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E.8 The chlor-alkali industry

E.8.1

Chlorine is produced by the electrolysis of sodium chloride, because it is not easy to chemically separate them. This is, today, done in a diaphragm cell...

The diaphragm cell is basically a container separated down the center with a porous (commonly asbestos) diaphragm. The container has brine (NaCl in water) pumped into the left hand side. above is a Ti anode where Chlorine is produced, and piped off. Spent brine flows through and at the steel cathode on this side, water is reduced to form H₂ (and OH which forms NaOH with the sodium left over from before). This sodium hydroxide solution is then piped out from the bottom, while the H₂ is piped off the top. The two gases must be separated because the would react explosively if they came into contact.

E.8.2

The fact that the above cell produces Cl₂, H₂ and NaOH is very convenient, since all three are useful products, and the cell uses comparatively little energy...uses

Chlorine - Dry cleaning solvents, refrigerants, PVC, disinfectant

Hydrogen - Ammonium production, fuel cells, reducing agent.

NaOH - Neutralizing acids, producing soap, aluminum production

E.8.3

The diaphragm cell above has replaced a similar one using a flowing mercury cathode due to concerns about mercury poisoning. Also, many problems with Chlorine containing solvents have been found (specifically the ozone layer) which makes this process somewhat less favorable.

E.9 Silicon

E.9.1

Silicon is the second most abundant element in the earth's crust (28%)

Extraction -- Reduction of silica (sand) by coke in an electric furnace

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More pure Si can be obtained from SiCl₄ by reduction in H₂.

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Very pure silicon can be produced by zone refining the silicon above. Long rods of impure silicon are passed through a heating element, which melts the silicon as it passes through. Impurities dissolve in this molten portion, and so as the Silicon passes through, the impurities are left behind, and eventually the end is cut off. (Silicon does have a high melting point however, so this is expensive).

E.9.2 : Semiconductors

In it's natural state, silicon is a large covalent lattice of silicon molecules. Semiconductors are created by doping (or adding one to about every million) the silicon with either Phosphorus (P) or Boron (B). Because Phosphorus has one extra valence electron, it can be removed and effectively carry a voltage through the lattice. Boron, on the other hand, has one less, and so it is possible to excite a valence electron from an Si atom into the gap...this allows a positive charge to be carried by the hole. Phosphorus is an n-type semiconductor (n for negative) while boron is p-type (positive).

E.10 Ellingham diagrams

There is an ellingham diagram in the data book, it works like this. The graph shows gibbs free energies at various temperatures. If you want one reaction to force the other to occur in reverse, then it's line must be below the other for the given temperature (because the second will be reversed, and -ve will become +ve). In general, it is used to see if reaction can be used to reduce another.

E.11

E.11.1 : Thermal cracking vs Catalytic

Thermal cracking uses homolytic breaking of bonds, creating free radical carbon chains. These, therefore act with a free radical mechanism.

Catalytic cracking breaks the bonds homolytically, producing complex ions, and thus an ionic mechanism.

E.11.2 : LDPE vs HDPE polythene mechanisms

LDPE (Low density polythene) - this is produced by the polymerisation of ethene of 200^oC and 1200 atm, with trace amounts of O₂ present. This produces highly branched chains, and so is soft, malleable etc.

Mechanism:

Initiator - Benzene(=O)-O-O-(O=)Benzene

The central peroxide link breaks, and 2Benzene° + 2CO₂ is produced. These benzene radicals the react with ethene, forming a Benzene-CH₂-CH₂° free radical, and so on until it terminates with two free radicals meeting.

HDPE (high density polythene) is produced at 60^oC and 1 atm using a zieger catalyst, and an ionic mechanism. It has fewer branched chains, and so is less malleable and has a higher melting point...and obviously it's higher density.

E.11.3 : Principles of condensation polymers

• The forming of monomers with the expulsion of a small group (H₂O or H₂) to form a polymer.

• The monomers must have two functional groups to react and form a continual chain with other monomers.

Nylons ( 1,6-hexanedioic acid + 1,6-hexanedamide, expelling water) and Polyesters ( 1,4-Benzenediol and ethanedioic acid, expelling water)

Polyurethanes - I'll come back to this (probably never)...it's hard to describe without a diagram.

Silicones - Monomer is HO-(R)Si(R)-OH and produces ---O-(R)Si(R)-O-(R)Si(R)-O-(R)Si(R)-O- expelling water each time.

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