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7 Periodic Properties of the Elements Solutions to Exercises (b) K₂O₂(s) + H₂O₂(aq) + 2 KOH(aq) potassium peroxide hydrogen peroxide KO₂(s) + H₂O₂(aq) + 2KOH(aq) + O₂(g) potassium superoxide hydrogen peroxide Both potassium peroxide and potassium superoxide react with water to form hydrogen peroxide. The white solid could be either potassium salt. 7.73 (a) 2K(s) + Cl₂(g) 2KCl(s) (b) SrO(s) + Sr(OH)₂(aq) (c) 4Li(s) + 2Li₂O(s) (d) 2Na(s) + S(1) Na₂S(s) 7.74 (a) 2Cs(s) + 2CsOH(aq) (b) Sr(s) + Sr(OH)₂(aq) + (c) 2Na(s) + O₂(g) Na₂O₂(s) (See Equation [7.20].) (d) Ca(s) + CaI₂(s) 7.75 (a) The reactions of the alkali metals with hydrogen and with a halogen are redox reactions. In both classes of reaction, the alkali metal loses electrons and is oxidized. Both hydrogen and the halogen gain electrons and are reduced. The product is an ionic solid, where either hydride ion, or a halide ion, is the anion and the alkali metal is the cation. (b) Ca(s) + F₂(g) CaF₂(s) Ca(s) CaH₂(s) Both products are ionic solids containing Ca²⁺ and the corresponding anion in a 1:2 ratio. 7.76 Metallic hydrogen, in the form of an extended solid, will be much more like the other alkali metals, which are also extended solids. "Normal" gaseous hydrogen has a smaller atomic radius and larger first ionization energy than trends would predict for the first member of the alkali metals. Metallic hydrogen would have a larger atomic radius and smaller first ionization energy than "normal" hydrogen. Owing to its smaller first ionization energy, metallic hydrogen will be more reactive than "normal" hydrogen. 7.77 Br (a) (b) -1 -1 (c) (d) reacts slowly to form HBr+HOBr reacts slowly to form (e) (f) 1.14 Å 0.99 Å The n = 4 valence electrons in Br are farther from the nucleus and less tightly held than the n = 3 valence electrons in Therefore, the ionization energy of Cl is greater, the electron affinity is more negative and the atomic radius is smaller. 180