CH1020 Stream 1

Test 3

B02

 

NAME  __________________________________                                 

SECTION ญ___________ญญญญญญญญญญญญญญญญญญญญ_____________________                                                                         

 

Classroom Section (20 points):

1) (3 points) Use standard enthalpies of formation to determine the amount of energy released when the following reaction is carried out as written:

 

                2Na2CO3(s) + 4HCl(g) → 4NaCl(s) + 2H2O(l) + 2CO2(g)

 

∆H = 4(-411) + 2(-394) + 2(-286) – 2(-1131) – 4(-92) = -374 kJ

 

2) (3 points) Use the known reaction enthalpies to determine the unknown reaction enthalpy by Hess's Law:

 

                CO(g) + SiO2(s) → SiO(g) + CO2(g)                                   DHo = 521 kJ

                8 CO2(g) + Si3N4(s) → 3 SiO2(s) + 2N2O(g) + 8 CO(g)     DHo = 461 kJ

 

                5 CO2(g) + Si3N4(s) → 3 SiO(g) + 2N2O(g) + 5 CO(g)      DHo =

 

Reaction 3 = reaction 2 + 3*reaction 1 so ∆H for reaction 3 = 461 + 3(521) = 2024 kJ

 

3) (6 points) Starting with the situation shown, a gas expansion is carried out by opening both valves at the same time. Assume that T does not change.

 

Determine DVsys, w, q, DE, and DH for the process.

 

                DVsys =    8 L

                w = - 8 L-atm

                q = 8 L-atm

                DE = 0

DH = 0

 

 

 

 

 

 

4) (4 points) Sodium crystallizes in a body-centered cubic lattice. The edge length of the unit cell is 0.429 nm. What is the density of sodium metal? 0.967 g/mL

 

5) (4 points) A compound contains three elements, sodium, oxygen, and chlorine. It crystallizes in a cubic lattice. The unit cell has oxygen atoms at the vertices, a chlorine atom at the body center, and sodium atoms in the face centers. What is the formula of the compound?

 

The unit cell contains 8*(1/8) = 1 oxygen atom, 1 chlorine atom, and 6*(1/2) = 3 sodium atoms. Thus Na3OCl.

 

Lab Section (20 points):

1) In the Intermolecular Forces experiment, you mixed various pairs of liquids and looked for 3 things: mutual solubility (i.e., did they mix?), temperature change on mixing, volume change on mixing (observed total volume – expected total volume). For each pair of liquids below, indicate the expected results:

 

Liquids

Do they mix?

Delta T + or -?

Delta V + or -?

CCl4, H2O

No

0

0

(CH3)2CO, CH3CH2OH

Yes

+

-

CCl4, CH2Cl2

Yes

+, small

-, small

 

 

2) Each question is worth 2 points:

 

Where are the calcium ions located in a unit cell of calcium fluoride? At the vertices and face centers of a cubic unit cell.

What is the coordination number of a calcium ion in calcium fluoride? 8

Suppose you have two body-centered cubic unit cells side by side, sharing a face.  What is the net number of atoms contained in the 2 unit cells? 4

In which cubic unit cell (simple, body-centered, or face-centered) does each atom have a coordination number of  8? bcc

Why are the fcc and bcc lattices commonly found in metals, but sc is almost never found? Fcc is the most efficient packing possible for spheres, giving a coord. Number of 12. bcc gives CN of 8. sc gives coordination number of only 6 so would provide the least effective bonding between atoms.