Chapter 12: Chemical Equilibrium

The Dynamic Nature of Equilibrium A. What is equilibrium? 1. Definition a state of balance; no net change in a dynamic process

2. Definition of chemical equilibrium No net change with the total amount of reactants and products remaining constant, while the reaction continues Eek = equilibrium

14.1 The Dynamic Nature of Equilibrium 3. General Characteristics Double Headed Arrow Beginning of Rxn  Form lots of products (before eek is established) Moments Later  Forming both products and reactants (eek) **Not Necessarily equal proportions of both sides of the reaction!

14.1 The Dynamic Nature of Equilibrium B. Dynamic Equilibrium - Characteristics two opposing processes occur at exactly the same rate Rate of: Reactants  Products = Products  Reactants Dynamic Eek will not occur as soon as the reversible reaction begins Like all good things, it takes time!

C. Graph The N 2 O 4  2NO 2 Equilibrium System Initially there is a large amount of N 2 O 4 and no NO 2 Equilibrium is established when the amounts plateau

D. The 2NO 2  N 2 O 4 Equilibrium System 1. Description Dynamic, reversible, no net change 2. Equilibrium Conditions Specific for a Reaction (will be unique) MUST have the Balanced Written Equation Coefficients will matter! Temperature Dependent CONSTANT regardless of concentration

II.The Equilibrium Expression, K eq (Equilibrium Law or Law of Mass-Action) A. Writing Expressions for K eq aA + bB cC + dD K eq = (P C ) c (P D ) d _for gases use pressures (P A ) a (P B ) b K eq = [C] c [D] d _ for aqueous solutions use [A] a [B] b Molarities

The Equilibrium Expression Example: Write the K eq for: 4NH 3 (g) + 5O 2 (g)  4NO(g) + 6H 2 O(g) N 2 (g) + 3H 2 (g)  2NH 3 (g)

B. Characteristics of K c 1. Independent of : Pure solids and pure liquids as long as some of the substance is present 2. Dependent on: Gaseous substances and solutions

C. The meaning of K c 1. Kc = 1; reactants and products are present in equal amounts at eek 2. Kc > 1; products are present in greater proportion at eek 3. Kc < 1; reactants are present in greater proportion at eek

4. Examples: Who will be favored, in other words, be in greater proportion? Answer in a complete sentence. N 2 (g) + O 2 (g)  2 NO(g) K eq =55 N 2 (g) + 3H 2 (g)  2NH 3 (g) K eq =5x10 -6

III. Modifying Equilibrium Constant Expressions A. Reversible reactions Rule: The K eq value for the reverse of a reaction will be the reciprocal of the forward reaction.

III. Modifying Equilibrium Constant Expressions Example: Write the Keq for the following equation and its reverse. A(g) + B(g)  C(g)

III. Modifying Equilibrium Constant Expressions B. Summation of Reactions Rule: The K eq of summed reactions will be the product of the K eq ’s

Example: A(g) + B(g)  C(g)K p =3 A(g) + B(g)  D(g)K p =6 What is the K p for C(g)  D(g)?

Example: ½ N 2(g) + ½ O 2(g)  NO (g) K eq =6.9x NO 2(g)  NO (g) + ½ O 2(g) K eq =6.7x10 -7 N 2 O 4(g)  2NO 2(g) K eq =0.15 What is the K eq for N 2(g) +2O 2(g)  N 2 O 4(g)

C. Heterogeneous Systems Rule: Systems where all the substances are not in the same phase of matter. Specifically, the reaction includes at least one pure solid or liquid which would not be included in the K eq expression.

Example: Write the K eq for: a. Zn (s) + 2H + (aq)  Zn 2+ (aq) + H 2(g) b. CaCO 3(s)  CaO (s) + CO 2(g)

D. Relationship between Kc and Kp K p = K c (RT) ∆n g

IV. Determination of K A. Calculating K eq from experimental values (Intro to IRE problems). Example: Write the expression for the equilibrium reaction between solid ammonium chloride and gaseous products, hydrogen chloride and ammonia (NH 3 ). At equilibrium in a 1 liter container, the following amounts are present: 12.0 mol ammonium chloride, 3.0 mol of ammonia and 5.0 mol of hydrogen chloride. Determine the Keq.

Example: When 4.29 moles of PCl 3 (g) and 4.29 moles of Cl 2 (g) are placed in a 1.00 Liter container at 250 o C, the following equilibrium is established: PCl 3 (g) + Cl 2 (g)  PCl 5 (g) The equilibrium concentration of phosphorus pentachloride is 2.59 mole/L. What are the equilibrium concentrations of the other two gases? Calculate K eq for the above reaction system.

