Review for Exam 2 Chem 1721/1821

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1 Review for Exam 2 Chem 1721/1821 The following are the major concepts with which you should be well acquainted from Chapters 13, 14, 15, : Chapter 13: Chemical Kinetics Reaction Rates The rate of a chemical reaction is the change of concentration per unit time - the units are always the same - know them! Relating rates of consumption of reactants and rates of formation of products, including stoichiometric relationships Instantaneous reaction rates vs. average reaction rates Initial rates of reaction Rate Laws Rate laws show the dependence of reaction rates on concentration Know the general form of a rate law: rate = k[a] x [B] y You should know what is meant by orders of reaction with respect to reactants and overall, including being able to identify a reaction order from a rate law, and being able to write a rate law given the orders. Determination of reaction order and rate law by the method of initial rates Know the units of k for different order reactions!!! Integrated Rate Equations Integrated rate equations show the relationship between concentration and time Know the integrated rate equations for zero- first- and second-order reactions Know the plots of data that correspond to straight lines for each order of reaction, also know what the slope equals (k or -k) Half lives - definition and equations for each order of reaction Controlling Reaction Rates Temperature dependence of rates and the rate constant - including Arrhenius behavior, the Arrhenius equation and Arrhenius plots of data - know them Determination of the rate constant at one temperature if you know that rate constant at another temperature (Arrhenius equation) Collision Theory, Activation energy and frequency factor, reaction profiles, activation barriers, transition states (or activated complex) - all that good stuff that deals with how reactions actually occur Catalysts - how they work, homogeneous vs. heterogeneous, how to identify a catalyst in a reaction mechanism Reaction Mechanisms What is one? What is the difference between an elementary step and an overall reaction? Writing rate laws for elementary steps Molecularity of elementary steps Identifying reaction intermediates and catalysts in a reaction mechanism - what is the difference between them? Rate determining step Determination of a rate law from a reaction mechanism with a slow first step Chapter 14: Chemical Equilibrium Chemical reactions at equilibrium - what does that mean? Do you understand how that would look on a molecular level? Equilibrium Constant, K Relationship between the reaction quotient, Q, and the equilibrium constant (Q C vs K C and Q P vs. K P ) Writing equilibrium expressions - and the law of mass action Calculation of K C or K P and converting between them Making some judgments about a reaction and it's equilibrium position based on the magnitude of K, or comparing Q and K

2 Heterogeneous equilibrium - writing equilibrium expressions and calculating K C or K P based on them Equilibrium Calculations and the use of equilibrium tables to solve problems Calculation of an unknown concentration given other equilibrium concentrations and K C Calculation of K C given initial concentrations and one equilibrium concentration Determination of equilibrium concentrations given initial concentrations and K C Determination of the direction of a reaction and equilibrium concentrations given arbitrary initial data & K C Le Chatelier's Principle - How does the position of an equilibrium shift in response to changes in concentration of reactants or products, pressure and temperature or the addition of a catalyst? Chapter : Acids and Bases Fundamentals Bronsted-Lowry definitions of acids and bases as H + donors and acceptors Acid and base ionization (dissociation) equations including identification of conjugate pairs K a and K b expressions relationship of acid strength to: K a value, [H 3 O + ], [HA] eq, equilibrium position, strength of conjugate base relationship of base strength to: K b value, [OH ], [B] eq, equilibrium position, strength of conjugate acid relative [H 3 O + ] and [OH ] in acidic, basic, and neutral solutions identification of strong acids and bases (only a small number so KNOW THEM!!) Water Water is amphoteric (or amphoprotic). What does that mean? Auto-ionization of water, and water auto-ionization constant K w K w = 1.0 x at 25 C; K w = [H 3 O + ][OH ] ph (and poh, pk a, pk b ) scale logarithmic scale ph = log[h 3 O + ] and [H 3 O + ] = 10 ph ; poh = log[oh ] and [OH ] = 10 poh pk a = logk a and K a = 10 pka ; pk b = logk b and K b = 10 pkb ph + poh = pk w Where do acidic, basic, and neutral solutions fall on the ph scale? You should be able to do the following kinds of calculations from this chapter: ph of strong acid and strong base solutions ph of weak acid and weak base solutions: this includes identifying the major species in solution, identifying the major source of H 3 O + or OH in solution and setting up an equilibrium calculation ph of mixtures of weak acids or weak bases Calculation of % dissociation or % ionization or weak acids and weak bases Determination of K a or K b using ph or % ionization values Some Review Problems: 1. a. The rate of formation of O 2 is 1.5 x 10 3 Ms 1 in the reaction 2 O 3 (g) 3 O 2 (g). What is the rate of decomposition of ozone? b. The rate of formation of dichromate ions is 0.14 Ms 1 in the following reaction: 2 CrO 2 4 (aq) + 2 H + (aq) Cr 2 O 2 7 (aq) + H 2 O (l). What is the rate of reaction of CrO 2 4 in this reaction? 2. Calcualte the initial rate for the decomposition of N 2 O 5 when 2.0 g N 2 O 5 are confined in a 1.0 L container and heated to 65 C. k = 5.2 x 10 3 s 1 at 65 C. 3. In the reaction CH 3 Br (aq) + OH (aq) CH 3 OH (aq) + Br (aq), when the [OH ] is doubled the rate doubles; when the [CH 3 Br] increases by a factor of 1.2, the rate increases by a factor of 1.2. Write the rate law for the reaction.

