Questions 1, 3,7 & Mt. Choice 1,2,4,5,6 Are on Ch. 4,5,6

Questions 1, 3,7 & Mt. Choice 1,2,4,5,6 are on Ch. 4,5,6 Chemistry 1151, Fall 2005 Name:______

Test 3. Form B, 100 pts. I.D.#______Disc. Sec.______

SCORE: Part 1.______Part. 2.______Total: ______

Part 1. Useful Info: PV = nRT, R = 0.0821 L atm/mol K

1. The meals-ready-to-eat (MREs) in the military can be heated on a flameless heater: The source of energy in the heater is: Mg(s) + 2 H2O(L)  Mg(OH)2)(s) + H2(g)

o a. (5) Calculate ΔH rxn for the reaction given the following: o ΔH f : H2O(L) ; -285.5 kJ/mole, Mg(OH)2(s); -924.5 kJ/mole

b. (8) How many grams of Mg are required to provide enough heat to warm 200. ml water (1 cup) from 25 oC to 80 oC using your calculation in (a) above. o Sp. Ht. H2O(l) = 4.18 J/g C, Density H2O = 1.00 g/mL

O c. (7) How many liters of H2(g) is produced from 3.00 grams of Mg at 25 C and 0.92 atm pressure? 2. (4ea.) Draw the Lewis structures for the following and fill in the blanks which refer to the central atom. Also determine the hybridization around central atom Follow the octet rule when possible.

-1 a. SCl2 b. NO3 c. KrF2

# bonds :_____ # of bonds______# bonds ______

# lone e- pairs:______# lone e- pairs:______# lone e- pairs: ______

3. (8) The standard entropy of formation of diborane, B2H6, cannot be determined directly. o 2 B(s) + 3 H2(g)  B2H6(g) ΔH f = ?

o Use the following reactions with measurable heat terms, to determine the above ΔH f in kJ /mole of B2H6. Show work clearly for partial credit.

o 4 B(s) + 3 O2(g)  2 B2O3(s) ΔH rxn = -2544 kJ

o H2(g) + ½ O2(g)  H2O (g) ΔH rxn = - 242 kJ

o B2H6(g) + 3 O2(g)  B2O3(s) + 3 H2O(g) ΔH rxn = -2033 kJ 4. (20) points. Given the following Lewis structures, determine the geometry (shape) of the molecule or ion. (NOT THE CHARGE CLOUD GEOMETRY). Lone pairs have been omitted from terminal atoms. LEWIS STRUCTURE DOES NOT SHOW MOLECULAR GEOMETRY. Also determine the hybridization. Geometries: linear, bent,triangular planar and pyramidal, square planar and pyramidal, tetrahedral, trigonal bipyramidal, T-shaped, see-saw, octahedral.

Lewis Structure Molecular Geometry (3) Hybridization (2)

-1 a. IO3 ______

b. SF4 ______

c. SO2 ______

d. BrF5 ______

e. (2) How many equivalent resonance structures should be written for SO2 ?______f. (2) Determine the average bond order in SO2 .______g. (2) How many σ bonds and π bonds are in SO2? σ bonds:______, π bonds:______

H C O 5. Given the following Lewis structure: O H

(2 ea) Determine the indicated bond angles: The Lewis structure does not indicate the bond angles or molecular geometry. a. angle OCO : ______b. angle COH : ______6. Three unequivalent resonance structures can be written for the OCN-1 ion (cyanate): a. (3) Determine the formal charge on the oxygen in each structure (show work) and write it below the O in each structure;

Otr = ______Odb = ______Osin =______

[ O C N ]-1 [ O C N ]-1 [ O C N ]-1

I. II. III. b. (3) The formal charge on the C is 0 in all three structures. Write the formal charge of each N below the N in each structure. Determine the BEST structure for OCN-1 based on electronegativity and magnitude of formal charge and explain.

c. 3 points extra credit: Explain why the formal charge on C is the same in all of the structures.

7. (6) Calculate the density of a HCN gas at 685 torr and 35 oC. Part 2. Multiple Choice. 3 Points Each. Mark letter of correct choice on blank at left

_____1. Two metals of equal mass, with different heat capacities are subjected to the same amount of heat. Which undergoes the smallest change in temperature? a. The metal with the higher heat capacity. b. The metal with the lower heat capacity. c. Both under go the same change in temperature.

_____2. The pressure in an oxygen tank stored at 25.0 oC is 15.0 atm. What pressure will develop in the tank if it is heated in an oven to 450. oC.? a. 6360 atm b. 6.18 atm c. 270 atm d. 36.4 atm e. 54.6 atm

_____3. In which pair do both compounds exhibit predominantly ionic bonding? a. PCl5 and HF b. Na2SO3 and SbCl3 c. KI and O2 d. NaF and H2O e. RbCl and CaO

_____4. A chemical reaction that absorbs heat from the surroundings is said to be ______and has a ______ΔH at constant pressure. a. endothermic, positive b. endothermic, negative c. exothermic, positive d. exothermic, negative

_____5. A gaseous mixture containing 1.5 mol Ar and 3.5 mol CO2 has a total pressure of 7.0 atm. What is the partial pressure of CO2 in atm. ? a. 1.8 b. 2.1 c. 3.5 d. 4.9 e. 2.4

_____6. Consider four identical 1.0 L flasks containing the following gases each at 25o C and 1 atm pressure. Which gas has the highest density? a. H2 b. O2 c. NH3 d. SO2 e. same for all gases.