Chemical Equilibrium Chapter 6 Web-site:

Chemical Equilibrium – Ch. 6 N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g)

Chemical Equilibrium – Ch. 6

1. Consider the following reaction: N 2 (g) + 3 Cl 2 (g) ⇌ 2 NCl 3 (g) a. Write the equilibrium expression (K and K p ) b. What is the value of K p at 25°C if the equilibrium pressures for N 2, Cl 2 and NCl 3 were 1.8 atm, 0.5 atm and atm respectively. c. What is the value of K at 25°C? K p = K(RT) Δn d. Does the reaction favor reactants or products?

Chemical Equilibrium – Ch. 6 Where does the equilibrium lie? If the products are favored ⇒ K > 1 If the reactants are favored ⇒ K < 1 Where does the equilibrium lie? If the products are favored ⇒ K > 1 If the reactants are favored ⇒ K < 1

Chemical Equilibrium – Ch Which reaction does the K p = K? a. P 4 (s) + 6 Cl 2 (g) ⇌ 4 PCl 3 (g) b. H 2 O (l) ⇌ H 2 O (g) c. H 2 (g) + Cl 2 (g) ⇌ 2 HCl (g) d. C 3 H 8 (g) + 5 O 2 (g) ⇌ 3 CO 2 (g) + 4 H 2 O (g) K p = K(RT) Δn

Chemical Equilibrium – Ch Given the following information: H 2 (g) + I 2 (g) ⇌ 2 HI(g)K = 54.0 N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g)K = 96.0 Determine the value of the equilibrium constant for the following reaction: 2 NH 3 (g) + 3 I 2 (g) ⇌ 6 HI(g) + N 2 (g)

Chemical Equilibrium – Ch. 6

4. Consider the following reaction: UO 2 (s) + 4HF (g) ⇌ UF 4 (g) + 2H 2 O (g) K p = 11 If you fill a 5 L container with 15 g of UO 2, 1.1 atm of HF, 0.75 atm of UF 4 and 5.9 atm of H 2 O, what will be the mass of the solid UO 2 at equilibrium? a. 15 g b. > 15 g c. < 15 g d. not enough information

Chemical Equilibrium – Ch. 6 K K K K K K Q Q Q Q Q Q K > Q System has too much reactant K = Q System is at equilibrium K < Q System has too much product Shift reaction to the right Shift reaction to the left causes Q to increase causes Q to decrease

Chemical Equilibrium – Ch Consider the following reaction: N 2 (g) + O 2 (g) ⇌ 2 NO (g) K p = ? Initially the partial pressures of N 2 and O 2 are 1 atm and 3 atm respectively. At equilibrium the pressure of NO is 1.5 atm. What is the K p for this reaction?

Chemical Equilibrium – Ch Pure PCl 5 is introduced into an empty rigid 5-L flask at 24 °C with a pressure of 0.45 atm. The PCl 5 is heated to 90 °C upon which it decomposes into solid P and gaseous Cl 2. 2 PCl 5 (g) ⇌ 2 P (s) + 5 Cl 2 (g) At equilibrium the total pressure is measured to be 0.93 atm. What is the equilibrium constant (K p ) at 90 °C? What percentage of the PCl 5 decomposed? How many grams of P are in the flask at equilibrium?

Chemical Equilibrium – Ch At a certain temperature the partial pressures of an equilibrium mixture of N 2 O 4 (g) and NO 2 (g) are 0.34 atm and 1.2 atm respectively. N 2 O 4 (g) ⇌ 2 NO 2 (g) The volume of the container is doubled. Find the new partial pressures when the equilibrium is re-established.

Chemical Equilibrium – Ch Consider the following reaction: 2 NOBr (g) ⇌ 2 NO (g) + Br 2 (g) K p = 34 An unknown pressure of NOBr is put into a rigid container at 40 °C, equilibrium is reached when 86 % of the original partial pressure of NOBr has decomposed. What was the initial pressure of the NOBr? What is the total pressure in the flask at equilibrium?

Chemical Equilibrium – Ch At a particular temperature K p = 5.7x10 -8 for the following reaction: 2 C (s) + 3 H 2 (g) ⇌ C 2 H 6 (g) What is the pressure of C 2 H 6 at equilibrium if initially there's 5 g of C and 3 atm of H 2 ?

Chemical Equilibrium – Ch Consider the following endothermic reaction at equilibrium: CCl 4 (g) ⇌ C (s) + 2 Cl 2 (g) a. Is heat absorbed or released as the reaction goes forward? b. Which way will the reaction shift if the temperature is decreased? c. What happens to K as the temperature is increased? d. Which way will the reaction shift if the partial pressure of CCl 4 is increased? e. Which way will the reaction shift if you add more C? f. Which way will the reaction shift if you remove Cl 2 ? g. Which way will the reaction shift if you compress the system? h. Which way will the reaction shift if you add a catalyst?

