The Foundations of Chemistry

CaCO heat

3 (s)888nCaO(s)CO 2 (g) lime production

CaO(s)H 2 O(
)888nCa(OH) 2 (s) slaking lime

Ca(OH) 2 (s)Mg^2 (aq)888nCa^2 (aq)Mg(OH) 2 (s) precipitation

The last reaction occurs because Kspfor Mg(OH) 2 , 1.5 10 ^11 , is much smaller than that

for Ca(OH) 2 , 7.9 10 ^6. The milky white suspension of Mg(OH) 2 is filtered, and the
solid Mg(OH) 2 is then neutralized with HCl to produce a MgCl 2 solution. Evaporation
of the H 2 O leaves solid MgCl 2 , which is then melted and electrolyzed (Figure 22-6) under
an inert atmosphere to produce molten Mg and gaseous Cl 2. The products are separated
as they are formed, to prevent recombination.

Magnesium is cast into ingots or alloyed with other light metals. The Cl 2 by-product is
used to produce more HCl for neutralization of Mg(OH) 2.

ALUMINUM

Aluminum is the most commercially important nonferrous metal. Its chemistry and uses
will be discussed in Section 23-7. Aluminum is obtained from bauxite, or hydrated
aluminum oxide, Al 2 O 3
xH 2 O. Aluminum ions can be reduced to Al by electrolysis only
in the absence of H 2 O. First the crushed bauxite is purified by dissolving it in a concen-
trated solution of NaOH to form soluble Na[Al(OH) 4 ]. Then Al(OH) 3
xH 2 O is
precipitated from the filtered solution by blowing in carbon dioxide to neutralize the unre-
acted NaOH and one OHion per formula unit of Na[Al(OH) 4 ]. Heating the hydrated
product dehydrates it to Al 2 O 3.

22-6

Mg(OH) 2 (s)2[H(aq)Cl(aq)] [Mg^2 (aq)2Cl(aq)] 2H 2 O

MgCl 2 (
)ClMg(
) 2 (g)

evaporate solution, electrolysis

then melt solid

22-6 Aluminum 909

Mg dipped out

Molten MgCl 2

Steel cathode

Graphite anode

Chlorine gas

Inert

atmosphere

Molten Mg

• –

Figure 22-6 A cell for electrolyzing molten MgCl 2. The magnesium metal is formed on
the steel cathode and rises to the top, where it is dipped off periodically. Chlorine gas is
formed around the graphite anode and is piped off.

A bed of limestone, CaCO 3 , along

the Verde River, Arizona.

Recall that Al 2 O 3 is amphoteric.

Impurities such as oxides of iron,

which are not amphoteric, are left

behind in the crude ore.