2. Explain why the ph of unbuffered water is generally around 5.7

Transcription

1 CHEM 108b FINAL EXAM The exam is worth a total of 100 points. Each problem is worth five points. Show all work for partial credit. Don t forget to label units and check whether equations are balanced if they are not balanced and need to be balanced, balance them!!!!!!!! Multiple choice/short answer/true-false (30 points) 1. Given a reaction with ΔH o = 20 kj/mol and ΔS = -100J/Kmol, which of the following statements are true?: a. ΔG o > 0 under all circumstances b. ΔG > 0 under all circumstances c. ΔG o < 0 under most circumstances d. a and b are both true e. We don t have enough information to answer the question. 2. Explain why the ph of unbuffered water is generally around Chloride concentrations in a salt water environment are typically: a. 35 parts per thousand b. 35 parts per million c. 35 parts per trillion d. 19 parts per million e. 19 parts per thousand 4. Acetate anion (CH 3 COO - ) has a K b = 5.5 x M. At what ph would you expect the base to be completely associated? a. ph 1 b. ph 10 c. ph As ph increases, the oxidizing power of oxygen increases/decreases (circle one). Explain why. 1

2 6. Nuclear power plants produce thermal pollution, which is a concern because: a. manatees love hot water and manatees are invasive species b. hot water causes salts to precipitate and clog pipes c. less oxygen gas dissolves in water when it is warm d. it carries with it high amounts of radioactivity Problems (70 points) 7. Which of the following compounds are ionic and which are molecular? How do you know? For compounds that are ionic, identify the anion and the cation. Identify the oxidation state of all elements in all compounds. Cl 2 O LiNO 3 Na 2 CrO 4 Si 2 Br 6 ZnCO 3 8. Given a solution of [Fe(phen) 3 ] 2+ with an absorbance of 0.5 and an extinction coefficient of 11.1 mm -1 cm -1, calculate the concentration of [Fe(phen) 3 ] 2+. The path length is 1 cm. 2

3 9. Some organisms consume hydrogen sulfide (H 2 S) as a fuel. (a) Given the information at the end of the exam, calculate the ΔH o associated with consuming 5 g of H 2 S in an oxygen-rich environment in the following reaction. (O 2 is not limiting.) The reaction is H 2 S (g) + O 2(g) S (s) + H 2 O (l). (b) Identify the oxidation number for sulfur in both the reactants and products (c) Is sulfur being reduced or oxidized in this reaction? 10. Given the reaction above for hydrogen sulfide consumption, calculate the cell potential for the reaction and the free energy change under standard state conditions. Calculate the cell potential and free energy change for the reaction under aqueous environmental conditions where [H 2 S] = 1 x 10-7 atm, and the partial pressure of oxygen in the atmosphere is [O 2 ] = 0.21 atm. 3

4 11. Some organisms use methane as an energy source and convert it to methanol. (a) Determine the free energy change for methane oxidation under standard conditions. (CH 4(g) + O 2(g) CH 3 OH (l) ) (b) Determine the free energy change for methane oxidation when the pressure of methane is 5 atm, oxygen is 5 atm and methanol is 0.05 M. Assume T = 298 K. 12. Determine the enthalpy change for the conversion of methanol (liquid) to methane (gas). 4

5 13. Given the following table of bond energies, estimate the enthalpy change when one mole of methane is burned in air. Note: part of what the question is asking you to do is to write down the correct equation. If you don t know the correct equation, write an equation and do the problem based on the equation you have guesses. Identify the oxidation number of carbon in reactants and products. Bond Energy (kj mol 1 ) Bond Energy (kj mol -1 ) C-H 414 C=O 799 O=O 498 C-O 360 O-O 142 O-H If 0.1 mole of methane were burned in the presence of oxygen and all heat transferred to 2 L of water, by how many degrees would the temperature of the water rise or fall? 5

