FINAL EXAM REVIEW KEY

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Lawrence Higgins
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1 FINAL EXAM REVIEW KEY Leaders: Deborah Course: CHEM 178 Instructor: Date: 1. What is the rate-determining step and why is it important? The slowest step of the elementary steps; looks the most like the rate law 2. Write the mass action expression for the reaction C (s) + CO2 (g) 2 CO (g) Kp = PCO 2 / PCO2 3. Calculate the ph of 0.1 M HNO2 at 298 K. Ka = 4.5 x 10-4 ph = The following data were measured for the reaction of BF3 (g) + NH3 (g) F3BNH3 (g): Experiment [BF3] (M) [NH3] (M) Initial Rate (M/s) a. What is the rate law for the reaction? Rate = k[bf3][nh3] using experiments 1&2 and 4&5 b. What is the overall order of the reaction? 2 c. Calculate the rate constant with proper units. Using experiment 1: k = ( M/s) / (0.250 M)(0.250 M) = M -1 s -1 d. What is the rate when [BF3] = 0.100M and [NH3] = M? Rate = (3.408 M -1 s -1 )(0.100 M)(0.500 M) = M/s 5. Complete the balanced equation: Cf B n Lr 6. In the reaction 2 NO2 (g) N2O4 (g), the initial concentration of NO2 was M and N2O4 was M. At equilibrium, the concentration of N2O4 was measured as M. a. Calculate the equilibrium concentration of NO2. [NO2]equilibrium = M

b. Calculate Kc. Kc = 0.27 7. Label if the following will favor products or reactants. a. K > 1: products b. K < 1: reactants c. K > Q: products d. K < Q: reactants 8.

2 b. Calculate Kc. Kc = Label if the following will favor products or reactants. a. K > 1: products b. K < 1: reactants c. K > Q: products d. K < Q: reactants 8. Write the rate law for a reaction between A, B, and C that is the first order in A, zero order in B, and second order in C. Rate = k[a][c] 2 a. What happens to the rate when [A] doubles and all other concentrations stay the same? Rate doubles b. What happens to the rate when C is tripled and all other concentrations stay the same? Rate increases by a factor of 9 c. By what factor does the rate change if all three are tripled? Rate increases by a factor of 27 d. By what factor does the rate change if all three are cut in half? Rate decreases by a factor of ⅛ 9. Is the following reaction spontaneous at 25 C under the following conditions: N2 (g) + O2 (g) 2 NO (g) Nitrogen and oxygen have a pressure of 1 atm and nitrogen oxide has a pressure of 4 atm at some point during the reaction. (ΔG = kj) **need to find ΔG because NO is at a nonstandard temperature** ΔG = 180 kj so nonspontaneous 10. What is the order of these reactions based on their graphs? a. Reaction 1 Zeroth order b. Reaction 2

Overall Kc = 0.14 12. How long (in hours) must a 20.0 amp current flow through a solution of ZnSO4 in order to produce 25 g of Zn metal? t = ~3689 s = ~1.02 hr 13. Calculate the ph of 0.

3 Second order c. Reaction 3 First order 11. Find the Kc value for the overall reaction given: 1) CaO (s) + H2 (g) Ca (s) + H2O (g) K1 = 67 2) CaO (s) + CO (g) Ca (s) + CO2 (g) K2 = 490 overall: H2 (g) + CO2 (g) CO (g) + H2O (g) Overall Kc = How long (in hours) must a 20.0 amp current flow through a solution of ZnSO4 in order to produce 25 g of Zn metal? t = ~3689 s = ~1.02 hr 13. Calculate the ph of M HNO3 at 298 K. ph = -log(0.046 M) = For each pair, which molecule has the greater entropy? a. CO2 (s) or CO2 (l) b. NH3 (l) or NH3 (g) c. Pure magnesium at 0 K or 200 K 15. Are the following isotopes stable? If unstable, what type of decay will they undergo? a C: stable b N: unstable β + emission or electron capture