3 4. The following kinetic data were obtained for the reaction A 2 (g) + 2 B 3 (g) product: Experiment Initial [A 2 ], M Initial [B 3 ], M Initial Rate, Ms ?? a. What is the order of the reaction with respect to each reactant, and the overall order of the reaction? b. Write the rate law for the reaction. c. Determine the value of the rate constant. d. Use the data to calculate the reaction rate for experiment The following kinetic data were obtained for the reaction NO 2 (g) + O 3 (g) NO 3 (g) + O 2 (g): Experiment Initial [NO 2 ], M Initial [O 3 ], M Initial Rate, Ms x x x x x x x x 10 3?? a. Write the rate law for the reaction. b. What is the overall order of the reaction? c. Determine the value of the rate constant. d. Use the data to calculate the reaction rate for exp't Determine the rate constants for the following first-order reactions: a. A B, given that the concentration of A decreases to one-half its initial value in 1000s. b. 2 A B + C, given that [A] 0 = M and that after 120 s [B] = M. 7. Dinitrogen pentoxide, N 2 O 5 decomposes by first-order kinetics with a rate constant of 3.7 x 10 5 s 1 at 298 K. a. What is the half-life (in hours) for the decomposition of N 2 O 5 at 298 K? b. Calculate the concentration of N 2 O 5 after 2.0 h if [N 2 O 5 ] 0 = 2.33 x 10 2 M. c. How much time, in min, will elapse before the N 2 O 5 concentration decreases from 2.33 x 10 2 to 1.76 x 10 2 M? 8. For a second order reaction A products, t 1/2 = 50.5 s (with [A] 0 = 0.84 M). Calculate the time required for the concentration of A to decrease to (a) one-sixteenth; (b) one-fourth; (c) one-fifth of its original value. 9. Determine the rate constant for the following second-order reactions: a. 2 A B + 2 C, given that the concentration of A decreases from 2.5 x 10 3 to 1.25 x 10 3 M in 100s. b. 3 A 2 D + C, given that [A] 0 = 0.30 M and that the concentration of C increases to M in 200 s. 10. The following data were obtained on the reaction 2 A B: Time, s [A], mm a. Plot the data to determine the order of the reaction. b. Determine the rate constant. 11. a. Calculate the activation energy for the conversion of cyclopropane to propene from an Arrhenius plot of the following data: T, K k, s x x x b. Calculate the value of the rate constant at 600 C. 12. The rate constant of the first-order reaction 2 N 2 O(g) 2 N 2 (g) + O 2 (g) is 0.38 s 1 at 1000 K and 0.87 s 1 at 1030 K. Calculate the activation energy for this reaction. 13. At 800 C, k = 9.7 x M 1 s 1 for the following reaction: O (g) + N 2 (g) NO(g) + N (g) The activation energy for the reaction is 315 kj/mol. Determine the rate constant at 700 C.

