Chemistry 1A Fall 2000

Transcription

1 Chemistry 1A Fall 2000 Midterm Exam III, versin B Nvember 14, 2000 (Clsed bk, 90 minutes, 155 pints) Name: SID: Sectin Number: T.A. Name: Exam infrmatin, extra directins, and useful hints t maximize yur scre: Write yur name n all 8 pages. There are tw parts t the exam: 1) multiple chice and 2) shrt answer prblems. Fr the multiple chice prblems, fill in the Scantrn TM frm AND circle the answer n yur exam. Answer the questins yu knw hw t d first, then wrk n the questins yu skipped. Shw all wrk n the shrt answer prblems fr which yu want credit and d nt frget t include units! Unit Prefixes milli, m (x 10-3 ) micr, µ (x 10-6 ) nan, n (x 10-9 ) kil, k (x 10 3 ) mega, M (x10 6 ) giga, G (x 10 9 ) Sme pssibly useful infrmatin: H S = H f (prducts) = S (prducts) S H f (reactants) (reactants) G = H - T S C p (H 2 O, l)= 1 cal/k g E = q + w S = k B lnw 1 Cal = 1000 cal Bnd Average Bnd Enthalpy (kj/ml) C-H 413 C-C 348 C-O 360 O=O 497 O-H 463 H-H 436 Cmpund H f (kj/ml) at 25 C S (J/ml-K) at 25 C CO (gas) CO 2 (gas) H 2 (gas) H 2 O (liquid) CH 3 OH C 2 H 5 OH CH 3 COOH (D nt write in the bx belw.) Page Pints 2-4 / / / 47 Ttal / 155

2 Page 2 f 2 Part 1: Multiple Chice. (4 pts each, 48 pts ttal) Instructins: Bubble in the crrect answer n yur Scantrn sheet AND circle the answer n yur exam. Each questin has ne crrect answer. 1.) The answer t questin 1 is B. Bubble in B n yur Scantrn frm. 2.) Which has K P CO2 at equilibrium? A.) Na 2 CO 3 (slid) B.) CaCO 3 (slid) C.) C(slid) + O 2 (gas) D.) CO 2 (slid) Na 2 O(slid) + CO 2 (gas) CaO(slid) + CO 2 (gas) CO 2 (gas) CO 2 (gas) E.) 2 MgO(slid) + C(slid) 2 Mg(slid) + CO 2 (gas) 3.) Fr the exthermic frmatin f ammnia, N 2 (gas) + 3 H 2 (gas) 2 NH 3 (gas) which will act t favr the frmatin f ammnia? A.) high pressure B) high vlume C.) high temperature D.) catalyst E.) all f the abve 4.) Which f the fllwing is true fr the isthermal expansin f an ideal gas? A.) w > 0 B.) T > 0 C.) V < 0 D.) S Sys > 0 E.) S Surr > 0 5.) Which f the fllwing has the greatest entrpy? A.) 1 mle f water at 25 C and 1 atm B.) 1 mle f water at 100 C and 1 atm C.) 0.5 mles f water plus 0.5 mles f steam at 100 C and 1 atm D.) 1 mle f steam at 100 C and 1 atm E.) 1 mle f steam at 100 C and 0.5 atm

