Chapter 23

Explain why aluminum, magnesium, and sodium metals are obtained by electrolysis instead of by reduction with chemical reducing agents.

Write balanced chemical equations for each of the following verbal descriptions: (a) Vanadium oxytrichloride \(\left(\mathrm{VOCl}_{3}\right)\) is formed by the reaction of vanadium(III) chloride with oxygen. (b) Niobium(V) oxide is reduced to the metal with hydrogen gas. (c) Iron(III) ion in aqueous solution is reduced to iron(II) ion in the presence of zinc dust. (d) Niobium(V) chloride reacts with water to yield crystals of niobic acid \(\left(\mathrm{HNbO}_{3}\right)\).

Write a balanced chemical equation to correspond to each of the following verbal descriptions: (a) \(\mathrm{NiO}(s)\) can be solubilized by leaching with aqueous sulfuric acid. (b) After concentration, an ore containing the mineral carrollite \(\left(\mathrm{CuCo}_{2} \mathrm{~S}_{4}\right)\) is leached with aqueous sulfuric acid to produce a solution containing copper ions and cobalt ions. (c) Titanium dioxide is treated with chlorine in the presence of carbon as a reducing agent to form \(\mathrm{TiCl}_{4}\) (d) Under oxygen pressure \(\mathrm{ZnS}(s)\) reacts at \(150^{\circ} \mathrm{C}\) with aqueous sulfuric acid to form soluble zinc sulfate, with deposition of elemental sulfur.

The crude copper that is subjected to electrorefining contains tellurium as an impurity. The standard reduction potential between tellurium and its lowest common oxidation state, \(\mathrm{Te}^{4+}\), is $$\mathrm{Te}^{4+}(a q)+4 \mathrm{e}^{-} \longrightarrow \mathrm{Te}(s) \quad E_{\mathrm{red}}^{\circ}=0.57 \mathrm{~V}$$ Given this information, describe the probable fate of tellurium impurities during electrorefining.

Why is the \(+2\) oxidation state common among the transition metals? Why do so many transition metals exhibit a variety of oxidation states?

Write balanced chemical equations that correspond to the steps in the following brief account of the metallurgy of molybdenum: Molybdenum occurs primarily as the sulfide, \(\mathrm{MoS}_{2}\). On boiling with concentrated nitric acid, a white residue of \(\mathrm{MoO}_{3}\) is obtained. This is an acidic oxide; when it is dissolved in excess hot concentrated ammonia, ammonium molybdate crystallizes on cooling. On heating ammonium molybdate, white \(\mathrm{MoO}_{3}\) is obtained. On further heating to \(1200^{\circ} \mathrm{C}\) in hydrogen, a gray powder of metallic molybdenum is obtained.

Introduction of carbon into a metallic lattice generally results in a harder, less ductile substance with lower electrical and thermal conductivities. Explain why this might be so.

The thermodynamic stabilities of the three complexes \(\mathrm{Zn}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}^{2+}, \mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\), and \(\mathrm{Zn}(\mathrm{CN})_{4}^{2-}\) increase from the \(\mathrm{H}_{2} \mathrm{O}\) to the \(\mathrm{NH}_{3}\) to the \(\mathrm{CN}^{-}\) complex. How do you expect the reduction potentials of these three complexes to compare?

Indicate whether each of the following compounds is expected to be diamagnetic or paramagnetic, and give a reason for your answer in each case: (a) \(\mathrm{NbCl}_{5}\), (b) \(\mathrm{CrCl}_{2}\), (c) \(\mathrm{CuCl}\), (d) \(\mathrm{RuO}_{4}\), (e) \(\mathrm{NiCl}_{2}\).

Associated with every ferromagnetic solid is a temperature known as its Curie temperature. When heated above its Curie temperature, the substance no longer exhibits ferromagnetism but rather becomes paramagnetic. Use the kinetic- molecular theory of solids toexplain this observation.

Associated with every antiferromagnetic solid is a temperature known as its Néel temperature. When heated above its Néel temperature the magnetic behavior changes from antiferromagnetic to paramagnetic. In contrast diamagnetic substances do not generally become paramagnetic upon heating. How do you explain this difference in behavior?

Write balanced chemical equations for each of the following reactions characteristic of elemental manganese: (a) It reacts with aqueous \(\mathrm{HNO}_{3}\) to form a solution of manganese(II) nitrate. (b) When solid manganese(II) nitrate is heated to \(450 \mathrm{~K}\), it decomposes to \(\mathrm{MnO}_{2}\). (c) When \(\mathrm{MnO}_{2}\) is heated to \(700 \mathrm{~K}\), it decomposes to \(\mathrm{Mn}_{3} \mathrm{O}_{4}\). (d) When solid \(\mathrm{MnCl}_{2}\) is reacted with \(\mathrm{F}_{2}(g)\), it forms \(\mathrm{MnF}_{3}\) (one of the products is \(\mathrm{ClF}_{3}\) ).

Based on the chemistry described in this chapter and others, propose balanced chemical equations for the following sequence of reactions involving nickel: (a) The ore millerite, which contains NiS, is roasted in an atmosphere of oxygen to produce an oxide. (b) The oxide is reduced to the metal, using coke. (c) Dissolving the metal in hydrochloric acid produces a green solution. (d) Adding excess sodium hydroxide to the solution causes a gelatinous green material to precipitate. (e) Upon heating, the green material loses water and yields a green powder.

