Improving Processes & Products (OCR GCSE Chemistry A (Gateway)): Flashcards

Phytomining (phytoextraction) is a method of extracting metals in which plants absorb metal ions from soil through their roots. The plants are harvested, dried and burned, and the useful metals are extracted from the ash.

Traditional mining is only economically viable for high-grade ores. As ores of metals like copper and nickel become scarcer, biological methods such as phytomining and bioleaching provide ways to extract metals from low-grade ores without the large-scale environmental damage caused by conventional mining.

False.

Only specific parts of the plant — the shoots and leaves — concentrate the metal and are harvested and burned. The ash then contains metal compounds from which the metal is extracted.

Bioleaching is a method of extracting metals using bacteria that break down metal ores to form an acidic solution (leachate) containing metal ions, which are then recovered by displacement or electrolysis.

Bacteria produce an acidic solution called a leachate. The metal ions are then extracted by displacement or electrolysis.

The metal ions in the leachate are reduced to solid metal during extraction.

Advantage: They cause significantly less environmental damage — no large-scale digging or disposal of rock.

Disadvantage: They are very slow processes. Bioleaching also produces toxic substances that must be treated to prevent environmental contamination.

True.

During bioleaching, bacteria produce an acidic leachate containing dissolved metal ions. This solution can be toxic and must be carefully treated or contained to prevent it from polluting the surrounding environment.

In both methods, the metal is obtained by displacement reactions or electrolysis. In phytomining, these processes act on metal compounds in the ash. In bioleaching, they act on the metal ions dissolved in the leachate.

The Haber Process is the industrial method used to manufacture ammonia from nitrogen and hydrogen gases, using an iron catalyst at 450°C and 200 atmospheres of pressure.

Nitrogen is obtained from the air by fractional distillation (air is approximately 78% nitrogen). Hydrogen is obtained from methane (natural gas) by a process called steam reforming.

The Haber Process uses an iron catalyst at a temperature of 450°C and a pressure of 200 atmospheres.

These are compromise conditions balancing yield, rate and cost.

False.

The reaction is reversible, so equilibrium is established. Only about 30% of the gases are converted to ammonia at any one time. Unreacted gases are recycled back into the system.

The unreacted nitrogen and hydrogen are recycled back into the reactor. This prevents waste and improves the overall efficiency of the process.

The gas mixture is passed into a cooling tank. Ammonia liquefies at a higher temperature than nitrogen and hydrogen, so it condenses and is removed as a liquid. The remaining gases are recycled.

More than 80% of ammonia produced is used to make fertiliser.

Ammonia is also used to make explosives and dyes.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

The ⇌ symbol shows the reaction is reversible — both the forward and reverse reactions occur simultaneously.

NPK fertilisers are formulations containing appropriate ratios of the three essential plant nutrients: nitrogen (N), phosphorus (P) and potassium (K), needed for healthy plant growth.

Nitrogen promotes healthy leaves.

Phosphorus promotes healthy roots.

Potassium promotes growth and healthy fruit and flowers.

False.

Plant roots can only absorb elements in water-soluble form from the soil. Plants cannot use nitrogen gas directly from the air.

Fertilisers are made by neutralisation — a reaction between an acid and an alkali to form a salt. For example, ammonia reacts with sulfuric acid to produce ammonium sulfate.

The indicator (methyl orange) turns yellow at the start. The endpoint is reached when it turns red sharply.

The final preparation is repeated without the indicator to avoid it acting as an impurity in the product.

In a batch process (laboratory scale), a small amount of product is made at a time and equipment must be cleaned between batches.

In a continuous process (industrial scale), product is made constantly as long as raw materials are available, allowing large quantities to be produced.

True.

Fertiliser factories integrate multiple processes: the Haber Process provides ammonia and the Contact Process provides sulfuric acid, which react together to form ammonium sulfate fertiliser.

The mixture is heated gently in a water bath to evaporate water slowly. Heating too strongly could cause the ammonium sulfate to decompose. The solution is concentrated to about one-third its volume, then left to crystallise in a warm place.

Fertiliser factories make multiple fertilisers simultaneously using raw materials such as sulfur (for sulfuric acid) and phosphate rock.

This integrated approach improves efficiency and reduces production costs.

200 atmospheres is a compromise. Higher pressures would increase the yield of ammonia (equilibrium shifts right, towards fewer gas molecules), but very high pressures are hazardous and expensive to maintain. 200 atm gives a reasonable yield at acceptable cost and risk.