4 14. The contribution to the destruction of the ozone layer caused by high flying aircraft has been attributed to the following mechanism: Step 1: O 3 + NO NO 2 + O 2 Step 2: NO 2 + O NO + O 2 a. What is the catalyst in this reaction? b. What is the reaction intermediate? 15. The presence of a catalyst reduces the activation energy of a certain reaction from 100 to 50 kj/mol. By what factor does the rate of the reaction increase at 400 K, all other factors being equal? 16. Each of the following steps is an elementary reaction. Write the rate law for each step and indicate its molecularity. a. 2 NO N 2 O 2 b. Cl 2 2 Cl c. 2 NO 2 NO + NO Consider the following reaction: 2 NO (g) + Cl 2 (g) 2 NOCl (g). This reaction obeys the following rate law: rate = k[no] 2 [Cl 2 ]. The following mechanism has been proposed for this reaction: step 1: NO (g) + Cl 2 (g) NOCl 2 (g) step 2: NOCl 2 (g) + NO (g) 2 NOCl (g) a. What would the rate law be if the first step were rate determining? b. Based on the observed rate law, what can you conclude about the relative rates of the two steps? 18. The rate law for the reaction H 2 SeO 3 (aq) + 6 I (aq) + 4 H + (aq) Se (s) + 2 I 3 (aq) + 3 H 2 O (l) is: Rate = k[h 2 SeO 3 ][I ] 3 [H + ] 2 with k = 5.0 x 10 5 M 5 s 1. What is the initial reaction rate when [H 2 SeO 3 ] 0 = [I ] 0 = M, and [H + ] 0 = M 19. The half-life for the (first-order) decomposition of azomethane (CH 3 N=NCH 3 ) is 1.02 s at 300 C: CH 3 N=NCH 3 (g) N 2 (g) + C 2 H 6 (g). A 45.0 mg sample of azomethane is placed in a 300 ml reaction vessel and heated to 300 C. a. What mass (in mg) of azomethane remains after 10 s? b. Determine the partial pressure exerted by the N 2 (g) in the reaction vessel after 3.0 s. 20. Tertiary buytl chloride reacts in basic solution according to the equation: (CH 3 ) 3 CCl + OH (CH 3 ) 3 COH + Cl The experimentally determined rate law for this reaction is Rate = k[(ch 3 ) 3 CCl]. Is the following a plausible mechanism for this reaction? Why or why not? Step 1: (CH 3 ) 3 CCl (CH 3 ) 3 C + + Cl (slow) Step 2: (CH 3 ) 3 C + + OH (CH 3 ) 3 COH (fast) 21. Write the equilibrium expressions for the following reactions: a. CO (g) + Cl 2 (g) COCl (g) + Cl (g) b. H 2 (g) + Br 2 (g) 2 HBr (g) c. 2 O 3 (g) 3 O 2 (g) 22. Consider the following equilibrium: N 2 (g) + 3 H 2 (g) 2 NH 3 (g); K P = 41 at 400 K. Calculate the values of K P at the same temperature for the following reactions: a. 2 NH 3 (g) N 2 (g) + 3 H 2 (g) b. 1/2 N 2 (g) + 3/2 H 2 (g) NH 3 (g) 23. Use the following experimental data collected at 460 C to determine the value of K C for the following reaction (these are equilibrium concentrations): H 2 (g) + I 2 (g) 2 HI (g) [H 2 ], M [I 2 ], M [HI], M 6.47 x x x x x x Determine K C at 45 C for each of the following equilibria: a. 2 NOCl (g) 2 NO (g) + Cl 2 (g); K P = 1.8 x 10 2 b. CaCO 3 (s) CaO (s) + CO 2 (g); K P = 167