Indicate whether each of the following solids is likely to be an insulator, a metallic conductor, or a semiconductor: (a) \(\mathrm{TiO}_{2}\), (b) \(\mathrm{Ge}\), (c) \(\mathrm{Cu}_{3} \mathrm{Al}\), (d) \(\mathrm{Pd}\), \((\mathrm{e}) \mathrm{SiC}\), (f) \(\mathrm{Bi}\).

(a) A charge of \(3.3 \times 10^{6} \mathrm{~kg}\) of material containing \(27 \%\) \(\mathrm{Cu}_{2} \mathrm{~S}\) and \(13 \%\) FeS is added to a converter and oxidized. What mass of \(\mathrm{SO}_{2}(g)\) is formed? (b) What is the molar ratio of \(\mathrm{Cu}\) to \(\mathrm{Fe}\) in the resulting mixture of oxides? (c) What are the likely formulas of the oxides formed in the oxidation reactions, assuming an excess of oxygen? (d) Write balanced equations representing each of the oxidation reactions.

In an electrolytic process nickel sulfide is oxidized in a two-step reaction: $$ \begin{aligned} \mathrm{Ni}_{3} \mathrm{~S}_{2}(\mathrm{~s}) & \longrightarrow \mathrm{Ni}^{2+}(a q)+2 \mathrm{NiS}(s)+2 \mathrm{e}^{-} \\ \mathrm{NiS}(s) & \longrightarrow \mathrm{Ni}^{2+}(a q)+\mathrm{S}(s)+2 \mathrm{e}^{-} \end{aligned} $$ What mass of \(\mathrm{Ni}^{2+}\) is produced in solution by passing a current of 67 A for a period of \(11.0 \mathrm{hr}\), assuming the cell is \(90 \%\) efficient?

Copper(I) is an uncommon oxidation state in aqueous acidic solution because \(\mathrm{Cu}^{+}(a q)\) disproportionates into \(\mathrm{Cu}^{2+}\) and \(\mathrm{Cu}\). Use data from Appendix \(\mathrm{E}\) to calculate the equilibrium constant for the reaction $$ 2 \mathrm{Cu}^{+}(a q) \rightleftharpoons \mathrm{Cu}^{2+}(a q)+\mathrm{Cu}(s) $$

The reduction of metal oxides is often accomplished using carbon monoxide as a reducing agent. Carbon (coke) and carbon dioxide are usually present, leading to the following reaction: $$\mathrm{C}(\mathrm{s})+\mathrm{CO}_{2}(g) \rightleftharpoons 2 \mathrm{CO}(g)$$ Using data from Appendix \(\mathrm{C}\), calculate the equilibrium constant for this reaction at \(298 \mathrm{~K}\) and at \(2000 \mathrm{~K}\), assuming that the enthalpies and entropies of formation do not depend upon temperature.

An important process in the metallurgy of titanium is the reaction between titanium dioxide and chlorine in the presence of carbon, which acts as a reducing agent, leading to the formation of gaseous \(\mathrm{TiCl}_{4}\). (a) Write a balanced chemical equation for this reaction, and use it with the values listed in Appendix \(C\) to calculate the standard enthalpy change of this reaction. Is this reaction exothermic or endothermic? (b) Write a reaction for the direct reaction between titanium dioxide and chlorine to form \(\mathrm{TiCl}_{4}\) and oxygen. Is this reaction exothermic or endothermic?

Magnesium is obtained by electrolysis of molten \(\mathrm{MgCl}_{2}\). (a) Why isn't an aqueous solution of \(\mathrm{MgCl}_{2}\) used in the electrolysis? (b) Several cells are connected in parallel by very large copper buses that convey current to the cells. Assuming that the cells are \(96 \%\) efficient in producing the desired products in electrolysis, what mass of \(\mathrm{Mg}\) is formed by passing a current of \(97,000 \mathrm{~A}\) for a period of \(24 \mathrm{hr} ?\)

The galvanizing of iron sheet can be carried out electrolytically using a bath containing a zinc sulfate solution. The sheet is made the cathode, and a graphite anode is used. Calculate the cost of the electricity required to deposit a 0.49-mm layer of zinc on both sides of an iron sheet \(2.0 \mathrm{~m}\) wide and \(80 \mathrm{~m}\) long if the current is \(30 \mathrm{~A}\), the voltage is \(3.5 \mathrm{~V}\), and the energy efficiency of the process is \(90 \%\). Assume the cost of electricity is \(\$ 0.082\) per kilowatt hour. The density of zinc is \(7.1 \mathrm{~g} / \mathrm{cm}^{3}\).

The heats of atomization, \(\Delta H_{\text {atom }}\), in \(\mathrm{kJ} / \mathrm{mol}\), of the first transition series of elements are as follows: \(\begin{array}{lllllllllll} \text { Element } & \text { Ca } & \text { Sc } & \text { Ti } & \text { V } & \text { Cr } & \text { Mn } & \text { Fe } & \text { Co } & \text { Ni } & \text { Cu } \\ \hline \Delta H_{\text {atom }} & 178 & 378 & 471 & 515 & 397 & 281 & 415 & 426 & 431 & 338 \end{array}\) (a) Write an equation for the process involved in atomization, and describe the electronic and structural changes that occur. (b) \(\Delta H_{\text {atom }}\) varies irregularly in the series following \(\mathrm{V}\). How can this be accounted for, at least in part, using the electronic configurations of the gaseous atoms? (Hint: Recall the discussions of Sections \(6.8\) and \(6.9 .)\)