4.7 Industrial, Environmental, and Atmospheric Inorganic Chemistry and Advanced Materials

4.7 Industrial, Environmental, and Atmospheric Inorganic Chemistry and Advanced Materials

Quick Answer: The last Inorganic TOS area (6 items) asks you to apply inorganic principles to industrial processes, environmental and atmospheric chemistry, and advanced materials. Section 4.1 covers the Haber-Bosch, Ostwald, and Contact processes. This section adds chlor-alkali and Solvay chemistry, extractive metallurgy, stratospheric ozone, smog and acid rain, water-quality parameters, and materials such as zeolites, superconductors, and metal-organic frameworks.

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Major Inorganic Industries

Chlor-Alkali Process

Electrolysis of brine produces three commodities at once:

$2NaCl + 2H_2O \xrightarrow{\text{electrolysis}} 2NaOH + Cl_2 + H_2$

Chlorine forms at the anode and hydrogen at the cathode. Modern plants use membrane cells: a cation-exchange membrane passes $Na^+$ but blocks $Cl^-$ and $OH^-$, giving pure caustic soda. The older mercury cell (Section 9.4) is being phased out because of mercury pollution.

The Solvay (Ammonia-Soda) Process

Sodium carbonate is made from brine and limestone, with ammonia recycled:

1. $CaCO_3 \rightarrow CaO + CO_2$ (lime kiln)
2. $NaCl + NH_3 + CO_2 + H_2O \rightarrow NaHCO_3\downarrow + NH_4Cl$ (sodium hydrogen carbonate is the least soluble salt and precipitates)
3. $2NaHCO_3 \rightarrow Na_2CO_3 + CO_2 + H_2O$ (calcination)
4. $2NH_4Cl + CaO \rightarrow 2NH_3 + CaCl_2 + H_2O$ (ammonia recovery)

Overall: $CaCO_3 + 2NaCl \rightarrow Na_2CO_3 + CaCl_2$. Calcium chloride is the main by-product.

Extractive Metallurgy and Ellingham Diagrams

An Ellingham diagram plots $\Delta G^\circ$ for forming oxides (per mole of $O_2$) against temperature. A metal whose line lies lower can reduce the oxide of a metal whose line lies higher. The line for $2C + O_2 \rightarrow 2CO$ slopes downward because gas moles increase. At high temperature carbon therefore reduces many oxides, which is the basis of smelting.

Aluminium cannot be made by carbon reduction because its oxide line lies far below carbon's at practical temperatures. Cryolite matters because pure alumina melts above 2000 C.

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Atmospheric Chemistry

Stratospheric Ozone

The Chapman cycle forms and destroys ozone naturally:

$O_2 + h\nu\,(\lambda < 242\text{ nm}) \rightarrow 2O$ $O + O_2 + M \rightarrow O_3 + M$ $O_3 + h\nu \rightarrow O_2 + O$ $O + O_3 \rightarrow 2O_2$

Chlorofluorocarbons (CFCs) are photolysed in the stratosphere, releasing chlorine atoms that destroy ozone catalytically:

$Cl + O_3 \rightarrow ClO + O_2$ $ClO + O \rightarrow Cl + O_2$

Net: $O_3 + O \rightarrow 2O_2$, with $Cl$ regenerated, so one chlorine atom can destroy thousands of ozone molecules. Polar stratospheric clouds release active chlorine, causing the Antarctic ozone hole. The Montreal Protocol (1987) phases out CFCs and other ozone-depleting substances.

Tropospheric Pollution

• Photochemical smog: $NO_2 + h\nu \rightarrow NO + O$, then $O + O_2 \rightarrow O_3$. With volatile organic compounds this produces ground-level ozone and peroxyacetyl nitrate (PAN), both eye and lung irritants.
• Acid rain: $SO_2$ and $NO_x$ are oxidized to $H_2SO_4$ and $HNO_3$. Unpolluted rain is already slightly acidic, with a pH of about 5.6, because dissolved $CO_2$ forms carbonic acid.
• Greenhouse gases: $CO_2$, $CH_4$, $N_2O$, and halocarbons absorb infrared radiation from the Earth. Methane and nitrous oxide have much larger global-warming potentials per molecule than $CO_2$.
• Particulates: $PM_{10}$ and $PM_{2.5}$ come from combustion and secondary sulfate and nitrate formation.

The Philippine Clean Air Act of 1999 (RA 8749) regulates air emissions, and the Clean Water Act of 2004 (RA 9275) regulates water quality and effluents.

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Water Chemistry Parameters

Toxic metals such as $Hg$, $Cd$, $Pb$, and $As$ concentrate up food chains (bioaccumulation and biomagnification). Methylmercury caused the Minamata disaster in Japan.

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Advanced Inorganic Materials

• Zeolites: crystalline aluminosilicates built from $SiO_4$ and $AlO_4$ tetrahedra. Each Al introduces a negative framework charge balanced by exchangeable cations. Uniform pores make them molecular sieves, water softeners, and solid-acid catalysts for petroleum cracking.
• Ceramics: alumina, silicon carbide, and silicon nitride offer hardness and heat resistance. Glass is an amorphous silicate network whose properties are tuned by network modifiers such as $Na_2O$ and $CaO$.
• Superconductors: below a critical temperature resistance drops to zero and magnetic fields are expelled (the Meissner effect). The cuprate $YBa_2Cu_3O_7$ superconducts below about 92 K, conveniently above liquid nitrogen's 77 K.
• Semiconductors and photovoltaics: doped silicon (Section 4.3), GaAs, and perovskite solar absorbers.
• Nanomaterials: quantum dots such as CdSe show size-dependent band gaps (smaller dots emit bluer light). Carbon nanotubes and graphene have exceptional strength and conductivity.
• Metal-organic frameworks (MOFs): metal nodes linked by organic ligands into porous crystals with very high surface areas, used for gas storage, separation, and catalysis. MOF chemistry received the 2025 Nobel Prize in Chemistry.