6 15. Calculate the ph of a 0.50 M solution of the weak base methylamine (CH 3 NH 2 ) given that the K a of its conjugate acid is 2.22 x Write the chemical expression for the base association reaction. 16. What would the ph be of a 1L solution with 0.25 M methylamine and 0.25 M CH 3 NH 3 + (methyl ammonium)? What would it be if 2 ml of 1.0 M NaOH was added to the solution? (Note, you can neglect the volume change from the addition of the base). What would the ph of a solution be if you added 2 ml of 1 M NaOH solution to 1 L of water? 6

7 17. The first order decomposition of N 2 O 5 has been studied at several temperatures. Temp C k, s x x x 10-4 a. Calculate the expected value of k and also the reaction half-life, at 75 C. b. How much N 2 O 5 would you expect to have left after 5 minutes if you initially began with 1 mm at 25 C? at 75C? 18. Balance the following redox equation in basic conditions S 2 O CrO 4 2- Cr(OH) 3 + SO 3 2-7

8 19. Radioactive decay is a first order process. The decay of the radioactive isotope 14 C is used to data organic material. The rate constant for the decay of 14 C is 1.2 x 10-4 yr -1. Suppose the original concentration of 14 C in an artifact was 2.4 x atoms per gram and it is acceptable to sacrifice 1 gram of sample for dating analysis. What is the maximum age of a sample that can be dated if the scintillation counter cannot detect fewer than 80 disintegrations of 14 C per minute? 20. K sp for calcium fluoride, CaF 2 (s), is 3.9 x at 25 O C. Calculate [Ca 2+ ] and [F - ] in M and ppm for an aqueous solution in equilibrium with CaF 2 (s) at that temperature in the presence of 0.1 M Ca 2+. 8

9 Numbers and Formulas you may need: First order rate expression Δ[ A] Δt A = k[ A] [ ] t = [ A] 0 e kt ln[ A] t = kt + ln A t 1/ 2 = ln2 k [ ]0 Zero order rate expression Δ[ A] Δt A = k [ ] t = [ A] 0 kt Second order rate expression Δ[ A] Δt = k[ A] 2 1 = kt + 1 [ A] t [ A] 0 1 t 1/ 2 = k[ A] o Determining activation energy lnk = E a 1 + ln A R T R = J mol -1 K -1 k = Ae E a RT ΔG = ΔH TΔS K = 0 C ΔG = ΔG 0 + RtlnQ Buffer equation (Henderson-Hasselbach) ΔΗ = nc p ΔT = gc s ΔT ph = pk a + log [A - ]/[HA] C p for water = 75.3 J mol -1 K -1 K w = 1 x C s for water = J g -1 K -1 x = b ± b2 4ac 2a E = E logq n E = E RT nf lnq ΔG = RT lnk ΔG = nfe F = 96485C(mol) 1 ln k 2 = E a 1 1 k 1 R T 1 T 2 Δ G f (kj) Δ H f (kj) S (J K -1 ) NH 3(g) NH 3(aq) NH 4 (aq)

10 NO (g) Δ G f (kj) Δ H f (kj) S (J K -1 ) NO 2(g) N 2 O (g) N 2 O 4(g) N 2 O 5(g) N 2 H 4(l) N 2 H 3 CH 3(l) HNO 3(aq) HNO 3(l) NH 4 ClO 4(s) NH 4 Cl (s) C 6 H 12 O 6(s) CO 2(g) H 2 O (l) H 2 O (g) O 2(g) N 2(g) NO 3 (aq) H 2(g) CO (g) SO 2 (g) H 2 S (g) CH 4 (g) CH 3 OH (l) O 2 + 4H + + 4e - 2H 2 O E o = 1.23 V Ag + + e - Ag E 0 = Fe 3+ + e - Fe 2+ E o red =.77 V Ni e - Ni E o = V ΔG = ΔG o + RTlnQ K H (CO2) = 3.4 x 10-2 M atm -1 Fe(OH) 2 K sp = 1.8 x Fe(OH) 3 K sp = 4.0 x Fe 3+ + e - Fe 2+ E o red =.77 V S + 2e - + 2H + H 2 S E o red =.123V A = bcε 10