True.

The forward reaction (formation of ammonia) is exothermic. This means a lower temperature would favour a higher yield. However, lower temperatures also give a slower rate, so 450°C is used as a compromise.

Although a lower temperature would favour a higher equilibrium yield of ammonia (the forward reaction is exothermic), the rate of reaction would be too slow. 450°C is a compromise — it gives a reasonable yield within a reasonable time frame.

Increasing pressure shifts the equilibrium to the right because there are fewer moles of gas on the right-hand side.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g): 4 moles of gas on the left, 2 on the right.

The process is viable because the unreacted nitrogen and hydrogen are recycled back into the reactor, so no raw material is wasted. The ammonia is continuously removed as it liquefies, which also helps drive the process forward overall.

False.

The iron catalyst increases the rate at which equilibrium is reached but does not change the position of equilibrium or the yield. It allows the process to operate profitably at 450°C.

Manufacturers should consider the raw materials used (whether they are renewable or non-renewable) and the amount of energy required for the process. Both factors affect cost and sustainability.

In the Haber Process, unreacted nitrogen and hydrogen are recycled back into the reactor, so that no raw material is wasted.

The Contact Process is the industrial method used to manufacture sulfuric acid from sulfur, air and water, involving three stages including a catalytic oxidation of sulfur dioxide to sulfur trioxide.

1. The manufacture of fertilisers.\n2. Metal extraction.\n3. Making paints and dyes.

Sulfur dioxide and oxygen react to form sulfur trioxide using vanadium(IV) oxide (V₂O₅) as a catalyst.

2SO₂ (g) + O₂ (g) ⇌ SO₃ (g)

False.

Although the overall conversion involves water, sulfur trioxide reacts with water to produce sulfuric acid:

H₂O (l) + SO₃ (g) → H₂SO₄ (aq)

The forward reaction is exothermic, so lower temperatures favour a higher yield of sulfur trioxide. However, very low temperatures give a slow rate. 450°C is a compromise — low enough for a reasonable yield, high enough for a reasonable rate.

The equilibrium in Stage 2 already gives a high yield of sulfur trioxide, so very high pressures are not needed. A higher pressure would substantially increase energy costs without significantly improving the yield, making it uneconomical.

True.

Sulfur is burned to make SO₂, air provides the oxygen for Stage 2, and water reacts with SO₃ in Stage 3 to form sulfuric acid.

1. Burning sulfur in oxygen to make SO₂.

2. Oxidising SO₂ to SO₃ using a vanadium(IV) oxide catalyst.

3. Reacting SO₃ with water to form sulfuric acid (H₂SO₄).

A life cycle assessment (LCA) is an analysis of the overall environmental impact of a product throughout its entire lifetime — from raw material extraction to disposal.

1. Raw materials

2. Manufacture

3. Usage

4. Disposal

False.

Extracting raw materials causes environmental damage — including depleting finite resources such as ores and crude oil and damaging habitats through mining or deforestation.

Factories use up land and production relies on fossil-fuelled machines for manufacturing and transport, releasing greenhouse gases and consuming energy.

The disposal stage considers whether waste ends up in landfill sites or can be recycled.

Every stage of a product's life cycle has some environmental cost. A compromise is always made between environmental impact and economic factors.

False.

LCAs can be biased because it is difficult to quantify each stage — the choice of criteria and weighting can reflect the views of whoever commissioned the assessment.

A plastic bag has a much longer lifespan than a paper bag, so it causes less environmental harm per use if it is reused many times.

During the usage stage of an LCA, the environmental impact depends on what the product does — a wooden desk has very little impact, whereas a car causes significant air pollution.

These materials come from finite (limited) natural sources, so recycling conserves raw materials, reduces harmful substances released into the environment and reduces waste sent to landfill.

Recycling is the process of collecting and reprocessing used materials so that they can be made into new products rather than being sent to landfill or incinerated.

Glass bottles are washed and sterilised, then refilled and used again — no remelting or reprocessing is required.

To recycle metals, they are first crushed into smaller pieces, then melted and recast into new shapes.

True.

Iron and steel can be recycled together in a blast furnace, which reduces the need for iron ore to be mined.

The glass is separated by colour and composition, then crushed and melted before being remoulded into its new form.

1. Collection and transport of recyclable materials requires energy and fuel.

2. Materials must be sorted before recycling, which demands labour and additional energy.

False.