4 c Ta: unstable (heavy nuclei) alpha emission 16. A reaction has an activation energy of 137 kj/mol. At 420 C, k = 6.3 M -1 s -1. What is the value of k at 480 C? ln (k1 / k2) = (Ea / R)([1 / T2] - [1 / T1]) k2 = 41.7 M -1 s What is the ph and chemical reaction of a solution containing 0.25 M HCO3 - and 0.34 M Na2CO3? Ka(HCO3 - ) = 5.6 x This creates a buffer so use Henderson-Hasselbalch equation; ph = Initially moles of SO2, 0.5 moles of SO2Cl2, and 0.08 moles of Cl2 are combined in a 5-liter flask. What is the value of Q and which direction will the reaction proceed to establish equilibrium? SO2Cl2 (g) SO2 (g) + Cl2 (g) Kc = Q = 1.12 x 10-3 ; K > Q so rxn shifts towards products 19. For nitrous acid, HNO2, the Ka = 4.5 x What is the Kb value for the nitrite ion, NO2 -? Kb = Kw / Ka = 2.2 x Consider the reaction PbCl2 (s) Pb 2+ (aq) + 2Cl - (aq) with an equilibrium constant of 2.47x10-3. What is the value of K for 5 PbCl2 (s) 5 Pb 2+ (aq) + 10 Cl - (aq)? Kc = (2.47 x 10-3 ) 5 = 9.19 x What s the difference between the Arrhenius, Lewis, and Bronsted-Lowry definitions of acids and bases? Arrhenius definitions: acids increase [H + ] in water & bases increase [OH - ] in water; limited to aqueous solutions Bronsted-Lowry definitions: acids donate protons & bases accept protons; requires hydrogen atoms Lewis definitions: acids accept electron pairs & bases donate electron pairs 22. Is a spontaneous process reversible? a. Yes, always b. No, only if ΔG < 0 c. No, never d. Yes, if ΔSuniv < Write the reduction half-reaction for the redox reaction: Mn 2+ (aq) + NaBiO3(s) Bi 3+ (aq) + MnO4 - (aq). NaBiO3 (s) + 2 e - Bi 3+ (aq) 24. How do the following changes affect an equilibrium reaction of the following: 2 CH3CH3 (g) + 5 O2 (g) 6 H2O (l) + 2 CO2 (g) ΔH < 0 Increasing [O2], decreasing volume of container, increase temperature

5 Shift right; shift right; shift left (because exothermic) 25. Calculate the concentration of OH - (aq) in a solution in which [H + ] is a 100 times greater than [OH - ]. [OH - ] = 10-8 M 26. A voltaic cell utilizes the following reaction and operates at 298 K: 3 Ce 4+ (aq) + Cr(s) 3 Ce 3+ (aq) + Cr 3+ (aq) What is the emf of this cell under standard conditions? E cell = V 27. What particle is released during the decay of sodium-24 into magnesium Na 24 12Mg + 0-1β 28. Given the following elementary steps: 1) H2O2 + I - HO - + HOI 2) HO - + H2O2 H2O + HOO - 3) HOI + HOO - H2O + O2 + I - a. What is the overall reaction? 2 H2O2 2 H2O + O2 b. What are the intermediate(s) and catalyst(s)? Intermediates: HO -, HOI, & HOO - (produced then consumed) Catalyst: I - (consumed then produced) c. If the overall reaction rate is rate=k[h2o2] what is the rate determining step? The first step because it looks the most like the overall rate law 29. Calculate the ph of a 0.15 M NaNO2 at 298 K. (Ka = 4.5 x 10-4 ) ph = Cobalt-60, which undergoes beta decay, has a half-life of 5.26 years. What is the rate for 3.76 x atoms of 60 Co? Rate = kn = (0.132 yrs-1)(3.76 x atoms 60 Co) = 4.95 x atoms 60 Co/yr

Chapter 12: Chemistry of Solutions

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