Example: Consider the equilibrium system: 2NO(g) + Br 2 (g)  2NOBr(g) At a given temperature, 1.6 mol of NO and 1.6 mol of Br 2 are added to a 1.00 Liter flask and the equilibrium concentration of NOBr is found to be 0.53 mol/L. Calculate the equilibrium concentrations of the other 2 gases and the value of K eq.

14.5 Some Illustrative Equilibrium Calculations D. Size of the Equilibrium Constant 1. For N 2 (g) + O 2 (g)  2 NO(g) k p at 298 K = 4.76 x

14.5 Some Illustrative Equilibrium Calculations 2. For N 2 (g) + 3H 2 (g)  2NH 3 (g) k p at 298 K = 104

14.5 Some Illustrative Equilibrium Calculations E. Direction of Shift to Reach Equilibrium 1. Example 14L.11 For N 2 O 4 (g)  2NO 2 (g) K c = 11.0 at 100 o C. Is the system at equilibrium if the concentration of both gases is 0.20 moles/L? In which direction will the system shift to reach equilibrium?

14.5 Some Illustrative Equilibrium Calculations 2. Example 14L.12 Answer the same question for the system above if the gas concentrations are 0.07 M for N 2 O 4 and 1.00 M for NO 2.

14.5 Some Illustrative Equilibrium Calculations 3. Example 14L.13 For the system H 2 (g) + I 2 (g)  2HI(g) K p = 55.5 If the system is started with 0.50 atm of each of the reactants and 9.30 atm of the HI in a 5.0 L container, predict the direction the system will shift to reach equilibrium.

14.5 Some Illustrative Equilibrium Calculations F. Equilibrium Partial Pressures or Concentrations 1. Description of Problems Start with Initial Reactant Conc. and “k” value Use “x” value to identify changes in concentration Substitute “x ” values into equation and solve *May have to use Quadratic Formula

14.5 Some Illustrative Equilibrium Calculations 2. Example 14L.14 Carbon dioxide and hydrogen gas at concentrations of 1.00 M each are introduced into a container and the following system is established: CO 2 (g) + H 2 (g)  CO(g) + H 2 O(g) K c = 0.64 What are the equilibrium concentrations of all the species?

14.5 Some Illustrative Equilibrium Calculations 3. Example 14L.15 For the system: H 2 (g) + I 2 (g)  2HI(g) at 425 o C, K = If 2.5 atm of hydrogen and 2.5 atm of iodine are placed in a vessel and heated to 425 o C and the system reaches equilibrium, what are the pressures of all species?

14.5 Some Illustrative Equilibrium Calculations 4. Example 14L.16 For the previous system at the same temperature, calculate the equilibrium partial pressures of all gases if the system is started with 2.3 atm of hydrogen gas and 1.4 atm of iodine gas.

14.4 Qualitative Treatment of Equilibrium: Le Chatelier A. Statement of Le Chatelier’s Principle “When a system is stressed (changes in concentration, temp, gas pressure, or volume of container), the system will respond by attaining new equilibrium conditions that counteract the change”

14.4 Qualitative Treatment of Equilibrium: Le Chatelier B. For the system, N 2 O 4 (g)  2NO 2 (g)  H = 57.2 kJ 1. Changes in amounts of species a. Adding or removing reactant Add Reactant  Eek shifts toward products (use it up) Remove Reactant  Eek shifts toward reactants (make more)

14.4 Qualitative Treatment of Equilibrium: Le Chatelier b. Adding or removing product Add Product  Eek shifts toward reactants (use it up) Remove Product  Eek shifts toward products (make more) c. Adding or removing pure solid or solvent Has NO effect on equilibrium (no shift) d. Adding inert substance (inert = non-reactive) Has NO effect on equilibrium (no shift)

14.4 Qualitative Treatment of Equilibrium: Le Chatelier 2. Changes in pressure Pressure increased  Eek shifts in the direction producing the smaller number of moles of gas Pressure decreased  Eek shifts in the direction producing the larger number of moles of gas Moles of gas equal on both sides = pressure does not effect Eek.

14.4 Qualitative Treatment of Equilibrium: Le Chatelier 3. Changes in volume Decreases in volume = increase in pressure Increases in volume = decrease in pressure

14.4 Qualitative Treatment of Equilibrium: Le Chatelier 4. Changes in temperature Increase Temp  Eek shifts in the direction of the endothermic rxn; away from heat Decrease Temp  Eek shifts in the direction of the exothermic rxn; toward heat

14.4 Qualitative Treatment of Equilibrium: Le Chatelier Example List all of the ways to shift the equilibrium of the following systems to the right: a. N 2 (g) + 3H 2 (g)  2NH 3 (g)

14.4 Qualitative Treatment of Equilibrium: Le Chatelier b. CO(g) + H 2 O(g)  CO 2 (g) + H 2 (g)

End of Chapter 14!