrations of ions A, B, C,... and ZA, ZB, ZC,... are their charges. EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Ionic Strength EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Ionic Strength EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Ionic Strength EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Ionic Strength  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Ionic Strength ➔ The ionic strength of a solution of a strong electrolyte consisting solely of singly charged ions is identical to its total molar salt concentration. ➔ The ionic strength is greater than the molar concentration if the solution contains ions with multiple charges. ➔ For solutions with ionic strengths of 0.1 M or less, the electrolyte effect is independent of the kind of ions and dependent only on the ionic strength.  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Activity Coefficients Chemists use the term activity, a, or effective concentration to account for the effects of electrolytes on chemical equilibria. ax=γx [X] where AX is the activity of reaction species X, [X] is its molar concentration, and γx is a dimensionless quantity called the activity coefficient.  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Properties of Activity Coefficients 1) In solutions that are not too concentrated, the activity coefficient for a given species is independent of the nature of the electrolyte and dependent only on the ionic strength. 1) The activity coefficient of a species is a measure of the effectiveness with which that species influences an equilibrium in which it is a participant. As the solution approaches infinite dilution, aX → [X] and K'sp → Ksp.  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Properties of Activity Coefficients 1). 2). EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Properties of Activity Coefficients 1). 2).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Properties of Activity Coefficients 1). 2).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Activity Coefficients 1).Use EqUse Equation 10-5 to calculate the activity coefficient for Hg21 in a solution 2) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 3) the ion.uation 10-5 to calculate the activity coefficient for Hg21 in a solution 4) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 5) the ion. 6).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM 1).Use EqUse Equation 10-5 to calculate the activity coefficient for Hg21 in a solution 2) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 3) the ion.uation 10-5 to calculate the activity coefficient for Hg21 in a solution 4) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 5) the ion. 6).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM 1).Use EqUse Equation 10-5 to calculate the activity coefficient for Hg21 in a solution 2) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 3) the ion.uation 10-5 to calculate the activity coefficient for Hg21 in a solution 4) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 5) the ion. 6).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM 1).Use EqUse Equation 10-5 to calculate the activity coefficient for Hg21 in a solution 2) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 3) the ion.uation 10-5 to calculate the activity coefficient for Hg21 in a solution 4) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 5) the ion. 6).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Properties of Activity Coefficients Calculate the activity coefficient for Hg2+ in a solution that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of the ion. 1).Use EqUse Equation 10-5 to calculate the activity coefficient for Hg21 in a solution 2) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 3) the ion.uation 10-5 to calculate the activity coefficient for Hg21 in a solution 4) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 5) the ion. 6).  EFFECTS OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM Properties of Activity Coefficients Calculate the activity coefficient for Hg2+ in a solution that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of the ion. 1).Use EqUse Equation 10-5 to calculate the activity coefficient for Hg21 in a solution 2) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 3) the ion.uation 10-5 to calculate the activity coefficient for Hg21 in a solution 4) that has an ionic strength of 0.085 M. Use 0.5 nm for the effective diameter of 5) the ion. 6).  SOLVING MULTIPLE-EQUILIBRIUM PROBLEMS BY A SYSTEMATIC METHOD Three types of algebraic equations used in solving multiple- equilibrium problems: 1) Equilibrium Constant Expression 2) Mass Balance Expression 3) Charge Balance Expression  SOLVING MULTIPLE-EQUILIBRIUM PROBLEMS BY A SYSTEMATIC METHOD Equilibrium-constant expression, Keq - the concentration ratio of products and reactants. - [solids] and [liquids] are considered constant, therefore, not included in writing K. Keq = [products]/[reactants]  MASS-BALANCE EQUATIONS (MBE) - relate the equilibrium concentrations of various species in a solution to one another and to the analytical concentrations of various solutes. - equilibrium concentration, [X]eq, refers to the molar concentration of a particular species in a solution at equilibrium. Analytical concentration, CX, is the total number of moles of a solute, regardless of its chemical state, in 1 L of solution. - the sum of the molar concentration of all species in a solution containing a particular atom (or group of atoms) is equal to the amount of that atom (or group) delivered to the solution.  MASS-BALANCE EQUATIONS (MBE) Writing mass balance equations may be as straightforward as the case of a weak acid: MBE: This type of mass-balance expression is often referred to as the proton balance equation because it accounts for all sources of protons.  MASS-BALANCE EQUATIONS (MBE) Write the mass balance expression for a solution that is: (a) 0.20 M H3AsO4 0.20 = [H3AsO4] + [H2AsO4- ] + [HAsO42-] + [AsO43-] (b) 0.10 M Na2HAsO4 0.10= [H3AsO4] + [H2AsO4- ] + [HAsO42-] + [AsO43-] 2(0.10) =[Na+] = 0.20  CHARGE-BALANCE EXPRESSIONS (CBE) ❏ Electrolyte solutions are electrically neutral even though they may contain up to several moles per liter of charged ions. ❏ Solutions are neutral because the molar concentration of positive charge in an electrolyte solution always equals the molar concentration of negative charge.  CHARGE-BALANCE EXPRESSIONS (CBE) Write the charge- balance expression for a solution that is: (a) 0.20 M H3AsO4 [H3O+] = [OH-] + [H2AsO4- ] + 2[HAsO42-] + 3[AsO43-] (b) 0.10 M Na2HAsO4 [Na+] + [H3O+] = [OH-] + [H2AsO4-] + 2[HAsO42-] + 3[AsO43-] Write the charge balance expression. Write the charge balance expression. CHEM 326A – ANALYTICAL CHEMISTRY MODULE 2 EFFECT OF ELECTROLYTES ON CHEMICAL EQUILIBRIUM EQUILIBRIUM CALCULATION IN COMPLEX SYSTEMS