solut18 - CHAPTER EIGHTEEN THE REPRESENTATIVE ELEMENTS:...

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532 CHAPTER EIGHTEEN THE REPRESENTATIVE ELEMENTS: GROUPS 1A THROUGH 4A 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. H ° = -110.5 - [-242 - 75] = 207 kJ; S ° = 198 + 3(131) - [186 + 189] = 216 J/K b. G ° = H ° - T S ° ; G ° = 0 when T = J/K 216 J 10 _ 207 = S H 3 ° ° = 958 K At T > 958 K and standard pressures, the favorable S ° term dominates and the reaction is spontaneous ( G ° < 0). 3. a. H ° = 2(-46 kJ) = -92 kJ; S ° = 2(193 J/K) - [3(131 J/K) + 192 J/K] = -199 J/K; G ° = H ° - T S ° = -92 kJ - 298 K(-0.199 kJ/K) = -33 kJ b. Since G ° is negative, then this reaction is spontaneous at standard conditions. c. G ° = 0 when T = kJ/K 0.199 - kJ 92 - = S H ° ° = 460 K At T < 460 K and standard pressures, the favorable H ° term dominates and the reaction is spontaneous ( 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.
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533 7. a. 2 Rb(s) + 2 H 2 O(l) 2 RbOH(aq) + H 2 (g) b. Na 2 O 2 (s) + 2 H 2 O(l) 2 NaOH(aq) + H 2 O 2 (aq) c. LiH(s) + H 2 O(l) H 2 (g) + LiOH(aq) d. 2 KO 2 (s) + 2H 2 O(l) 2 KOH(aq) + O 2 (g) + H 2 O 2 (aq) 8. 2 Li(s) + 2 C 2 H 2 (g) 2 LiC 2 H(s) + H 2 (g); This is an oxidation-reduction reaction. 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, while 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. 10. NaH(s) + H 2 O(l) Na + (aq) + OH - (aq) + H 2 (g); Acid-base: A proton is transferred from an acid, H 2 O, to a base, H - , forming the conjugate base of water, OH - , and the conjugate acid of H - , H 2 . Oxidation-reduction: The oxidation state of H is -1 in NaH, +1 in H 2 O, and zero in H 2 . Thus, an electron is transferred from the hydride ion to a hydrogen in water when forming H 2 (g). 11. 4 Li(s) + O 2 (g) 2 Li 2 O(s) 16 Li(s) + S 8 (s) 8 Li 2 S(s); 2 Li(s) + Cl 2 (g) 2 LiCl(s) 12 Li(s) + P 4 (s) 4 Li 3 P(s); 2 Li(s) + H 2 (g) 2 LiH(s) 2 Li(s) + 2 H 2 O(l) 2 LiOH(aq) + H 2 (g); 2 Li(s) + 2 HCl(aq) 2 LiCl(aq) + H 2 (g) 12. a. lithium oxide; b. potassium superoxide; c. sodium peroxide 13. 4 KO 2 (s) + 2 CO 2 (g) 2 K 2 CO 3 (s) + 3 O 2 (g); Potassium superoxide can react with exhaled CO 2 to produce O 2 , which then can be inhaled.
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solut18 - CHAPTER EIGHTEEN THE REPRESENTATIVE ELEMENTS:...

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