Improving processes and products

Welcome to section C6.1 of your GCSE Chemistry course! In this topic, we will explore how chemists extract metals from the Earth, how we design and optimise industrial chemical processes, and how we assess the environmental footprint of everyday items using Life-Cycle Assessments.

What you'll learn:

• How a metal's position in the reactivity series determines how we extract it.
• How chemical industries balance reaction rate and percentage yield to make processes profitable and safe.
• The differences between how fertilisers are synthesised in a school laboratory versus a massive chemical plant.
• How to evaluate materials, alloys, and prevention methods against corrosion.

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1. Extracting Metals from Ores

Most metals are found in the Earth's crust as chemical compounds within rocks. We call these rocks ores.

The method used to extract a metal depends on its chemical reactivity. We use the reactivity series—and specifically the position of carbon—to decide on the extraction route.

Reduction by Carbon

If a metal is less reactive than carbon, we can extract it by heating its metal oxide ore with carbon. In this reaction, carbon acts as a reducing agent because it removes oxygen from the metal oxide.

For example, iron is extracted from hematite (iron oxide) in a blast furnace:

$$iron(III)\; oxide+carbon\to iron+carbon\; dioxide$$ $$2{Fe}_2{O}_3(s)+3C(s)\to 4Fe(l)+3{CO}_2(g)$$

Because carbon has gained oxygen, we say it has been oxidised. Because the iron ions have lost oxygen (and gained electrons), they have been reduced.

Extraction by Electrolysis

If a metal is more reactive than carbon (such as aluminium or sodium), carbon is not a strong enough reducing agent to take the oxygen away from it. Instead, we must use electrolysis.

To perform electrolysis on an ore:

1. The metal ore must be melted (or dissolved in a solvent like molten cryolite) so that the ions are free to move.
2. An electric current is passed through the liquid.
3. Positive metal ions migrate to the negative electrode (cathode), where they gain electrons to become neutral metal atoms.

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2. Alternative Biological Extraction Methods

As high-grade ores (rocks with a high percentage of metal) run out, we must find ways to extract metals from low-grade ores (rocks with very low percentages of metal). Standard mining of low-grade ores produces vast amounts of waste rock and is not economical.

To solve this, we can use two biological extraction techniques: phytoextraction and bioleaching.

Phytoextraction

1. Plants are grown on soil containing low-grade metal compounds.
2. The plants absorb the metal ions through their roots and concentrate them in their leaves and shoots.
3. The plants are harvested, dried, and burned in a furnace.
4. The resulting ash contains a high concentration of the metal compound, which can then be extracted using electrolysis or displacement.

Bioleaching (Bacterial Extraction)

1. Specific strains of bacteria are mixed with low-grade ores.
2. The bacteria carry out chemical reactions that break down the ore, producing a solution called a leachate.
3. The leachate contains a high concentration of metal ions.
4. We can extract the metal from the leachate by reacting it with cheap scrap iron (a displacement reaction) or via electrolysis.

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3. Industrial Equilibria: Rate vs Yield (Separate Chemistry)

This section is exclusive to Separate Chemistry, and key trade-off arguments are Higher Tier only.

Many industrial processes involve reversible reactions, which can reach a state of dynamic equilibrium.

In industry, chemical engineers want to produce chemicals as quickly as possible (high rate) and get as much product as possible from their reactants (high percentage yield). However, changing reaction conditions often affects rate and yield in opposite ways. This requires a trade-off (a compromise).

The Haber Process

The Haber process is used to manufacture ammonia, which is essential for agricultural fertilisers.

$$N_2(g)+3H_2(g)\rightleftharpoons 2{NH}_3(g)(forward\; reaction\; is\; exothermic,\Delta H=-92kJ/mol)$$

Let's look at how pressure and temperature affect this equilibrium:

1. Temperature Trade-off

• Equilibrium Yield: Because the forward reaction is exothermic, Le Chatelier's principle states that a lower temperature will shift the equilibrium position to the right, producing a higher yield of ammonia.
• Reaction Rate: However, at low temperatures, the reacting gas particles have less kinetic energy. They collide less frequently and with less energy, resulting in an incredibly slow rate of reaction.
• The Compromise: A compromise temperature of $450{}^{\circ }C$ is used. This is high enough to ensure a rapid reaction rate, but low enough to still give a reasonable yield of ammonia (around 15% per pass; unreacted gases are recycled).

