8. Relax and do well.

Transcription

1 CHEM Exam I John I. Gelder February 5, 2002 Name TA's Name Section INSTRUCTIONS:. This examination consists of a total of 8 different pages. The last three pages include a periodic table, a table of vapor pressures for water, a solubility table and a table of thermodynamic values. All work should be done in this booklet. 2. PRINT your name, TA's name and your lab section number now in the space at the top of this sheet. DO NOT SEPARATE THESE PAGES. 3. Answer all questions that you can and whenever called for show your work clearly. Your method of solving problems should pattern the approach used in lecture. You do not have to show your work for the multiple choice or short answer questions. 4. No credit will be awarded if your work is not shown in 4a, 4d and Point values are shown next to the problem number. 6. Budget your time for each of the questions. Some problems may have a low point value yet be very challenging. If you do not recognize the solution to a question quickly, skip it, and return to the question after completing the easier problems. 7. Look through the exam before beginning; plan your work; then begin. 8. Relax and do well. Page 2 Page 3 Page 4 Page 5 TOTAL SCORES (23) (35) (8) (24) (00)

2 CHEM 55 EXAM I PAGE 2 (9). Write the chemical formula(s) of the product(s) and balance the following reactions. Identify all products phases as either (g)as, (l)iquid, (s)olid or (aq)ueous. Soluble ionic compounds should be written in the form of their component ions. a) 2K(s) + 2H 2 O(l) 2K + (aq) + 2OH (aq) + H 2 (g) b) Na 2 CO 3 (aq) + Ca(NO 3 ) 2 (aq) 2Na + (aq) + 2NO 3 (aq) + CaCO3 (s) c) Ba(OH) 2 8H 2 O(s) + 2NH 4 SCN(s) Ba 2+ (aq) + 0H 2 O(l) + 2NH 3 (aq) + 2SCN (aq) (4) 2a. Write the ionic and net ionic chemical equation for a) or b). Ionic equation 2Na + (aq) + CO 3 2- (aq) + Ca 2+ (aq) + 2NO3 (aq) 2Na + (aq) + 2NO3 (aq) + CaCO3 (s) Net Ionic equation CO 3 2- (aq) + Ca 2+ (aq) CaCO3 (s) (0) 3. Predict whether the entropy change in the system is positive or negative for each of the following processes. a) CH 3 OH(l) + O 2 (g) HCO 2 H(l) + H 2 O(l) The phases are both liquid, while the reactants are a liquid and a gas. There are equal numbers of moles, and the liquid phase is more ordered compared to the gas phase, so the products are more ordered compared to the reactants and S is. b) 2SO 3 (g) 2SO 2 (g) + O 2 (g) The reactants and products are all in the same phase, but there are more moles of products compared to reactants. So there are more possible arrangements in the products compared to the reactants and S is +.

3 CHEM 55 EXAM I PAGE 3 4. Methanol, CH 3 OH, can be synthesized by reacting hydrogen gas with carbon monoxide, according to the chemical equation, CO(g) + 2H 2 (g) CH 3 OH(l) (8) a) Calculate H rxn, S rxn and G rxn. H f kj mol S J K. mol G f kj mol CO(g) + 2H 2 (g) CH 3 OH(l) H rxn = Σ H f (products) - Σ H f (reactants) = [ H f (CH 3OH(l)) ] [ H f (CO(g)) + 2 H f (H 2(g)) ] = (-239 kj) - [(-0.5 kj) + 2(0 kj)] = 28 kj S rxn = ΣS (products) - ΣS (reactants) = [S (CH 3 OH(l)) ] [S (CO(g))+ 2S (H 2 (g))] = [(27 J K )] - [ (98 J K ) + 2(3 J K )] = 333 J/K G rxn = Σ G f (products) - Σ G f (reactants) = [ G f (CH 3OH(l)) ] [ G f (CO(g)) + 2 G f (H 2(g)) ] = (-66 kj) - [(-37 kj) + 2(0 kj)] = 29 kj (4) b) Which factor, the change in enthalpy, H, or the change in entropy, S, provides the principal driving force for the reaction at 298 K? Explain. H is the principal driving force, because the sign of H in the reaction is negative. A H favors spontaneity. S is also negative. S does not favor spontaneity. Since G = H T S a spontaneous reaction has a negative G. In this equation spontaneity is favored by a H and a + S. In the reaction in part a only the sign of H favors spontaneity. (5) c) For the reaction, how is the value of the standard free energy change, G, affected by an increase in temperature? Explain. Using the equation G = H T S, since H is negative and S is negative as the temperature increases the T S term is becoming more negative, and T S is more positive, making G more positive and less spontaneous. (8) d) Calculate the S f for CH 3 OH(l). S J K. mol S rxn C(s) + 2H 2 (g) + 2 O 2 (g) CH 3 OH(l) = ΣS (products) - ΣS (reactants) = [S (CH 3 OH(l)) ] [S (C(s))+ 2S (H 2 (g)) + 2 S (O 2 (g)) ] = [(27 J K )] - [ (6 J K ) + 2(3 J K )+ 2 (205 J K )] = 243 J/K

