Reduction with carbon vs electrolysis

How a metal is extracted depends on its place in the reactivity series — specifically, where it sits relative to carbon:

• Metals less reactive than carbon (e.g. iron, zinc, copper, tin, lead) can be reduced by carbon — the carbon removes the oxygen from the metal oxide.
• Metals more reactive than carbon (e.g. aluminium, magnesium, sodium) cannot — carbon won't take their oxygen, so they are extracted by electrolysis of the molten compound.

Watch out: a metal more reactive than carbon is the one that needs electrolysis (more expensive, lots of energy). It's a common slip to assume the more reactive metal is the easier one to get — the opposite is true.

Which extraction method?

Phytomining & bioleaching

High-grade copper ores are running low, so we use biological methods on low-grade ores (too little metal to mine traditionally):

• Phytomining — plants are grown on low-grade soil; they absorb metal compounds. The plants are burned to ash, which is rich in the metal compound.
• Bioleaching — bacteria break down low-grade ores, producing a leachate solution containing metal ions (e.g. copper).

The metal is then displaced from solution (e.g. with scrap iron) or obtained by electrolysis.

Watch out: phytomining and bioleaching are for low-grade ores. They're slower than smelting but use less energy and cause far less land damage.

Why use plants and bacteria?

Recycling & life cycle assessment

Recycling a material (e.g. melting scrap metal) saves the energy and raw materials needed to extract it fresh, and reduces waste sent to landfill — though sorting and collecting costs energy too.

A life cycle assessment (LCA) works out the total environmental impact of a product across four stages:

Note: an LCA isn't fully objective — judging the impact of, say, pollutant effects involves value judgements, so results can be biased (e.g. by a manufacturer).

Making ammonia

The Haber process makes ammonia (NH₃) for fertilisers from nitrogen (from the air) and hydrogen (from natural gas). It is a reversible reaction that reaches equilibrium:

Watch out: the reaction never "finishes" — it sits at a dynamic equilibrium. The chosen conditions are a compromise, not the values that give the absolute highest yield.

The equilibrium compromise

Because the forward reaction is exothermic and produces fewer gas molecules (4 → 2), Le Chatelier's principle tells us the ideal yield would come from low temperature and high pressure. But:

• Low temperature gives more ammonia at equilibrium, but the rate becomes too slow. So ~450 °C is a compromise — a reasonable yield at a reasonable rate.
• High pressure shifts equilibrium toward ammonia (the side with fewer molecules), but very high pressure is expensive and dangerous. So ~200 atm is a practical compromise.
• The iron catalyst speeds up the reaction but does not change the position of equilibrium.

Key idea: industry balances yield, rate and cost. Removing the ammonia and recycling the unused gases pushes the equilibrium forward and raises the overall yield.

Why not just use a low temperature?

Feeding crops with N, P and K

Plants need three key elements to grow well, supplied by NPK fertilisers:

• N — Nitrogen (e.g. ammonium nitrate, ammonium sulfate) for healthy leaves and growth.
• P — Phosphorus (phosphate compounds) for roots.
• K — Potassium (e.g. potassium chloride/sulfate) for flowers and fruit.

Ammonia from the Haber process is the starting point for nitrogen-rich fertilisers — neutralising it with nitric acid gives ammonium nitrate. Industrially this is done on a huge scale from cheap raw materials; in the lab the same salts are made in small batches by titration.

Why it matters: the Haber process underpins modern agricultural production — without manufactured fertilisers, food yields could not feed today's population.

Alloys and stopping rust

An alloy is a mixture of a metal with other elements. The different-sized atoms disrupt the regular layers, so the layers can't slide easily — alloys are usually harder than the pure metal. Examples: steel (iron + carbon), brass (copper + zinc), bronze (copper + tin).

Corrosion is the reaction of a metal with substances in its environment. Rusting is the corrosion of iron, and needs both water and oxygen.

Prevent rust with a physical barrier (paint, oil, plastic) or with sacrificial protection — a more reactive metal (e.g. zinc, in galvanising) corrodes instead of the iron.

Carbon or electrolysis?

Tap a metal, then tap the box for how it is extracted from its ore.

Crude oil & hydrocarbons

Crude oil is a finite resource formed over millions of years from the remains of ancient sea organisms. It is a mixture of mostly hydrocarbons — molecules made only of hydrogen and carbon.