5 25. An analysis of a reaction mixture at 500 K showed that it had the composition 4.8 x 10 3 M H 2, 2.4 x 10 3 M I 2, and 2.4 x 10 3 M HI. Is this system at equilibrium? If not, is there a tendency to form more reactants or more products? The reaction is: H 2 (g) + I 2 (g) 2 HI (g); K C = Consider the following equilibrium: 2 SO 2 (g) + O 2 (g) 2 SO 3 (g); K P = 3.0 x A 500 ml reaction vessel at 700 K contains 1.20 x 10 3 mol SO 2 (g), 5.0 x 10 4 mol O 2 (g) and 1.0 x 10 4 mol SO 3 (g). Will P SO3 increase with time? 27. A 25.0 g sample of solid ammonium carbamate, NH 4 (NH 2 CO 2 ), was placed in an evacuated 250 ml flask and kept at 25 C. At equilibrium, 17.4 mg of CO 2 was present. What is the value of K C for the decomposition of ammonium carbamate into ammonia and carbon dioxide? 28. In a gas-phase equilibrium mixture of H 2, I 2 and HI at 500 K, [HI] = 2.21 x 10 3 M and [I 2 ] = 1.46 x 10 3 M. What is the molar concentration of H 2 at equilibrium? K C = 160 at 500 K. 29. In a gas-phase equilibrium mixture of PCl 5, PCl 3, and Cl 2 at 500 K, P PCl5 = 0.15 atm, and P Cl2 = 0.20 atm. What is the partial pressure of PCl 3 at equilibrium? PCl 5 (g) PCl 3 (g) + Cl 2 (g); K P = 25 at 500 K. 30. The initial concentration of HBr in a reaction vessel is 1.2 x 10 3 M. The vessel is heated to 500 K. 2 HBr (g) H 2 (g) + Br 2 (g); K C = 7.7 x at 500 K Determine the percentage decomposition of the HBr and the equilibrium composition of the mixture. 31. Consider the following equilibrium: PCl 5 (g) PCl 3 (g) + Cl 2 (g); K P = 0.36 at 400 K. If 1.0 g of PCl 5 is initially placed in a 250 ml reaction vessel, determine the molar concentrations in the mixture at equilibrium. What percentage of PCl 5 is decomposed at 400 K? 32. A reaction mixture consisting of mol of H 2 and 1.60 mol of I 2 is placed into a 3.00 L flask and heated. H 2 (g) + I 2 (g) 2 HI (g) At equilibrium 60.0% of the H 2 had reacted. Determine K C for this reaction at this temperature. 33. For each of the following equilibria, state whether reactants or products will be favored by an increase in the total pressure resulting from a decrease in the volume of the container. If no change occurs, explain why. a. 2 O 3 (g) 3 O 2 (g) b. H 2 O (g) + C (s) H 2 (g) + CO (g) c. 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g) 34. Four gases, NH 3, O 2, NO, and H 2 O are mixed in a reaction vessel and allowed to reach equilibrium in the following reaction: 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g). Certain changes are then made to the mixture (see table below). Considering each change separately, state the effect (increase, decrease, no effect) that each change has on the quantity in the second column. The temperature and volume are constant unless otherwise noted: Change Quantity add NO [H 2 O] remove H 2 O [NO] remove O 2 [NH 3 ] remove NH 3 K C add NH 3 [O 2 ] 35. Predict whether each of the following equilibria will shift toward the products or reactants with a temperature increase, and whether the value of K C will increase or decrease. a. N 2 O 4 (g) 2 NO 2 (g); ΔH = +57 kj b. Ni (s) + 4 CO (g) Ni(CO) 4 (g); ΔH = 161 kj 36. A reaction mixture consisting of 2.00 mol of CO and 3.00 mol of H 2 are placed into a 10.0 L reaction vessel and heated to 1200 K and the following equilibrium is established: CO (g) + 3 H 2 (g) CH 4 (g) + H 2 O (g). At equilibrium, mol of CH 4 was present in the system. Determine the values of K C and K P for the reaction: g of N 2 O 4 and 0.33 g of NO 2 are placed in a 2.0 L reaction vessel and equilibrium is established: N 2 O 4 (g) 2 NO 2 (g); K C = 4.66 x 10 3 at 25 C. Determine the equilibrium concentrations of N 2 O 4 & NO 2 at 25 C.

