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Chemistry 30 Chemical Equilibrium 50 I. Multiple Choice 1. 2. 3. 4. 5. 15 B D C A A 6. 7. 8. 9. 10. B B B A A 11. 12. 13. 14. 15. II. Short Answer C B C C A 35 1. Write the equilibrium constant expression for the following equilibrium. Be sure to pay attention to the physical state. a. 3 O2(g) 2 O3(g) b. 2 NO(g) + Cl2(g) c. CaCO3 (s) Keq = 2 NOCl(g) CaO(s) + CO2(g) Chemical Equilibrium Exam Key Keq = [O 3 ] 2 [O 2 ] 3 [ NOCl ] 2 [ NO ] 2 [Cl 2 ] Keq = [CO 2 ] Page 1 of 4 Page 2 of 4 2. For systems involving gases, the equilibrium constant is often determined by using partial pressure instead of concentration. Given the following reaction at equilibrium at the partial pressures of the participants, H2(g) + I2(g) 2 HI(g) Partial Pressures: PHI = 4 10-3 atm PH2 = 7.5 10-3 atm PI2 = 4.3 10-5 atm a. Calculate Keq for this reaction, carried out at a constant temperature. Begin by writing the equilibrium constant expression for the reaction. Show your work. K eq 3 (4 10 3 ) 2 [HI ] 2 49 .6 [H 2 ][ I 2 ] (7.5 10 3 )( 4.3 10 5 ) b. Are the reactants (H2 and I2) or products (HI) favored at equilibrium at this temperature? 2 How do you know? Products are favored; Keq > 1 3. For the equilibrium system at a certain temperature, described by the equation PCl3(g) + Cl2(g) Keq = 60 PCl5(g) [PCl3] = 0.2 M [Cl2] = 0.1 M a. Calculate the equilibrium concentration of PCl5. K eq [PCl 5 ] [PCl 3 ][ Cl 2 ] 60 x (0.2)( 0.1) 3 x [PCl5 ] 1.2 M b. Are reactants or products favored at equilibrium for this system? 1 products are favored; Keq = 60 is greater than 1 Chemical Kinetics Exam Key Page 2 of 4 Page 3 of 4 4. Each of the following systems has come to equilibrium. The systems are then subjected to the changes indicated. Tell how the concentrations of the following substances will change as a result of the stress – will their concentrations increase, decrease, or undergo no change when the indicated change is made: a. Cu2+(aq) + 4 NH3(g) Cu(NH3)42+(aq) More Cu2+ is added [NH3] will decrease [Cu(NH3)42+] will increase b. CO(g) + ½ O2(g) 5 CO2(g) + energy The system is put on ice [CO] will decrease [O2] will decrease [CO2] will increase 5. For the reaction PBr3(g) + Br2(g) PBr5(g) + heat 3 How will the reaction shift (forward; reverse; no change) if: a. the pressure is increased forward b. concentration of Br2 is decreased reverse c. reverse temperature is increased 6. The following system is allowed to reach equilibrium: CO(g) + 2 H2(g) CH3OH (g) At a given temperature, Keq for the reaction = 12, and the following concentrations are noted: [CO] = 0.02 M [H2] = 0.35 M [CH3OH] = ??? Calculate the concentration of CH3OH at equilibrium. Begin by writing the equilibrium constant expression. K eq [CH 3 OH ] [CO ][ H 2 ] 2 12 x (0.02 )( 0.35 ) 2 x [CH 3 OH] 2.94 10 2 M 3 Chemical Kinetics Exam Key Page 3 of 4 Page 4 of 4 7. Consider the following system at equilibrium: 2 NaOH (aq) + CaCO3 (aq) Na2CO3 (aq) + Ca(OH)2 (s) + energy a. Write the equilibrium constant expression for this reaction. K eq 1 [ Na 2 CO 3 ] [ NaOH ] 2 [CaCO 3 ] b. Predict the effect of increasing the concentration of NaOH to the system: 4 Select the appropriate answer in each case c. The reaction will shift to the (right / left) right [CaCO3] will (increase / decrease / no change) decrease [Na2CO3] will (increase / decrease / no change) increase The value of Keq will (increase / decrease / not change) not change What will happen to the system if the temperature of the system is increased and the volume is kept constant? The reaction will shift to the (right / left) left [CaCO3] will (increase / decrease / no change) increase [Na2CO3] will (increase / decrease / no change) decrease The value of Keq will (increase / decrease / no change) decrease 4 8. A flask is filled with some HI(g) and allowed to reach equilibrium. 2 HI(g) H2(g) + I2(g) Keq = 0.25 At equilibrium the concentration of [HI] = 0.80 M. What is the concentration of H 2 at equilibrium? [Hint: you will also be finding [I2] – consider how [H2] and [I2] compare to each other] K eq [HI ] 2 [H 2 ][ I 2 ] Chemical Kinetics Exam Key x2 x2 0.25 (0.8) 2 0.64 3 x 2 0.16 x [ H 2 ] 0 .4 M Page 4 of 4