Please Pick Up Dynamic Equilibria Problem Set

Dynamic Equilibrium Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

6/27/2015 Chemical Reactions in Dynamic Equilibrium Reading Assignment: ·Zumdahl Chapter Processes which do not proceed to completion, but which appear to stop on a macroscopic scale. The Law of Mass Action and the equilibrium constant are introduced as means to interpret this process.

6/27/2015 Dynamic versus Static Equilibrium There are always two children in front of the chairs. The forces on both sides of the teeter-totter are the same.

6/27/2015 Dynamic Equilibrium  There is both a forward and reverse process.  The process can be described with an equilibrium arrow (actually two arrows). favorableunfavorableequally favorable

6/27/2015 Dynamic Equilibrium  Forward rate equals reverse rate.  Not all reactions are 100% complete.  Note: it does not mean that reactants and products are equally stable. reactants and products have the same or equal concentrations. H 2 O (l)H 2 O (g) H2OH2O H2OH2O

6/27/2015 Law of Mass-Action Mass-Action Expression Concentration or Pressure of Each Product Concentration or Pressure of Each Reactant Q = Reaction Quotient uses the current concentrations or pressures If the reactants and products are in dynamic equilibrium, the ratio equals the Equilibrium Constant = K

6/27/2015 Mass-Action Expression Balance the equation Write the mass-action expressionmass-action expression ·products over reactants Hydrogen gas reacts with oxygen gas to give steam.

6/27/2015 Mass-Action Expression 2 H 2 (g) + O 2 (g)2 H 2 O(g) () ()() Q P 2 HO 2 PP 2 H 2 O 2 = Hydrogen gas reacts with oxygen gas to give steam.

6/27/2015 Mass-Action Expression Write the mass-action expression Carbon dioxide gas dissolves in water to give aqueous carbon dioxide.

6/27/2015 Mass-Action Expression CO 2 (g)CO 2 (aq) Q [] CO 2 () P CO 2 = square brackets are used to indicate molar concentrations What would be the units of this mass-action expression? Carbon dioxide gas dissolves in water to give aqueous carbon dioxide.

6/27/2015 The Reaction Quotient  The pressure of CO 2 gas is 2.0 atm  The concentration of CO 2 is M What is the numeric value of the reaction quotient?numeric value

6/27/2015 The Reaction Quotient  The pressure of CO 2 gas is 2.0 atm  The concentration of CO 2 is M Q [] CO 2 () P CO 2 = = M 2.0 atm = 1.5 x M · atm -1 What is the numeric value of the reaction quotient?

6/27/2015 Determining If a System is at Equilibrium  The equilibrium constant for CO 2 gas dissolved in water is 3.4 x M·atm -1 Q [] CO 2 () P CO 2 = = M 2.0 atm = 1.5 x M · atm -1 Is the reaction at equilibrium? What must occur to attain equilibrium?

6/27/2015 Equilibrium Constant  K = Q when the system is at equilibrium  Special equilibrium constants K or K eq no special conditions K p partial pressures K c molarity concentrations

6/27/2015 Mass-Action Expression CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO ¯ (aq) K = [CH 3 COOH] [H 2 O] [H 3 O + ] [CH 3 COO ¯ ] Write the mass-action expression for the reaction of aqueous acetic acid with water to give hydronium ion and acetate ion.

6/27/2015 Mass-Action Expression Simplifications  If the concentration of a reactant or product does not or cannot vary, the mass-action expression can usually be simplified.  The term which doesn’t vary is “constant” and becomes part of the K.  A special subscript for K is added

6/27/2015 CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO ¯ (aq) CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO ¯ (aq) K = [CH 3 COOH] [H 2 O] [H 3 O + ] [CH 3 COO ¯ ] [CH 3 COOH] [H 2 O] [H 3 O + ] [CH 3 COO ¯ ] Mass-Action Expression Simplifications What is the molarity of water in pure water? In this equation which terms can vary? (i.e., which can have different concentrations)

6/27/2015 [CH 3 COOH] [H 3 O + ] [CH 3 COO ¯ ] Mass-Action Expression Simplifications CH 3 COOH(aq) + H 2 O(l) [H 2 O] · K = H 3 O + (l) + CH 3 COO ¯ (aq) CH 3 COOH(aq) + H 2 O(l) = Ka= Ka K a is the acid dissociation constant.

6/27/2015 Mass-Action Expression Simplifications  solvent  insoluble or sparingly soluble solid  insoluble or sparingly soluble liquid

6/27/2015 Write the Simplified Mass-Action Expression for  Aqueous iron(III) ion reacts with water to give insoluble iron(III) hydroxide and aqueous hydrogen ions. Write a balanced and annotated equationbalanced and annotated equation Write the mass-action expressionmass-action expression Simplify the mass-action expressionSimplify

6/27/2015 Write the Simplified Mass-Action Expression for Fe(OH) 3 (s) + 3 H + (aq) Fe 3+ (aq) + 3 H 2 O(l)  Aqueous iron(III) ion reacts with water to give insoluble iron(III) hydroxide and aqueous hydrogen ions. K = [Fe 3+ ] [H 2 O] 3 [Fe(OH) 3 ] [H + ] 3 coefficients are raised to exponential powers

6/27/2015 Write the Simplified Mass-Action Expression for Fe(OH) 3 (s) + 3 H + (aq) Fe 3+ (aq) + 3 H 2 O(l)  Simplify the expression Which terms can vary? Fe(OH) 3 (s) + 3 H + (aq) Fe 3+ (aq) + 3 H 2 O(l) [Fe 3+ ] [H + ] 3 K · [H 2 O] 3 · [Fe(OH) 3 ] 1 = = K hyd hydrolysis This part of the term we won’t write very often

6/27/2015 Simplified Mass-Action Expression Fe(OH) 3 (s) + 3 H + (aq) Fe 3+ (aq) + 3 H 2 O(l) [Fe 3+ ] [H + ] 3 = K eq use K eq if you aren’t sure what to call the constant Why isn’t water in the mass-action expression? Why isn’t Fe(OH) 3 in the MAE

6/27/2015 Factors Which May Result in a System Failing to Attain Equilibrium  Reaction coordinate diagram  Thermodynamic stability  Kinetic stability activation energy catalyst

6/27/2015 Factors Which May Result in a System Failing to Attain Equilibrium Energy  Reactants Products Reaction Coordinate Diagram Thermodynamically Unstable 2 H 2 (g) + O 2 (g)2 H 2 O(g)

6/27/2015 Energy  Reactants Products Reaction Coordinate Diagram Factors Which May Result in a System Failing to Attain Equilibrium Thermodynamically Stable Large activation barrier The activation barrier may be overcome by heating the reactants

6/27/2015 Energy  Reactants Products Reaction Coordinate Diagram Large activation barrier Factors Which May Result in a System Failing to Attain Equilibrium The activation barrier may be lowered by the addition of a catalyst.

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