Chapter 13 Chemical Equilibrium  The state where the concentrations of all reactants and products remain constant with time  Equilibrium is reached when rates of forward and reverse reactions are equal

Equilibrium Condition  Dynamic Condition- rates of forward and reverse reactions are equal  Law of Mass Action- general description of the equilibrium condition  - generic equation jA + kB  lC + mD A,B,C,D  chemical substances J,k,l,m  coefficients

Equilibrium Expression K=[C] l [D] m [A] j [B] k Write the equilibrium expression for: 1) PCl 5 (g)  PCl 3 (g) + Cl 2 (g) 2) Cl 2 O 7 (g) + 8H 2 (g)  2HCl(g) + 7H 2 O(g)

Calculating the Values of K  Calculate the equilibrium constant, K, for the following reaction at 25°C. H 2 (g) + I 2 (g)  2HI(g) If the equilibrium concentrations are [H 2 ] = M, [I 2 ]= 0.022M, and [HI]=1.29M Using the same example, calculate the equilibrium concentration of HI if H 2 =.81M and I 2 =.035 M and K= 7.1x 10 2

Equilibrium Expressions involving Pressures Ideal Gas Law PV= nRT P= pressure(atm) V= volume(L) n= # of moles of gas(mol) R= universal gas constant=.08206Latm/Kmol T= temp. in K(273)

Rearrange the Ideal Gas Law for Pressure P=(n/v) RT or P= CRT where C= molar concentration of the gas Kp= [C] l [D] m /[A] j [B] k

What is the relationship between K c and K p Kp= K(RT)  n  n= sum of the gaseous products coefficients minus the sum of the gaseous reactants coefficients

Ex Calculating Values of Kp Calculate the value of Kp for the following reaction at 25°C if P NOCl = 1.2 atm, P NO = 5.0x10 -2 atm, and P Cl2 = 3.0x10 -1 atm. 2NO(g) + Cl 2 (g)  2NOCl(g)

Calculating K from Kp  Calculate the value of K at 25°C for the reaction 2NO(g) + Cl 2 (g)  2NOCl(g) If Kp= 1.9x10 3 Kp= K(RT)  n T = = 298K  n= 2-(2+1)= -1

Heterogeneous Equilibria  Involves more than one phase CaCO 3 (s)  CaO(s) + CO 2 (g) K= [CaO] [CO 2 ] / [CaCO 3 ] K= [CO 2 ]

Write the expressions for K and Kp  NH 4 NO 2 (s)  N 2 (g) + 2H 2 O(g)  HCl(g) + NH 3 (g)  NH 4 Cl(s)

Applications of the Equilibrium Constant  Reaction Quotient(Q)- using initial concentrations in the law of mass action N 2 (g) + 3H 2 (g)  2NH 3 (g) Compare Q to K  3 Possible Cases 1) Q= K, equilibrium, no shift 2) Q  K, system shifts to left 3) Q  K, system shifts to right

Predicting the shift H 2 (g) + I 2 (g)  2HI(g) K= 7.1x10 2 at 25°C a. Q= 427 Q  K shift to right b. Q=1522 Q  K, shift left

Con. [H 2 ] 0 =.81M [I 2 ] 0 =.44M [HI] 0 =.58M Q= ? [H 2 ] 0 =.078M [I 2 ] 0 =.033M [HI] 0 =1.35M Q=?

Example 13.8 N 2 O 4 (g)  2NO 2 (g) Kp=.133atm P N 2 O 4 =2.71atm What is P NO 2 ?

Example 2,p. 627  1.00 L flask initially contained mol PCl 3 (g) and 8.70x10 -3 mol PCl 5 (g). After reaching equilibrium 2.00x10 -3 mol of Cl 2 (g) was found in the flask. Calculate the equilibrium concentrations of all species and value of K. PCl 5 (g)  PCl 3 (g) + Cl 2 (g) K= [PCl 3 ][Cl 2 ]/[PCl 5 ]

Solving Equilibrium Steps 1. Write a balanced equation 2. Write the equilibrium expression 3. List the initial concentrations 4. Calculate Q and determine the direction of the shift 5. Use ICE box 6. Solve for unknown(s)

Ex , p. 628 CO(g) + H 2 O(g)  CO 2 (g) + H 2 (g) K= 5.10 Calculate the equilibrium concentration of all species if 1.00 mol of each component is mixed in a L flask.

Example, p. 632 H 2 (g) + F 2 (g)  2HF(g) K= 1.15x10 2 Suppose mol of H 2 and mol of F 2 are mixed in L flask. Calculate the equilibrium concentration for all species.

Le Chatelier Principle  If a system at equilibrium is subjected to a stress(change), the equilibrium will shift in an attempt to reduce the stress 1. Effect of a change in concentration a. If a reactant/product is added, the system will shift away from the added component b. If a reactant/product is removed, the system will shift toward the removed component

2. Effect of a Change in Pressure a. Add or remove a gaseous reactant/product- add, shift away -remove, shift towards b. Add an inert gas - there is no effect on equilibrium c. Change the volume of the container - when the container volume is reduced, the system will shift toward the side involving the smaller # of gaseous molecules - when the volume is increased, it will shift toward the side with the larger # of molecules

Example of Pressure N 2 (g) + 3H 2 (g)  2NH 3 (g) If volume is decreased, then ? Shift right, 4  2 If volume is increased, then ? Shift left, 2  4 Determine the shift if the volume is reduced: P 4 (s) + 6Cl 2 (g)  4PCl 3 (l) Shift right, P 4 (s) and PCl 3 (l), look only at Cl 2 PCl 3 (g) + 3NH 3 (g)  P(NH 2 ) 3 (g) + 3HCl(g) No effect 4 on each side

3. Effect of a change in Temperature a. If  H is positive, it is an endothermic reaction  energy is viewed as a reactant b. If  H is negative, it is an exothermic reaction  energy is viewed as a product N 2 (g) + O 2 (g)  2NO(g)  H= 181kJ 181kJ + N 2 (g) + O 2 (g)  2NO(g) Shift right 2SO 2 (g) + O 2 (g)  2SO 3 (g)  H= -198kJ 2SO 2 (g) + O 2 (g)  2SO 3 (g) kJ Shift left

Summarizing Le chatelier N 2 O 4 (g)  2NO 2 (g)  H = 58kJ 58kJ + N 2 O 4 (g)  2NO 2 (g)

K sp The Solubility-Product Constant  Deals with equilibria associated with solids dissolving to form aqueous solutions AgCl(s)  Ag + (aq) + Cl - (aq) K= [Ag + ] [Cl - ]/ [AgCl] so Ksp= [Ag + ] [Cl - ]

K sp Examples Determine the Ksp of calcium fluoride given that its molar solubility is 2.14x M. CaF 2 (s)  Ca 2+ (aq) + 2F - (aq) Calculate the molar solubility(mol/L) of silver chloride that has a K sp = 1.77x AgCl(s)  Ag + (aq) + Cl - (aq)

Con. Calculate the molar solubility of Tin(II) hydroxide if Ksp=5.45x Sn(OH) 2 (s)  Sn 2+ (aq) + 2OH - (aq)