Revision notes: Manufactured Substances in Industry

These notes work through Manufactured Substances in Industry one content standard at a time. The chapter rewards two skills above all: explaining why each industrial condition is chosen as a compromise, and matching a material to its use through its properties. Keep these notes beside past-paper questions and use them to check your answers against the definitions the marking scheme expects.

8.1 Sulfuric acid and the Contact process

Sulfuric acid is one of the most widely used industrial chemicals. Its uses include making fertilisers such as ammonium sulfate, making detergents, paints and pigments, acting as the electrolyte in car accumulators, and cleaning (pickling) metal surfaces before plating.

The Contact process makes sulfuric acid in four stages:

• Stage 1, making sulfur dioxide. Sulfur is burned in dry air: S + O₂ → SO₂. Sulfur dioxide can also come from roasting sulfide ores.
• Stage 2, making sulfur trioxide. Sulfur dioxide and excess oxygen pass over a vanadium(V) oxide (V₂O₅) catalyst: 2SO₂ + O₂ ⇌ 2SO₃. This step is reversible and exothermic and is the heart of the process.
• Stage 3, making oleum. Sulfur trioxide is dissolved in concentrated sulfuric acid to form oleum: SO₃ + H₂SO₄ → H₂S₂O₇. It is not dissolved directly in water because that produces a dense, hard-to-condense mist of sulfuric acid.
• Stage 4, diluting oleum. Oleum is added to water to give concentrated sulfuric acid: H₂S₂O₇ + H₂O → 2H₂SO₄.

The reasoning that scores marks. Stage 2 conditions are a compromise. The forward reaction is exothermic, so a low temperature would favour a high yield of SO₃ but the rate would be too slow. A high temperature is fast but lowers the yield. A moderate temperature of about 450–550 °C balances a good yield against an acceptable rate. Because the forward reaction reduces the number of gas moles (3 → 2), high pressure would raise the yield, but the yield at about 1 atmosphere is already very high, so raising the pressure is not worth the extra cost of stronger plant. The catalyst speeds up the rate without changing the position of equilibrium or the yield.

Sulfur dioxide released to the air is a cause of acid rain, so industrial plants must control their emissions.

8.2 Ammonia and the Haber process

Ammonia is used mainly to make nitrogenous fertilisers (ammonium nitrate, ammonium sulfate and urea) and nitric acid, and as a raw material for cleaning agents.

In the Haber process, nitrogen and hydrogen combine: N₂ + 3H₂ ⇌ 2NH₃. The nitrogen comes from the fractional distillation of liquefied air; the hydrogen comes from natural gas. The forward reaction is reversible and exothermic.

The same compromise logic. Because the forward reaction is exothermic, a low temperature favours yield but is slow; a temperature of about 450–550 °C is the compromise. Because the forward reaction reduces the gas moles (4 → 2), high pressure favours the yield, so a high pressure of about 200–300 atmospheres is used, as high as is safe and economical. An iron catalyst speeds up the rate. Ammonia is removed by cooling and liquefying it, and the unreacted nitrogen and hydrogen are recycled to improve the overall conversion.

Notice that the Contact and Haber processes share the same skeleton of reasoning, exothermic forward reaction, moles of gas decreasing, a catalyst, but reach different pressure decisions because the yield gain from pressure differs. Being able to explain that difference is exactly what the higher-mark questions test.

8.3 Alloys and their uses

A pure metal is made of atoms of the same size arranged in orderly layers. When a force is applied, the layers slide over one another, so the metal is soft and easily bent. An alloy is a mixture of two or more elements, the main one a metal. The foreign atoms are a different size, so they disrupt the orderly arrangement and stop the layers from sliding. The alloy is therefore harder and stronger than the pure metal.

Alloying is also used to resist corrosion and to improve appearance. Learn a few clear examples with their uses:

• Bronze (copper + tin), medals, statues, artwork.
• Brass (copper + zinc), musical instruments, decorative fittings.
• Steel (iron + carbon), harder and stronger than iron; car bodies and building frames.
• Stainless steel (iron + carbon + chromium + nickel), resists rust; cutlery and surgical instruments.
• Duralumin (aluminium + copper + magnesium), light but strong; aircraft bodies.
• Pewter (tin + antimony + copper), souvenirs and decorative items.

8.4 Composite materials

A composite material is made by combining two or more materials so the product has better properties than any single component. Learn these standard examples and the property that makes each useful:

• Reinforced concrete (concrete + steel bars). Concrete is strong when compressed but cracks when stretched; steel is strong when stretched. Together they are strong under both forces, so they are used in buildings and bridges.
• Fibreglass (glass fibre + plastic resin). Light, strong and resistant to corrosion, so it is used for boat and car bodies, water tanks and helmets.
• Fibre optic (very pure glass). Carries data as light over long distances with little loss.
• Photochromic glass (glass + silver compounds). Darkens in bright light, so it is used for spectacle lenses.

The exam habit here is comparison: state the weakness of the original material, then say how the composite removes it.

8.5 Glass and ceramics

Glass is made mainly from silica (silicon dioxide, SiO₂). Different additives give different types with different uses:

• Fused (quartz) glass, pure silica; high melting point, withstands sudden temperature change; used for lenses and laboratory optics.
• Soda-lime glass (silica + sodium carbonate + limestone), cheap; used for bottles, windows and light bulbs.
• Borosilicate glass (silica + boron oxide), withstands heat and sudden temperature change; used for laboratory apparatus and cookware.
• Lead crystal glass (silica + lead(II) oxide), dense and sparkling; used for decorative glassware and lenses.

Ceramics are made from clay and hardened by firing at a high temperature. They are hard, brittle, chemically inert, have very high melting points and do not conduct electricity. These properties explain their uses: electrical insulators, furnace linings, tiles, bricks and tableware.

How to use these notes

Revise one content standard at a time and explain it aloud without looking. For 8.1 and 8.2, rehearse the compromise argument in full sentences, condition, effect on rate, effect on yield, decision. For 8.3, be able to explain hardness in terms of atoms and sliding layers. For 8.4 and 8.5, practise property-to-use reasoning. A one-to-one teacher can check that your rate-and-yield explanations are complete, because that is where marks are gained or lost in SPM Chemistry. Then attempt the practice questions for this chapter and mark your answers against these definitions.