4 CHEM 55 EXAM I PAGE 4 (2) 5. A sample of water is in the vacuum flask shown below. The vacuum flask side arm has vacuum rubber tubing with one end attached to the flask and the other end attached to a vacuum pump (can not see in this diagram). When the vacuum pump is on any gas/vapor in the flask is removed. In an experiment the water placed into the flask initially is at room temperature. The rubber tubing is attached to the flask and the vacuum pump, and the pump is turned on. After the pump has been on for several minutes the following two observations are made:. the volume of water in the flask has decreased; 2. what water remains has turned to ice. Explain both of these observations. The volume of liquid water decreases because as the vacuum pump removes vapor from above the liquid, particles escape the liquid phase to maintain the equilibrium vapor pressure. Since particles are evaporating and few particles are condensing, the volume of the liquid will decrease. The reason the liquid eventually turns to solid is because evaporation is an endothermic process, and heat is removed from the system (the liquid water). When enough heat is removed the liquid freezes. Evaporation is endothermic because the conversion of water in the liquid phase to water in the vapor phase requires breaking hydrogen-bonding forces. Breaking bonds is always an endothermic process. (6) 6. Give the name or draw the complete Lewis structure (showing all C-H(X) bonds) for each of the following compounds. 4-ethyl-2,3-dimethylhexane,-dichloro-2,2-difluoroethane 3, 3, 5 trimethylheptane

5 CHEM 55 EXAM I PAGE 5 (2) 7. Draw and name six different structural isomers for C 8 H 8. (NOTE: You may use condensed formulas when representing the different structural isomers.) n octane 2-methylheptane 3-methylheptane 4-methylheptane 2,3-dimethylhexane 3,3-dimethylhexane

6 CHEM 55 EXAM I PAGE 6 (2) 8. CH 3 I has a vapor pressure of 400 mm Hg at 25.3 C. Calculate the temperature that CH 3 I has a vapor pressure of 40.0 mm Hg. H vap for CH 3 I is 29.2 kj mol -. ln P P 2 = - H ( R T - T 2 ) J ln = mol J 8.34 mol K ln (0) = -352 K (3.35 x 0-3 T ) = -352 K (3.35 x 0-3 T ) T 6.56 x 0-4 = 3.35 x 0-3 T = 4.00 x 0-3 ( K - T ) T = 250 K

7 CHEM 55 EXAM I PAGE IA H Li 6.94 IIA 4 Be Na Mg IIIA IVA VA VIA VIIA B C N O F 9 0 Ne IIIB IVB VB VIB VIIB VIII IB IIB K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe (98) Cs Ba La Periodic Table of the Elements Hf Al Si P S Cl VIIIA 2 Ta W Re 75 Os Ir Pt Au Hg 80 Tl Pb Bi Po At Rn (209) (20) Fr Ra Ac (223) (26) (262) (263) He Ar (222) Lanthanides Actinides Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (45) Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr (244) (243) (247) (247) (25) (252) (257) (258) (259) (260) PV = nrt ln vp 2 vp = H vap R T 2 T density of H 2 O =.00 g cm 3 H rxn = n( H f (products)) m( H f (reactants)) S rxn = n(s (products)) m(s (reactants)) Useful Information R = L atm mol K = 8.34 density of H 2 O =.00 g cm 3 G rxn = n( G f (products)) m( G f (reactants)) G = H - T S J mol K

8 CHEM 55 EXAM I PAGE 8 Temperature ( C) Vapor Pressure(mmHg) Temperature ( C) Vapor Pressure(mmHg) Solubility Table Ion Solubility Exceptions NO 3 soluble none ClO 4 soluble none Cl soluble except Ag +, Hg 2+ 2, *Pb 2+ I soluble except Ag +, Hg 2+ 2, Pb 2+ SO 2 4 soluble except Ca 2+, Ba 2+, Sr 2+, Hg 2+, Pb 2+, Ag + CO 3 2 insoluble except Group IA and NH 4 + PO 3 4 insoluble + except Group IA and NH 4 - OH insoluble except Group IA, *Ca 2+, Ba 2+, Sr 2+ S 2 insoluble except Group IA, IIA and NH + 4 Na + soluble none NH + 4 soluble none K + soluble none *slightly soluble

9 CHEM 55 EXAM I PAGE 9 Substance H o f G o f S o and State J K. mol Carbon C(s) (graphite) C(s) (diamond) CO(g) CO2(g) CH4(g)? CH3OH(g) CH3OH(l) CH3Cl(g) CHCl3(g) CHCl3(l) -3.8 H2CO(g) HCOOH(g) HCN(g) C2H2(g) C2H4(g) CH3CHO(g) C2H5OH(l) C2H6(g) C3H6(g) C3H8(g) mol mol Thermodynamic Values (25 C) Substance H o f and State kj G o f kj S o J K. mol mol mol Oxygen O2(g) O(g) O3(g) Nitrogen N2(g) NCl3(g) NF3(g) NH3(g)? NH3(aq)? -27 NH 2 CONH2(aq)?? 74 NO(g) NO2(g) N2O(g) N2O4(g) N2O5(g) HNO3(aq) HNO3(l) NH4Cl(s) NH4ClO4(s) Bromine Br2(l) BrCl(g) Chlorine Cl2(g) Cl2(aq) Cl - (aq) HCl(g) Fluorine F 2 (g) F - (aq) HF(g) Hydrogen H2(g) H(g) H + (aq) OH - (aq) H2O(l) H2O(g) Magnesium Mg(s) Mg(aq) MgO(s) Silver Ag(s) Ag + (aq) Ag(S2O3) 3- (aq) AgBr(s) AgCl(s) Sulfur S(rhombic) SO 2 (g) SO 3 (g) H 2 S(g) Titanium TiCl4(g) TiO2(s) Aluminum AlCl3(s) Barium BaCl2(aq) Ba(OH)2 8H2O(s) Iodine I2(s) HI(g)

10 CHEM 55 EXAM I PAGE 0 H 2 O