Answer Key – Ch. 6 You have completed ch. 6

Answer Key – Ch. 6

1. …continued d. Does the reaction favor reactants or products? Since K p < 1 and/or K < 1 the reaction favors the reactants 2. Which reaction does the K p =K? a. P 4 (s) + 6Cl 2 (g) ⇌ 4PCl 3 (g) b. H 2 O (l) ⇌ H 2 O (g) c. H 2 (g) + Cl 2 (g) ⇌ 2HCl (g) d. C 3 H 8 (g) + 5O 2 (g) ⇌ 3CO 2 (g) + 4H 2 O (g) If Δn=1 ⇒ K p =K Δn=1 when there are the same number of moles of gaseous reactants as gaseous products

3. Given the following information: H 2 (g) + I 2 (g) ⇌ 2 HI(g)K = 54.0 N 2 (g) + 3 H 2 (g) ⇌ 2 NH 3 (g)K = 96.0 Determine the value of the equilibrium constant for the following reaction: 2 NH 3 (g) + 3 I 2 (g) ⇌ 6 HI(g) + N 2 (g) Answer Key – Ch. 6 Rxn 2 Flipped Rxn 1 x 3 Rxn 1 x 3 Rxn 2 Flipped 3 H 2 (g) + 3 I 2 (g) ⇌ 6 HI(g)K = NH 3 (g) ⇌ N 2 (g) + 3 H 2 (g) K = NH 3 (g) + 3 I 2 (g) ⇌ 6 HI(g) + N 2 (g) K = ( )( ) = 1640

Answer Key – Ch. 6 Since K < Q the reaction will go backwards thereby increasing the mass of the UO 2

Answer Key – Ch Consider the following reaction: N 2 (g) + O 2 (g) ⇌ 2 NO (g) K p = ? Initially the partial pressures of N 2 and O 2 are 1 atm and 3 atm respectively. At equilibrium the pressure of NO is 1.5 atm. What is the K p for this reaction? N2N2 O2O2 ⇌ 2 NO I1 atm3 atm0 Δ- x + 2x Eq1 – x3 – x2x

Answer Key – Ch Pure PCl 5 is introduced into an empty rigid flask at 24 °C with a pressure of 0.45 atm. The PCl 5 is heated to 90 °C upon which it decomposes into solid P and gaseous Cl 2. 2 PCl 5 (g) ⇌ 2 P (s) + 5 Cl 2 (g) At equilibrium the total pressure is measured to be 0.93 atm. What is the equilibrium constant (K p ) at 90 °C? What percentage of the PCl 5 decomposed? 2 PCl 5 (g) ⇌ 2 P (s)+ 5 Cl 2 (g) I0.55 atmN/A0 Δ- 2xN/A+ 5x Eq0.55 – 2xN/A5x Continue to next slide…

Answer Key – Ch. 6

…continue to next slide

Answer Key – Ch. 6 N 2 O 4 (g) ⇌ 2 NO 2 (g) I Δ- x+ 2x Eq x x 7. …continued P N 2 O 4 = 0.12 atm P NO 2 = 0.71 atm

Answer Key – Ch Consider the following reaction: 2 NOBr (g) ⇌ 2 NO (g) + Br 2 (g) K p = 34 An unknown pressure of NOBr is put into a rigid container at 40 °C, equilibrium is reached when 86 % of the original partial pressure of NOBr has decomposed. What was the initial pressure of the NOBr? What is the total pressure in the flask at equilibrium? 2 NOBr (g) ⇌ 2 NO (g)+ Br 2 (g) Ix00 Δ-0.86x+ 0.86x+0.43x Eq0.14x0.86x0.43x …continue to next slide

Answer Key – Ch. 6

9. At a particular temperature K p = 5.7x10 -8 for the following reaction: 2 C (s) + 3 H 2 (g) ⇌ C 2 H 6 (g) What is the pressure of C 2 H 6 at equilibrium if initially there's 5 g of C and 3 atm of H 2 ? 3 H 2 ⇌ C2H6C2H6 I30 Δ- 3x+ x Eq3 - 3xx Negligible b/c K p is so small

10. Consider the following endothermic reaction at equilibrium: CCl 4 (g) ⇌ C (s) + 2 Cl 2 (g) a. Is heat absorbed or released as the reaction goes forward? Since the reaction is endothermic heat is absorbed if the reaction goes forward and vice versa b. Which way will the reaction shift if the temperature is decreased? As the temperature is decreased the system is losing heat causing the reaction to shift left c. What happens to K as the temperature is increased? As the temperature is increased heat is being added causing the reaction to shift right producing more products thereby causing an increase in K d. Which way will the reaction shift if the partial pressure of CCl 4 is increased? More reactant pushes the reaction to the right … continue to next slide Answer Key – Ch. 6

e. Which way will the reaction shift if you add more C? Adding more solid will have no effect on the equilibrium f. Which way will the reaction shift if you remove Cl 2 ? Removing a product causes the reaction to shift to the right g. Which way will the reaction shift if you compress the system? Decreasing the volume of the system causes the reaction to shift to the side with fewer moles of gas – in this case the left h. Which way will the reaction shift if you add a catalyst? Catalysts will speed up the forward and reverse direction therefore having no effect on the equilibrium Answer Key – Ch. 6