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Chem 10171 Fall 2017

Tutorial Problem Set 2

Due September 8, 2017

(1) (a) As shown on the graph below, in general the electron affinity of an atom is correlated with its ionization energy. That is, atoms with high ionization energies usually have high electron affinities. Give a rationalization for this general trend. (b) Nitrogen (shown with a filled circle on the plot) is an exception to this general trend. Explain why the electron affinity of N is anomalously low for an atom with its ionization energy. (c) Hydrogen has almost the same ionization energy as oxygen, but the electron affinity of hydrogen is significantly different from the electron affinity of oxygen. Which atom would you predict to have the higher electron affinity, and why are they quite different? (Don't look up the values until after you've made your prediction!) (2)

Draw Lewis structures for the following molecules and ions. Remember to include formal charges where needed. The octet rule (duet rule for H) will be satisfied in all cases; do not draw structures that violate the octet rule. When relevant, draw all significant resonance structures and indicate their relative importance. (a) (c) (e) (g)

SO3 [SO3]2– [CN]– HN3

(b) (d) (f) (h)

H 2O 2 [HCO2]– N 2H 4 [NO3]-

(3)(a) Cyanamide has the formula CH2N2 and has a structure where both hydrogens are bonded to the same nitrogen and the carbon is bonded to both nitrogens (see picture below). Draw a Lewis structure of cyanamide, including all significant resonance structures.

(b) One C–N bond in cyanamide has been experimentally determined to be 115 pm long, while the other is 131 pm long. Which bond corresponds to which bond length? (c) Diazomethane has the same formula as cyanamide (CH2N2), but in diazomethane both hydrogens are bonded to carbon, which is bonded to only one of the nitrogens (see below). Draw a Lewis structure of diazomethane, including all significant resonance structures.

(d) Which molecule would you expect to be more stable, cyanamide or diazomethane? Explain your answer.

Chem 10171 Fall 2017 (4)

Tutorial Problem Set 2

Due September 8, 2017

Nitramide has the formula H2N2O2. There are two different kinds of nitrogen in nitramide: the first nitrogen is bonded to the second nitrogen and both oxygens, while both hydrogens are bonded to the second nitrogen (see below).

(a) Draw a Lewis structure, and any significant resonance structures, of nitramide. Make sure to give formal charges where appropriate, and rank the importance of the resonance structures you draw. (b) Are the two N-O bonds in nitramide the same length or different lengths? What are the bond order(s) of the N-O bonds? (c) The N-N bond in nitramide (138 pm) is significantly shorter than the N-N bond in hydrazine, N2H4 (145 pm; see problem 2[f] for the Lewis structure of hydrazine). Explain why nitramide has a shorter bond than hydrazine. (d) There is another compound with the formula H2N2O2, called hyponitrous acid, in which the atoms are connected in a chain as HONNOH. Draw a Lewis structure, and any significant resonance structures, of hyponitrous acid. Rank the importance of the resonance structures you draw. (e) Which would you expect to have a shorter N-N bond, nitramide or hyponitrous acid? Explain your reasoning....