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zumdahl_chemprin_6e_csm_ch18 - CHAPTER 18 THE...

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707 CHAPTER 18 THE REPRESENTATIVE ELEMENTS Group 1A Elements 1. The gravity of the earth is not strong enough to keep the light H 2 molecules in the atmosphere. 2. a. Δ H° = 110.5 ( 242 75) = 207 kJ; Δ S° = 198 + 3(131) (186 + 189) = 216 J/K b. Δ G° = Δ T Δ S°; Δ G° = 0 when T = K / J 216 J 10 207 S Δ H Δ 3 o o × = = 958 K At T > 958 K and standard pressures, the favorable Δ S° term dominates, and the reaction is spontaneous ( Δ G° < 0). 3. a. Δ H° = 2( 46 kJ) = 92 kJ; Δ S° = 2(193 J/K) [3(131 J/K) + 192 J/K] = 199 J/K Δ G° = Δ T Δ S° = 92 kJ 298 K( 0.199 kJ/K) = 33 kJ b. Because Δ G° is negative, this reaction is spontaneous at standard conditions. c. Δ G° = 0 when T = K / kJ 199 . 0 kJ 92 S Δ H Δ o o = = 460 K At T < 460 K and standard pressures, the favorable Δ H° term dominates, and the reaction is spontaneous ( Δ G° < 0). 4. (1) Ammonia production and (2) hydrogenation of vegetable oils. 5. Ionic, covalent, and metallic (or interstitial); the ionic and covalent hydrides are true compounds obeying the law of definite proportions and differ from each other in the type of bonding. The interstitial hydrides are more like solid solutions of hydrogen with a transition metal and do not obey the law of definite proportions. 6. The small size of the Li + cation results in a much greater attraction to water. The attraction to water is not so great for the other alkali metal ions. Thus lithium salts tend to absorb water. 7. Alkali metals have a n s 1 valence shell electron configuration. Alkali metals lose this valence electron with relative ease to form M + cations when in ionic compounds. They all are easily
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708 CHAPTER 18 THE REPRESENTATIVE ELEMENTS oxidized. Therefore, in order to prepare the pure metals, alkali metals must be produced in the absence of materials (H 2 O, O 2 ) that are capable of oxidizing them. The method of preparation is electrochemical processes, specifically, electrolysis of molten chloride salts and reduction of alkali salts with Mg and H 2 . In all production methods, H 2 O and O 2 must be absent. 8. Counting over in the periodic table, the next alkali metal will be element 119. It will be located under Fr. One would expect the physical properties of element 119 to follow the trends shown in Table 18.4. Element 119 should have the smallest ionization energy, the most negative standard reduction potential, the largest radius, and the smallest melting point of all the alkali metals listed in Table 18.4. It should also be radioactive like Fr. 9. Hydrogen forms many compounds in which the oxidation state is +1, as do the Group 1A elements. For example, H 2 SO 4 and HCl as compared to Na 2 SO 4 and NaCl. On the other hand, hydrogen forms diatomic H 2 molecules and is a nonmetal, whereas the Group 1A elements are metals. Hydrogen also forms compounds with a -1 oxidation state, which is not characteristic of Group 1A metals, e.g., NaH.
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zumdahl_chemprin_6e_csm_ch18 - CHAPTER 18 THE...

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