Products made from recycled materials may be of lower quality than those made from virgin raw materials, depending on the recycling process used.

1. How easy it is to collect and sort the waste.

2. The cost of recycling compared with incineration or landfill.

3. The energy required at each stage and the harmful by-products produced.

An alloy is a mixture of a metal with one or more other elements (metals or non-metals) that are not chemically combined.

Alloys contain atoms of different sizes, which distort the regular lattice arrangement. This makes it harder for layers to slide over each other, increasing hardness and strength.

Brass is an alloy made from copper and zinc and is used for making musical instruments and decorative fittings.

True.

Steel is made by adding small amounts of carbon to iron. The carbon atoms disrupt the regular iron lattice, making steel harder and stronger than pure iron.

Stainless steel contains iron, chromium and nickel. It is used in cutlery because it is hard and has excellent resistance to corrosion.

Bronze is an alloy of copper and tin. It is used to make statues, medals and bearings because it is hard and resistant to corrosion.

False.

Alloys are mixtures, not compounds. The metals (and any non-metals) are not chemically combined, so no new bonds are formed between the elements.

In an alloy, atoms of different sizes distort the regular arrangement of atoms in the metal lattice, making it harder for layers to slide over each other.

Alloys can have greater strength, hardness or improved resistance to corrosion or extreme temperatures compared with the pure metals they are made from.

Corrosion is the degradation of a metal at its surface caused by reactions with substances in the environment over a prolonged period.

Both oxygen (from the air) and water must be present for rusting to occur.

Rusting is a redox process in which iron reacts with oxygen and water to form iron(III) oxide, which has the formula Fe₂O₃.xH₂O.

False.

Rust is porous and flakes off the surface of iron, constantly exposing fresh iron underneath to further rusting — unlike aluminium oxide, which forms a protective layer.

Salt water speeds up rusting because it allows ions to move more easily, increasing the rate of the electrochemical reaction. Salt itself is not consumed — it acts as an electrolyte, not a catalyst.

Aluminium reacts with oxygen in the air to form aluminium oxide (Al₂O₃), which creates a tough, dense, protective layer on the surface that prevents further corrosion.

Galvanising is coating iron with a layer of zinc. Zinc protects by acting as a barrier and, if scratched, as a sacrificial metal — it corrodes in preference to iron because it is higher in the reactivity series.

False.

If a barrier coating such as paint is scratched or washed away, the iron is exposed to water and oxygen again and will begin to rust.

Sacrificial protection involves attaching a metal that is more reactive than iron — such as magnesium or zinc — so that it corrodes first, protecting the iron.

Ships' hulls are sometimes fitted with blocks of magnesium which slowly corrode to protect the steel hull — this is an example of sacrificial protection.

Iron's rust layer (iron(III) oxide) is porous and flakes off the surface, constantly exposing fresh iron underneath to further corrosion. Aluminium oxide, by contrast, forms a dense, adherent layer that seals the surface.

A composite material is made from two components: a reinforcement material embedded in a matrix material that acts as a binder, giving properties neither component has alone.

Polymers are poor conductors of heat and electricity, making them effective electrical and thermal insulators. This prevents electric shocks and overheating.

Clay is fired at high temperatures, producing a material that resists compressive forces.

When clay is fired, it hardens and becomes very strong under compression, allowing bricks to bear the downward weight and pressure of a wall.

True.

Borosilicate glass is made from sand and boron trioxide, and its higher melting point makes it more suitable for use in laboratory glassware and cookware.

LDP is produced at high pressure and moderate temperature. It is flexible and used for bags and bottles. HDP is produced at lower temperature and pressure with a catalyst. It is rigid and used for water tanks and drain pipes.

Clay hardens at high temperatures when fired, producing a very strong material that resists compressive forces, allowing bricks to bear the downward weight and pressure of a wall.

False.

Alloys are uniform mixtures of metals, whereas composites contain two or more distinguishable materials (reinforcement and matrix) that remain visibly separate.

Aluminium has a much lower density than copper, so cables made from it are significantly lighter. Its conductivity and tensile strength are sufficient for the purpose, making it the better overall choice.

Carbon fibre is used in aviation and professional racing bicycles because it is extremely strong and has a very low weight (density).

Make comparative statements between materials (e.g. copper has a higher density than aluminium) and end with a conclusion stating which material is most suitable and why.