Magnesium is a valuable, lightweight metal. It is used as a structural metal and in alloys, in batteries, and in chemical synthesis. Although magnesium is plentiful in Earth's crust, it is cheaper to "mine" the metal from seawater. Magnesium forms the second most abundant cation in the sea (after sodium); there are about 1.30 g of magnesium in 1.00 kg of seawater. The method of obtaining magnesium from seawater employs all three types of reactions discussed in this chapter: precipitation, acid-base, and redox reactions. In the first stage in the recovery of magnesium, limestone (CaCO3) is heated at high temperatures to produce quicklime, or calcium oxide (CaO): CaCO3(s) ----> CaO(s) + CO2(g) When calcium oxide is treated with seawater, it forms calcium hydroxide [Ca(OH)2], which is slightly soluble and ionizes to give Ca2* and OH ions: CaO(s) + H2Og- Ca? + 2 OH (aq) (aq) The surplus hydroxide ions cause the much less soluble magnesium hydroxide to precipitate: Mg2+ (aq) + 2 OH (ag) ----> Mg(OH)2(s) The solid magnesium hydroxide is filtered and reacted with hydrochloric acid to form magnesium chloride (MgCl2): Mg(OH)2(s) + 2HCI(ag) ---> MgCl2(aq) + 2H200 After the water is evaporated, the solid magnesium chloride is melted in a steel cell. The molten magnesium chloride contains both Mg2* and CH ions. In a process called electrolysis, an electric current is passed through the cell to reduce the Mg2 ions and oxidize the Cl ions. The half- reactions are: Mg* (ag) +2 e --> Mg(s) 2 Cr (ag) -----> Cl2) + 2 e The overall reaction is: MgCl20 -----> Mg(s) + Cl2g) This is how magnesium metal is produced. The chlorine gas generated can be converted to hydrochloric acid and recycled through the process. a) Identify the precipitation, acid-base, and redox processes. b) Instead of calcium oxide, why don't we simply add sodium hydroxide to precipitate magnesium hydroxide? c) Sometimes a mineral called dolomite (a combination of CaCOz and MgCO3) is substituted for limestone (CaCO3) to bring about the precipitation of magnesium hydroxide. What is the advantage of using dolomite? d) What are the advantages of mining magnesium from the ocean rather than from Earth's crust?