Most are alkanes, the family with the general formula:

Crude oil is the main source of hydrocarbons and the key feedstock for the petrochemical industry — modern life depends on it for fuels and materials.

Fractional distillation

Crude oil is separated into useful fractions by fractional distillation. The oil is heated to a vapour and fed into a column that is hot at the bottom, cool at the top. Each fraction condenses where the temperature matches its boiling point.

Short molecules have weaker intermolecular forces, so lower boiling points — they rise furthest up the column. They are also more volatile, more flammable and less viscous (runnier).

Reading the column

Cracking long chains

Distillation produces too much of the long, less useful fractions and not enough petrol. Cracking breaks long alkane molecules into shorter, more useful ones — using heat with a catalyst (or steam).

Cracking always makes a shorter alkane plus at least one alkene. Alkenes have a C=C double bond (general formula C_nH_2n) and are used to make polymers.

Test for an alkene: it decolourises bromine water (orange → colourless) because the double bond reacts by addition. Alkanes (no double bond) leave bromine water orange.

Complete vs incomplete combustion

Hydrocarbon fuels burn in oxygen. With plenty of oxygen you get complete combustion:

With too little oxygen you get incomplete combustion, producing carbon monoxide (CO) and soot (carbon particulates) as well:

Watch out: carbon monoxide is a toxic, colourless, odourless gas from incomplete combustion — it binds to haemoglobin so blood can't carry oxygen. Faulty gas appliances are a real danger.

Spot the dangerous product

Alkenes → polymers

Because alkenes have a reactive C=C double bond, many alkene monomers can join up to form a long addition polymer — the double bond "opens up" and the molecules link with no other product.

The repeat unit is the monomer with the double bond turned into a single bond. Addition polymers (poly(ethene), poly(propene), PVC) are cheap and durable — but most don't break down easily, causing waste problems.

Today's atmosphere

For about 200 million years the atmosphere has been roughly stable, made up of:

Early on, the atmosphere was mostly carbon dioxide from volcanoes (like Mars/Venus today). As oceans formed CO₂ dissolved, and as algae and plants spread, photosynthesis removed CO₂ and released the oxygen that built up over billions of years.

The greenhouse effect

The greenhouse effect is natural and essential — without it Earth would be far too cold for life. Greenhouse gases (mainly carbon dioxide, methane and water vapour) let short-wavelength sunlight through, but absorb the longer-wavelength infrared the warm Earth radiates back, keeping heat in.

Watch out: the greenhouse effect itself is natural and keeps Earth warm. The concern is the enhanced effect from extra CO₂ and methane released by human activity (largely burning fossil fuels), which correlates with rising temperatures.

Which is a greenhouse gas?

Carbon footprint & air pollutants

A carbon footprint is the total greenhouse gases (mainly CO₂ and methane) given off over the full life of a product, service or event. It can be reduced by using less energy, renewable resources, and efficient processes.

Burning fuels also releases atmospheric pollutants, each with its own problem:

• Carbon monoxide (CO) — toxic; from incomplete combustion.
• Sulfur dioxide (SO₂) — from sulfur impurities in fuels; causes acid rain and breathing problems.
• Oxides of nitrogen (NOₓ) — formed in hot engines; cause acid rain and respiratory problems.
• Particulates (soot) — from incomplete combustion; cause global dimming and lung damage.

Note: CO₂ and methane drive climate change; SO₂ and NOₓ cause acid rain — keep these effects separate.

Pollutant → problem

Tap a pollutant on the left, then its main problem on the right.

Making water safe to drink

Potable water is water that is safe to drink — low in dissolved salts and microbes. It is not the same as pure water (which is only H₂O); potable water still contains dissolved substances.

In the UK, fresh water is made potable by:

• Filtration — passing water through filter beds to remove solids.
• Sterilising — killing microbes with chlorine, ozone or UV light.

Where fresh water is scarce, seawater is treated by desalination — using distillation or reverse osmosis. These need lots of energy, so they are expensive.

Watch out: "potable" means safe to drink, not chemically pure. Pure water boils at exactly 100 °C and has no dissolved solids — most drinking water doesn't.

Potable or pure?

C6 in a nutshell

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