3 Page 3 f 3 6.) Which is true fr the fllwing exthermic reactin at 1 atm? Fe(slid) + CO(gas) FeO(slid) + C(slid) A.) Prceeds spntaneusly at all temperatures B.) Prceeds spntaneusly at n temperatures C.) Prceeds spntaneusly at high temperatures D.) Prceeds spntaneusly nly at 0 K E.) Prceeds spntaneusly at lw temperatures 7.) Hw many glasses f water (200 ml) can be heated frm 10 C t 35 C by the cmbustin f a mini-snickers bar (40 Cal)? A.) 1 B.) 8 C.) 40 D.) 500 E.) ) T the frmatin f which cmpund frm the elements in standard states des the fllwing prcess f bnd-breaking! bnd frmatin crrespnd? 2 C-C +2 H-H + ½ O=O! 3 C-H + C-O + O-H A.) ethanl B.) frmaldehyde C.) methanl D.) acetaldehyde E.) acetic acid 9.) Using the infrmatin n page 1, estimate H fr the prcess in questin 8. A.) 246 kj/ml B.) 45 kj/ml C.) 3 kj/ml D.) -45 kj/ml E.) -246 kj/ml 10.) When yu burn wd, energy is released. Which statement best describes where that energy cmes frm? A.) The reactin breaks the chemical bnds in the wd, releasing their energy. B.) Energy is released because wd and all rganic materials have high energy bnds. C.) When wd burns, its weak bnds break t cmbine with the strnger xygen-xygen duble bnds, releasing energy. D.) When yu burn wd, the prducts are mre stable than the reactants s energy is released. E.) When wd is a reactant, its high energy bnds release energy when they break.

4 Page 4 f 4 11.) Cnsider the dissciatin reactin A 2 (g) 2 A (g). The fllwing pictures represent ne pssible initial state and the equilibrium state fr the system. Initial State Equilibrium State Which f the fllwing statements is true fr this dissciatin reactin? A.) This reactin favrs prducts at all temperatures because the number f A atms increases and the number f A 2 mlecules decreases. B.) This reactin favrs reactants at all temperatures because the A 2 mlecules have a bnd which makes them mre stable. C.) This reactin favrs reactants at lwer temperatures because heat is absrbed. D.) This reactin favrs reactants at higher temperatures because heat is absrbed. E.) This reactin desn t favr prducts r reactants since the equilibrium cnstant equals ) Which prcess is accmpanied by the largest increase in entrpy? I II III IV V A.) I!V B.) I!II C.) II!III D.) III!IV E.) IV!V

5 Page 5 f 5 Part 2: Shrt Answer Prblems (107 pts ttal) Instructins: Enter answers fr all questins in the bxes prvided (r as therwise instructed). Shw yur wrk. Where requested write explanatins in fifteen wrds r less. 1.) (60 pts) Cnsider the fllwing reactin: CO 2 (gas) + H 2 (gas) CO(gas) + H 2 O(liquid) At equilibrium at 25 C, P CO2 = P H2 = 1.0 atm ; P CO = 3.2 x 10-4 atm. a.) The vlume is suddenly reduced by a factr f ½. What are Q and K? Hw will the reactin prceed tward equilibrium? Q: K: Shrt explanatin f re-establishment f equilibrium:. b.) What are the partial pressures f all the species in the reactin at the new equilibrium? P CO2 : P H2 : P CO :

6 Page 6 f 6 c.) Using the infrmatin n page 1, determine H fr the reactin frm H f s and determine S fr the reactin frm S s. Explain the sign f S. H : S Explanatin: d.) Determine G fr the reactin at 25 C. G e.) Belw what temperature des the reactin becme spntaneus? T:

7 Page 7 f 7 (47 pts) 2.) Fr questins a d cnsider the fllwing: Different hydrcarbn structural ismers burn differently in xygen. This is the basis fr the ctane rating f gasline. Cnsider three structural ismers, X,Y and Z, f a certain hydrcarbn. The hydrcarbn ismers can react as fllws at 298 K. 1) X Y K 1 = 20.0 H = kj/ml S = -120 J/K ml 2) Y Z K 2 = 0.4 H = 20.0 kj/ml S = 60 J/K ml a.) What is the equilibrium cnstant fr X Z? K eq : b.) Arrange the cmpunds X, Y, and Z n the relative enthalpy scale belw? H f

8 Page 8 f 8 c.) Upn cmbustin, which ismer, X r Z, burns htter? Circle ne and explain. X Z Same Explanatin: d.) Sketch a plt f G vs. T fr the equilibrium X Z. Gº 0 T