6 38. Give the conjugate base of the following acids: HSO 4, H 2 S, H 2 PO 4, NH Give the conjugate acid of the following bases: CN, HCO 3, SeO 4 2, PO What are the concentrations of H 3 O + and OH in each of the following: a M HCl b M NaOH c M Ca(OH) 2 d M HNO A solution of washing soda (sodium carbonate) has a hydroxide-ion concentration of M. Calculate the ph of this solution at 25 C. 42. The ph of a cup of coffee (at 25 C) was found to be Calculate the hydrogen ion concentration in the coffee, 43. Acrylic Acid, HC 3 H 3 O 2 is used in the manufacture of plastics. A 0.10 M aqueous solution of acrylic acid has a ph of Calculate K a and the percent dissociation of acrylic acid. 44. Write the balanced chemical equations describing the ionization of the following weak acids. In each case identify the acid, the base, the conjugate acid, and the conjugate base. a. HOCN (cyanic acid) b. HIO 3 c. HBrO d. HCO 2 H 45. C 6 H 4 NH 2 COOH, para-aminobenzoic acid (PABA), is used in some sunscreens. Calculate the concentrations of H 3 O + and para-aminobenzoate ion, C 6 H 4 NH 2 COO, in a M solution of the acid. K a = 2.2 x Calculate the % ionization and ph of propionic acid (HC 3 H 5 O 2 ) in a 0.45 M solution. For the acid, pk a = Write the chemical equation for the base ionization of ethylamine, C 2 H 5 NH 2. Write the K b expression for ethylamine. 48. What is the concentration of hydroxide ion in a M aqueous solution of methylamine, CH 3 NH 2? What is the ph of this solution? K b = 4.38 x 10 4 Review for Exam 1 - Answers to Review Problems 1. (a) 1.0 x 10 3 mol O 3 /Ls; (b) 0.28 mol CrO 2 4 /Ls mol N 2 O 5 /Ls 3. Rate = k[ch 3 Br][OH ] 4. (a) 2nd order WRT A 2, 1st order WRT B 3, 3rd order overall; (b) Rate = k[a 2 ] 2 [B 3 ]; (c) 1.2 x 10 2 M 2 s 1 ; (d) 0.87 Ms 1 5. (a) Rate = k[no 2 ][O 3 ]; (b) 1st order WRT each reactant, 2nd order overall; (c) 4.3 x 10 7 M 1 s 1 ; (d) 5.1 Ms 1 6. (a) 6.93 x 10 4 s 1 ; (b) 7.6 x 10 3 s 1 7. (a) 5.2 h; (b) 1.79 x 10 2 M; (c) 126 min 8. (a) 7.6 x 10 2 s; (b) 1.5 x 10 2 s; (c) 2.0 x 10 2 s 9. (a) 4.0 M 1 s 1 ; (b) 1.9 x 10 3 M 1 s (a) 1/[A] vs. t gives straight line, therefore 2nd order reaction; (b) slope = k = 12 M 1 s (a) plot ln k vs. 1/T, slope = E a /R = 275 kj/mol; (b) 8.8 x 10 2 s x 10 2 kj/mol x 10 9 M 1 s 1

7 14. (a) NO; (b) NO x (a) Rate = k[no] 2, bimolecular; (b) Rate = k[cl 2 ], unimolecular; (c) Rate = k[no 2 ] 2, bimolecular 17. (a) Rate = k[no][cl 2 ]; (b) Step 2 must be the slow (R.D.S.) step, step 1 a fast step x 10 6 Ms (a) mg; (b) atm 20. Yes, Rate = k[(ch 3 ) 3 CCl] 21. (a) K C = [COCl][Cl]/[CO][Cl 2 ]; (b) K C = [HBr] 2 /[H 2 ][Br 2 ]; (c) K C = [O 2 ] 3 /[O 3 ] ; ; 48.9; x 10 4 ; no, Q C < K C products 26. Q C < K C, yes x x 10 5 M atm % - approximately 0 % 31. [PCl 3 ] = [Cl 2 ] = 1.0 x 10 2 M; [PCl 5 ] = 9.0 x 10 3 M; 53 % decomposed Reactants; Reactants; Reactants 34. Decrease; Increase; Increase; No Change; Decrease 35. Products, K C will increase; Reactants, K C will decrease 36. K C = 3.88; K P = 4.00 x [NO 2 ] = 7.1 x 10 3 M; [N 2 O 4 ] = 1.2 x 10 2 M 38. SO 2 4 ; HS ; HPO 2 4 ; NH HCN; H 2 CO 3 ; HSeO 2 4 ; HPO (a) [H 3 O + ] = 1.25 M, [OH ] = 8 x M; (b) [H 3 O + ] = 4 x M, [OH ] = 0.25 M; (c) [H 3 O + ] = 1.4 x M, [OH ] = M; (d) [H 3 O + ] = 0.50 M, [OH ] = 2 x M x 10 6 M x 10 5, 2.3% 44. (a) HOCN (aq) + H 2 O (l) OCN (aq) + H 3 O + (aq); (b) HIO 3 (aq) + H 2 O (l) IO 3 (aq) + H 3 O + (aq) (c) HBrO (aq) + H 2 O (l) BrO (aq) + H 3 O + (aq); (d) HCO 2 H (aq) + H 2 O (l) CO 2 H (aq) + H 3 O + (aq) x 10 3 M %, ph = C 2 H 5 NH 2 (aq) + H 2 O (l) C 2 H 5 NH + 3 (aq) + OH (aq); K b = [C 2 H 5 NH + 3 ][OH ]/[C 2 H 5 NH 2 ] M, ph = 11.76