ΔH = -(EAEB + CACB)

1. (15 pts) (a) Define the term lattice energy.

(b) The Born-Landé equation is a sum of two terms. Explain these two terms in 1-2 sentences.

(c) Which of the following compounds has the largest lattice energy: NaCl, CsF, MgO, SrO? Explain.

(d) For which of the following compounds is the Born-Landé lattice energy likely to be in worstagreement with the experimental lattice energy: SrS, SrO, SrTe, SrSe? Explain.

2. (10 pts) Draw a simplified molecular orbital diagram for the reaction of BF3 with NH3. Labelthe HOMOs and LUMOs of the reactants and product as well as the “driving force” for adductformation. What is the point group of the adduct in the staggered and eclipsed conformations?

Energy needed to vaporize a crystal into a gas of its ions (per mole)

NaCl (s) → Na+ (g) + Cl- (g) , ΔHlattice ≈ U

Term 1: electrostatic (Coulomb) attraction/repulsion of ionsTerm 2: Born or Pauli repulsion term (repulsion of electron clouds)

MgO (highest charges, smallest ions)

SrTe (least ionic of the three compounds)

H3N-BF3 is C3v in both the staggeredand eclipsed conformations.

3. (10 pts) We discussed how Li and Mg are used to prepare reagents that are very strong Lewis bases (nucleophiles). Write down two balanced chemical reactions in which these reagents are employed to make new carbon-carbon bonds (use a Li reagent in the first reaction and a Mg reagent in the second).

4. (10 pts) Predict whether the equilibrium constants for the following reactions are greater than 1 or less than 1 at room temperature (298 K). Explain.

(a) CdI2(s) + CaF2(s) CdF2(s) + CaI2(s)

(b) SO2(g) + (C2H5)2S(g) SO2•(C2H5)2S(g)

(For this one, assume that ΔSo = -0.035 kcal / mol K)

nBuLi + CH3Cl C5H12 + LiCl(s)

Many possible answers to this question. Two examples:

RMgX HX MgX2

solvent

solvent

Use hard-soft acid-base principles. Cd2+ and I- are soft, while Ca2+ and F- are hard.

CdI2 is a soft-soft interaction and CaF2 is a hard-hard interaction, which are favorable compared to the soft-hard combinations of both products. Therefore, we expect the equilibrium to lie strongly toward reactants and Keq << 1.

Use Drago-Wayland parameters to estimate ΔHo of adduct formation. From the provided D-W equation and table of D-W values, we have:

-ΔHo = (0.920*0.399 + 0.808*7.4) = -6.35 kcal/mol

which gives, ΔG = ΔH - TΔS = -6.35 – (298)*(-0.035) = +4.08 kcal/mol

since ΔGo = -RTlnK, a positive ΔGo means that Keq < 1.

5. (15 pts) (a) Define the term Bravais lattice. How many such lattices exist? Name three of them.

(b) Name the crystal structures expected for cubic AB compounds with the following radiusratios: 0.25, 0.5, and 0.75. Draw the plan view diagram for the unit cell of the crystal with a radiusratio of 0.75.

6. (10 pts) (a) Which is the strongest base to H+ in the gas phase: NH3, CH3NH2, (CH3)2NH, (CH3)3N? Explain your answer in one sentence.

(b) Which is the strongest base to B(t-Bu)3 in the gas phase: NH3, CH3NH2, (CH3)2NH, (CH3)3N? Explain.

(c) Which is the strongest Brønsted acid in the gas phase: AsH3, PH3, H2Se, H2S? Explain.

0.25: CN = 4; zinc blende0.5: CN = 6: NaCl0.75: CN = 8: CsCl

An infinite array of points with an arrangement and orientation that is exactly the same at every lattice point. 14 Bravais lattices, including FCC, BCC, hexagonal, simple monoclinic, base-centered orthorhombic, etc.

1/2

Cesium chloride

=

Cesium chloride

(CH3)3N is the strongest base because it has the highest electron density on N (largest inductive effect from electron-donating alkyl groups).

Now the acid is very bulky, so steric effects become as important as inductive effects. CH3NH2 is the best base.

The best acid is the one with the smallest proton affinity. Acidity increases across a row (due to electronegativity) and down a column (due to anion size). H2Se is therefore the strongest acid.

8. EXTRA CREDIT (10 pts) Draw the energy level diagram for a p-n junction at zero appliedbias. Include the conduction and valence band positions and the Fermi levels, indicate thedirection of the diffusion and drift motion of electrons and holes, and label the p and n sides ofthe junction.

7. (15 pts) Consider the interhalogen compound IF7.

(a) Explain the formation of this compound in acid-base terms.

(b) What is the point group of this molecule?

(c) What is the oxidation state of iodine in this compound?

(d) Predict the products of the reaction between IF7 and Fe2O3.

Iodine acts as Lewis acid, fluorines Lewis bases

D5h

All fluorines are -1, so the oxidation state of iodine is +7

Interhalogens are oxidizing agents that evolve O2 from metal oxides:

IF7 + Fe2O3 FeF3 + I2 + O2 (unbalanced)

10. EXTRA CREDIT (10 pts). List the sequence of four (possibly five) chemical reactions used to produce nitric acid on an industrial scale from N2, CH4, H2O, and O2. Don’t bother balancing the reactions.

9. EXTRA CREDIT (10 pts). Derive the packing fraction (i.e., the percentage of filled space) for a simple cubic lattice. Show your work (as always!).

Make H2 :

CH4 + H2O H2 + CO

N2 + 3H2 2NH3

Fix N2 :

CO + H2O H2 + CO2 (this is the possible “extra” reaction)

Oxidize NH3 :

NH3 + O2 NO2 + H2O

NO2 + H2O HNO3 + NO

Make HNO3 :

a

Packing fraction = volume occupied by spheres

total volume of unit cell

1 sphere per unit cell for SC lattice

Volume of unit cell = a3

Volume of sphere =

packing fraction = 0.524, or52.4%