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7.6 End-of-Chapter Material
♦ Most plants, animals, and bacteria use oxygen as the terminal oxidant in their respiration process. In a few locations, however, whole ecosystems have developed in the absence of oxygen, containing creatures that can use sulfur compounds instead of oxygen. List the common oxidation states of sulfur, provide an example of any oxoanions containing sulfur in these oxidation states, and name the ions. Which ion would be the best oxidant?
Titanium is currently used in the aircraft industry and is now used in ships, which operate in a highly corrosive environment. Interest in this metal is due to the fact that titanium is strong, light, and corrosion resistant. The densities of selected elements are given in the following table. Why can an element with an even lower density such as calcium not be used to produce an even lighter structural material?
Element Density (g/cm3) Element Density (g/cm3) K 0.865 Cr 7.140 Ca 1.550 Mn 7.470 Sc 2.985 Fe 7.874 Ti 4.507 Co 8.900 V 6.110 Ni 8.908
♦ The compound Fe3O4 was called lodestone in ancient times because it responds to Earth’s magnetic field and can be used to construct a primitive compass. Today Fe3O4 is commonly called magnetite because it contains both Fe2+ and Fe3+, and the unpaired electrons on these ions align to form tiny magnets. How many unpaired electrons does each ion have? Would you expect to observe magnetic behavior in compounds containing Zn2+? Why or why not? Would you expect Fe or Zn to have the lower third ionization energy? Why?
♦ Understanding trends in periodic properties allows us to predict the properties of individual elements. For example, if we need to know whether francium is a liquid at room temperature (approximately 20°C), we could obtain this information by plotting the melting points of the other alkali metals versus atomic number. Based on the data in the following table, would you predict francium to be a solid, a liquid, or a gas at 20°C?
Li Na K Rb Cs Melting Point (°C) 180 97.8 63.7 39.0 28.5
Francium is found in minute traces in uranium ores. Is this consistent with your conclusion? Why or why not? Why would francium be found in these ores, but only in small quantities?
Problems marked with a ♦ involve multiple concepts.
Due to its 3s2 3p4 electron configuration, sulfur has three common oxidation states: +6, +4, and −2. Examples of each are: −2 oxidation state, the sulfide anion, S2− or hydrogen sulfide, H2S; +4 oxidation state, the sulfite ion, SO32−; +6 oxidation state, the sulfate ion, SO42−. The sulfate ion would be the best biological oxidant, because it can accept the greatest number of electrons.
Iron(II) has four unpaired electrons, and iron(III) has five unpaired electrons. Compounds of Zn2+ do not exhibit magnetic behavior, because the Zn2+ ion has no unpaired electrons. The third ionization potential of zinc is larger than that of iron, because removing a third electron from zinc requires breaking into the closed 3d10 subshell.