Equilibrium

The Concept of Equilibrium Chemical equilibrium occurs when opposing reactions are proceeding at equal rates The rate the products are formed from the reactants equals the rate the reactants are formed from the products

Example of Equilibrium N2(g) + 3H2(g)  2NH3(g)

Law of mass action expresses the relationship between the partial pressures for gases and molarities for solutions of the reactants and the products present at equilibrium

Law of mass action Kc=[C]c[D]d [A]a[B]b aA + bB cC + dD Kp =PCcPDd PAaPBb

Law of mass action The equilibrium-constant expression depends only on stoichiometry of the rxn The value of Keq depends only on T

Law of mass action p 15. 1 Write the equilibrium expression for Kc for the following reactions:

Law of mass action When Keq >>1, the equilibrium lies to the right When Keq<<1, the equilibrium lies to the left

P 15.3 The following diagrams represent three different systems at equilibrium, all in the same size containers. (a) Without doing any calculations, rank the three systems in order of increasing equilibrium constant, Kc.

Kp vs. Kc Kp is for gases (pressure) Kc is for solutions (molarity) Kp = Kc (RT)Dn Dn= products – reactants (gas moles)

Kp vs. Kc P15.2 In the synthesis of ammonia from nitrogen and hydrogen, Kc = 9.60 at 300 °C. Calculate Kp for this reaction at this temperature.

Manipulation of Keq Keq of a rxn in the reverse direction is the inverse of the Keq of the rxn in the forward direction

Manipulation of Keq Keq of a rxn that has been multiplied by a number is the equilibrium constant raised to the power equal to that number

Manipulation of Keq Keq for a net rxn made of 2 or more steps is the product of the equilibrium constants for the individual steps

P 15.4 Manipulation of Keq The equilibrium constant for the reaction of N2 with O2 to form NO equals Kc = 1 × 10–30 at 25 °C: Using this information, write the equilibrium constant expression and calculate the equilibrium constant for the following reaction:

P15.5 Manipulation of Keq Given the following information, determine the value of Kc for the reaction

Heterogeneous Equilibria Homogeneous equilibria = same phase Heterogeneous equilibria = different phases

Heterogeneous Equilibria If a pure s, pure l, or solvent is involved in an equilibrium, it is not included in the equilibrium-constant expression for the rxn Even though they are not part of the expression, they still must be present at equilibrium

P 15.7 Heterogeneous Equilibria Write the equilibrium-constant expression for Kc for each of the following reactions:

Calculating Equilibrium Constants If the conditions are at equilibrium, use the equilibrium expression If the conditions are not at equilibrium use ICE first

P 15.8 Calculating Equilibrium Constants A mixture of hydrogen and nitrogen in a reaction vessel is allowed to attain equilibrium at 472 °C. The equilibrium mixture of gases was analyzed and found to contain 7.38 atm H2, 2.46 atm N2, and 0.166 atm NH3. From these data, calculate the equilibrium constant Kp for the reaction

Calculating Equilibrium Constants ICE Diagrams I – Initial Concentration C – Change E - Equilibrium Concentration

p 15.9 Calculating Equilibrium Constants A closed system initially containing 1.000 × 10–3 M H2 and 2.000 × 10–3 M I2 at 448 °C is allowed to reach equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87 × 10–3 M. Calculate Kc at 448 °C for the reaction taking place, which is

Applications of Equilibrium Constants Keq allows us to predict the direction in which a rxn mixture will proceed to achieve equilibrium

Applications of Equilibrium Constants The reaction quotient (Q) is the substitution of concentration into the equilibrium expression under any condition

Applications of Equilibrium Constants If Q= Keq, the rxn is at equilibrium If Q>Keq, the rxn moves from right to left If Q<Keq, the rxn moves from left to right

P 15.10 Applications At 448 °C the equilibrium constant Kc for the reaction is 50.5. Predict in which direction the reaction will proceed to reach equilibrium at 448 °C if we start with 2.0 × 10–2 mol of HI, 1.0 × 10–2 mol of H2, and 3.0 × 10–2 mol of I2 in a 2.00-L container.

Calculating Equilibrium Concentrations Use ICE and Keq

P15.11 Calculating Equilibrium Concentrations In an equilibrium mixture of the three gases at 500oC, the partial pressure of H2 is 0.928atm and that of N2 is 0.432atm. What is the partial pressure of NH3 in the equilibrium mixture if Kp is 1.45x10-5?

Sample Exercise 15. 12 Calculating Equilibrium Concentrations from Sample Exercise 15.12 Calculating Equilibrium Concentrations from Initial Concentrations A 1.000-L flask is filled with 1.000 mol of H2 and 2.000 mol of I2 at 448 °C. The value of the equilibrium constant Kc for the reaction at 448 °C is 50.5. What are the equilibrium concentrations of H2, I2, and HI in moles per liter?

Don’t Forget! Keq= e-DG/RT Or DG=-RTlnKeq

Le Chatelier’s Principle If a system at equilibrium is disturbed by a change in T, P, or the conc., the system will shift its equilibrium position to counteract the effect of the disturbance

Le Chatelier’s Principle At equilibrium, if a substance is added, the rxn shifts to reestablish the equilibrium by consuming part of the added substance

Le Chatelier’s Principle At equilibrium, if a substance is removed, the rxn shifts in the direction to form more of that substance.

Le Chatelier’s Principle At constant T, reducing the V of a gaseous equilibrium, increases total P, and the system will shift in the direction that reduces the P which can be done by reducing the number of moles of gas

Le Chatelier’s Principle Endothermic rxn (absorbed) : reactants + heat  products Exothermic rxn (released): reactants  products + heat

Le Chatelier’s Principle When T is increased, we have added a reactant or product (heat) and the equilibrium shifts to consume the excess reactant or product (heat)

Le Chatelier’s Principle Endothermic: as T , Keq Exothermic: as T , Keq

Le Chatelier’s Principle When a catalyst is added to a system at equilibrium, the rate to achieve equilibrium increases, but the composition of the equilibrium mixture does not change

P 15.13 Consider the equilibrium Indicate the direction the equilibrium shift when (a) N2O4 is added (b) NO2 is removed (c) the total pressure is increased by addition of N2(g) (d) the volume is increased (e) the temperature is decreased

P 15.13 For the reaction Indicate the direction of the equilibrium shift when Cl2(g) is removed the temperature is decreased the volume of the reaction system is increased PCl3(g) is added Practice Exercise

P 15.14 (a) Using the standard heat of formation data in Appendix C, determine the standard enthalpy change for the reaction (b) Determine how the equilibrium constant for this reaction should change with temperature.