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solut19 - CHAPTER NINETEEN THE REPRESENTATIVE ELEMENTS...

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544 CHAPTER NINETEEN THE REPRESENTATIVE ELEMENTS: GROUPS 5A THROUGH 8A Group 5A Elements 1. NO 4 3- Both NO 4 3- and PO 4 3- have 32 valence electrons so both have similar Lewis structures. From the Lewis structure for NO 4 3- , the central N atom has a tetrahedral arrangement of electron pairs. N is small. There is probably not enough room for all 4 oxygen atoms around N. P is larger, thus, PO 4 3- is stable. PO 3 - PO 3 - and NO 3 - both have 24 valence electrons so both have similar Lewis structures. From the Lewis structure, PO 3 - has a trigonal arrangement of electron pairs about the central P atom (two single bonds and one double bond). P _ O bonds are not particularly stable, while N _ O bonds are stable. Thus, NO 3 - is stable. 2. H ° = 2(90. kJ) - [0 + 0] = 180. kJ; S ° = 2(211 J/K) - [192 + 205] = 25 J/K G ° = 2(87 kJ) - [0] = 174 kJ At the high temperatures in automobile engines, the reaction N 2 + O 2 2 NO becomes spontaneous since the favorable S ° term will become dominate. In the atmosphere, even though 2 NO N 2 + O 2 is spontaneous at the cooler temperatures of the atmosphere, it doesn’t occur because the rate is slow. Therefore, higher concentrations of NO are present in the atmosphere as compared to what is predicted by thermodynamics. 3- O N O O O P O O O
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