1 (232)

Prévia do material em texto

8 Chemical Bonding Solutions to Exercises (b) 16 8 pairs N -1 +1 -1 0 +1 -2 -2 +1 0 (c) The structure with two double bonds minimizes formal charges and is probably the main contributor. (d) The two N-N bond lengths will be equal. The two minor contributors would individually cause unequal N-N distances, but collectively they contribute equally to the lengthening and shortening of each bond. The N-N distance will be approximately 1.24 Å, the average N=N distance. 8.108 (a) C₂H₂: 10 5 pair N₂: 10 pair (b) The enthalpy of formation for N₂ is and for is 226.77 kJ/mol. N₂ is an extremely stable, unreactive compound. Under appropriate conditions, it can be either oxidized or reduced. is a reactive gas, used in combination with O₂ for welding and as starting material for organic synthesis. (c) 2N₂O₅(g) 2C₂H₂(g) (d) = rxn (C₂H₂) = CO₂ + - - O₂(g) The oxidation of is highly exothermic, which means that the energy state of the combined products is much lower than that of the reactants. The reaction is "downhill" in an energy sense, and occurs readily. The oxidation of N₂ is mildly endothermic (energy of products higher than reactants) and the reaction does not readily occur. This is in agreement with the general reactivities from part (b). Referring to bond enthalpies in Table 8.4, when the C-H bonds are taken into account, even more energy is required for bond breaking in the oxidation of than in the oxidation of The difference seems to be in the enthalpies of formation of the products. and H₂O(g) have extremely exothermic values, which cause the oxidation of to be energetically favorable. has an endothermic value, which causes the oxidation of N₂ to be energetically unfavorable. 226