NAME_________________________________

1) (4 points) Develop the distribution diagram for sulfurous acid, H₂SO₃.

pKa = 8.5 - 5.7(3-2) + 10.2(0) = 2.8. pKa of HSO3- is 4.5 units higher at 7.3. The distribution diagram should have crossovers at pH 2.8 and pH 7.3. H2SO3 predominates at pH < 2.8; HSO3- predominates at 2.8 < pH < 7.3; SO32- predominates at pH > 7.3.

Using the distribution diagram as a guide, calculate the numerical value of the ratio [HSO₃^-]/[SO₃^2-] at pH 9.0

Ka2 = 10^-7.3 = [H3O+][SO32-]/[HSO3-]. At pH = 9, [H3O+] = 10^-9 M. Thus [SO32-]/[HSO3-] = 50.1; the inverse ratio is 0.020. 

2) (8 points) In Test 1 we encountered the compound, potassium peroxydisulfate, with the formula K₂S₂O₈. The compound is ionic, consisting of K⁺ cations and S₂O₈^2- anions.

a) Propose a Lewis structure for the anion that is consistent with the following constraints: S has oxidation state 6+; six Os have oxidation state 2-; two Os have oxidation state 1-. Your Lewis structure should be the minimum formal charge resonance form.

Draw the chain, O-S-O-O-S-O, all single bonds. To each S, add two double-bonded oxygens. Fill in lone pairs.

b) How many resonance forms are possible for the anion? There are 49 resonance forms. 

c) State the stereochemistry at every atom that is bonded to two or more other atoms in the anion.

Both S atoms tetrahedral; peroxide oxygens are bent.

d) Indicate the hybrid orbital scheme employed by one of the sulfur atoms. Both are sp3.

4) (5 points) What hybridization is employed by the first-listed atom in each compound? Here you should develop the Lewis structure, then base hybridization on the number of electron groups around the central atom.

PO₂F: 3 electron groups around P, so sp2.

N₂O₅: 3 electron groups arount N, so sp2.

SO₂(OH)F: 4 electron groups around S, so sp3.

BH₃O₃: 3 electron groups around B, so sp2.

P₄O₆ (the structure consists of a tetrahedron of P atoms with O atoms bridging P atoms along each edge of the tetrahedron. P is bonded to O, not to other Ps): 4 electron groups around P, so sp3.

5) (3 points) Use bond energies to argue either for or against each statement:

F₂(g) and O₂(g) should react explosively to generate OF₂(g). NO, reaction is endothermic.

N₂(g) and P₄(g) should react spontaneously to form P₄N₂, a ring compound similar to benzene in which the two N atoms are at adjacent vertices of the hexagon. Here you have to guess at the P=P bond strength by averaging P-P and Pº P. NO, reaction is endothermic.

The formate anion, HCO₂^- , is more stable in the arrangement, C(=O)(OH)^- than in the arrangement, HC(=O)O^-. YES, bond energies support this.