2. Pressure Trade-off

• Equilibrium Yield: The left side of the equation has 4 moles of gas, while the right side has 2 moles. A higher pressure shifts the equilibrium to the right (the side with fewer gas molecules), increasing the yield of ammonia.
• Reaction Rate: Higher pressure also compresses the gas particles, increasing the frequency of collisions and increasing the rate.
• The Compromise: While extremely high pressures are ideal for both yield and rate, they require incredibly thick steel pipes, specialised pumps, and massive electricity costs to maintain. High pressures also pose a severe risk of explosions. Therefore, a compromise pressure of $200atm$ ($2\times 10^{7}Pa$) is used.

3. The Catalyst

An iron catalyst is used. A catalyst speeds up both the forward and reverse reactions equally. It has no effect on the position of equilibrium or the percentage yield, but it allows the system to reach equilibrium much faster, lowering the energy costs.

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4. Fertilisers and Agricultural Production (Separate Chemistry)

This section is exclusive to Separate Chemistry.

Plants absorb nitrogen, phosphorus, and potassium (NPK) from the soil to grow. When crops are harvested, these elements are removed from the soil. NPK fertilisers are chemical formulations containing compounds of these three elements to restore soil fertility.

Industrial vs Laboratory Preparation of Ammonium Sulfate

Ammonium sulfate, $({NH}_4{)}_2{SO}_4$, is a common nitrogen-rich fertiliser. It can be prepared on a small scale in a school laboratory, but industrial production is fundamentally different.

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5. Alloys and Corrosion (Separate Chemistry)

This section is exclusive to Separate Chemistry.

Alloys

A pure metal has giant structures of atoms arranged in regular, uniform layers. This makes pure metals malleable because the layers can easily slide over one another when hit. An alloy is a mixture of a metal with small amounts of other elements.

Common alloys you need to know:

• Steel: Iron alloyed with Carbon. Carbon steel is strong and hard. Stainless steel (alloyed with chromium and nickel) is highly resistant to corrosion.
• Brass: Copper and Zinc. It is acoustic, ductile, and resistant to corrosion (used in musical instruments and door taps).
• Bronze: Copper and Tin. Hard, resistant to corrosion (used in statues, medals, and ship propellers).
• Solder: Tin and Copper (historically lead). Has a low melting point (used to join electrical components).
• Duralumin: Aluminium, Copper, and Magnesium. Exceptionally lightweight but very strong (used in aircraft parts).

Corrosion

Corrosion is the destruction of materials by chemical reactions with substances in their environment. Rusting is a specific term used only for the corrosion of iron.

Mitigating Corrosion

1. Physical Barrier Methods: Coating the iron with a barrier to keep out oxygen and water. Examples include painting, oiling/greasing (for moving parts), and plastic coating.
2. Sacrificial Protection: Coating or connecting the iron with a more reactive metal (such as zinc or magnesium). The more reactive metal will react and oxidise preferentially, sacrificing itself to protect the iron underneath.
3. Galvanising: Coating iron with a layer of zinc. This acts as both a physical barrier and sacrificial protection. If the zinc coating is scratched, the remaining zinc still sacrifices itself to protect the exposed iron.

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6. Materials Comparison and Life-Cycle Assessments (LCAs)

Chemical engineers must select the right materials for products and evaluate their environmental impact.

Comparing Material Properties (Separate Chemistry)

You need to be able to quantitatively compare different classes of materials:

• Glass & Clay Ceramics: Brittle, high melting points, electrical insulators, resistant to chemical attack.
• Polymers: Flexible, low density, easily moulded, electrical insulators.
• Composites: Made of a reinforcing material (like carbon fibres or glass fibres) embedded in a binder or matrix. They can be engineered to be incredibly strong yet lightweight.
• Metals: Malleable, ductile, outstanding thermal and electrical conductors, high strength.

Life-Cycle Assessments (LCAs)

A Life-Cycle Assessment (LCA) evaluates the environmental impact of a product across its entire lifespan.

There are four key stages to assess:

[1. Extracting Raw Materials] ➔ [2. Manufacture & Packaging] ➔ [3. Use & Lifetime] ➔ [4. Disposal]

At every stage, we must account for:

• The energy required (especially if it comes from burning fossil fuels).
• The use of limited water and land resources.
• The production of waste products and pollution (greenhouse gases, toxic chemical run-off).

Recycling

Recycling materials (like glass, aluminium, and plastics) is a crucial way to improve sustainability.

• Benefits: Saves finite raw materials, reduces mining landscape destruction, uses significantly less energy than extracting virgin materials, and decreases landfill waste.
• Drawbacks/Challenges: Items must be sorted and washed, which can be labour-intensive. Transporting materials to recycling plants also consumes energy and produces carbon emissions.

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