SC11 · Obtaining and using metals · Topic 4 — Extracting metals and equilibria | GCSE Edexcel Separate Sciences

Revise the key ideas

The reactivity series orders metals by their tendency to form positive ions. A more reactive metal more readily loses electrons.
The required order is potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold. Carbon and hydrogen are non-metal comparison points, not metals.
Carbon guides extraction; hydrogen guides reactions with dilute acid. Neither is a metal.
Potassium, sodium and calcium react with cold water to make hydroxides and hydrogen. Magnesium reacts very slowly with cold water but more vigorously with steam; observations depend on conditions.
Metals above hydrogen generally react with suitable dilute acids to form a salt and hydrogen; copper, silver and gold do not produce hydrogen with dilute hydrochloric acid.
Compare water, acid and salt-solution reactions using controlled conditions and observations such as gas production or displacement. Protective oxide layers can affect apparent reaction rate.
Aluminium's oxide coating can prevent an immediate reaction despite its high position in the series. Distinguish underlying reactivity from a surface barrier.

A more reactive metal displaces a less reactive metal from its compound. A less reactive metal cannot usually displace a more reactive one under the same conditions.
Zinc plus copper sulfate gives zinc sulfate and copper: Zn + CuSO₄ → ZnSO₄ + Cu. A copper deposit forms and the blue solution loses colour as Cu²⁺ is removed.
The ionic equation is Zn + Cu²⁺ → Zn²⁺ + Cu. Zinc loses electrons and is oxidised; copper ions gain electrons and are reduced.
Half-equations are Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu. Both processes occur together, so displacement is a redox reaction.
Electrons lost by zinc are gained by copper ions.
Aluminium can reduce copper oxide: 2Al + 3CuO → Al₂O₃ + 3Cu. Such competition reactions can be very energetic and require a controlled demonstration.
Oxidation can also mean gain of oxygen and reduction loss of oxygen. Metal extraction from oxides is reduction; use electron transfer for reactions without oxygen.

An ore is a rock containing enough metal or metal compound to make extraction worthwhile financially. Whether it is worth extracting depends on the concentration, extraction costs and metal price.
Most metals occur as compounds in Earth's crust. Very unreactive metals such as gold can occur as uncombined elements.
For metals below carbon in the series, heating their oxides with carbon or carbon monoxide can remove oxygen and release the metal. This route includes iron.
For iron oxide, a useful reduction equation is Fe₂O₃ + 3CO → 2Fe + 3CO₂. Iron oxide loses oxygen while carbon monoxide gains it.
Metals above carbon, including aluminium, cannot be extracted from their oxides by heating with carbon in this GCSE model. Electrolysis of a molten ionic compound is needed.
Use reactivity and costs; electrolysis is of the compound, not molten metal.
Aluminium extraction uses aluminium oxide dissolved in molten cryolite, lowering the operating temperature compared with melting pure alumina. Electrical energy is a major cost.
Choose a route using reactivity and economic evidence. Extraction includes energy, raw material, plant and environmental costs, not just whether a reaction is possible.

As high-grade ores become less available, lower-grade materials can be considered. Biological extraction can recover metals such as copper from low-concentration sources.
In bioleaching, bacteria help produce soluble metal compounds from ores. The resulting leachate contains metal ions that can be recovered, for example by displacement or electrolysis.
Lower-grade ore can be used, but the process may be slow.
In phytoextraction, plants take up metal compounds from contaminated soil. Harvesting and burning the plants produces ash enriched in metal compounds, from which metal can be extracted.
Burning plants does not necessarily give pure metal directly. A further extraction step is required to recover the metal from the ash.
The ash is not automatically pure metal.
These routes may reduce energy demand or mining damage but can be slow and require large areas or careful treatment of solutions. Evaluate both advantages and limitations for the particular process.

Corrosion is reaction of a metal with its surroundings, often oxidation. More reactive metals generally have a greater tendency to oxidise, but protective surface coatings can change the observed behaviour.
Iron rusts when both oxygen and water are present. Rust is hydrated iron oxide and is porous, so it does not effectively protect the underlying iron.
Aluminium forms a thin oxide layer that sticks firmly to its surface. This keeps oxygen and water away from the metal underneath, explaining why aluminium resists corrosion despite being fairly reactive.
Paint, oil or a barrier coating can keep oxygen and water away from iron; protection can fail if the barrier is damaged. Galvanising uses zinc, which also offers sacrificial protection.
Sacrificial protection uses a more reactive metal connected to the iron. That metal oxidises instead of the iron, protecting it. It is gradually used up and eventually needs replacing.

Mining and extraction can damage landscapes and habitats, consume energy and produce waste and emissions. Recycling can reduce demand for ore and conserve finite metal resources.
Recycling often uses less energy than extracting a metal from ore, especially aluminium, and can reduce landfill. Collection, sorting, transport and remelting still have costs and impacts.
Separate metals and remove contaminants so the recycled material has suitable properties. Mixed alloys can be harder to reuse as a particular pure metal.
A life-cycle assessment considers obtaining raw materials, manufacturing, use and end-of-life disposal or recycling. Include transport and energy where relevant across these stages.
Consider the same useful service and impacts across all stages.
Compare products for the same useful service, such as a specified number of drinks served. A heavier reusable item may need repeated use to offset its initial manufacturing impact.
Use quantitative evidence for energy, emissions, waste and resource use; different indicators can favour different options. State assumptions and avoid treating a single number as a complete assessment.
Hypothetical example: a reusable item costs 100 impact units to make and 1 per use; a single-use alternative costs 6 per use. At 20 uses they tie at 120; beyond 20, the reusable option is lower in this simplified measure.