Unit 31: Materials and Energy — Short Questions

In nuclear fission, the splitting of a heavy nucleus such as U-235 by a neutron produces, besides energy, 2–3 new neutrons. These released neutrons go on to strike other U-235 nuclei and split them in turn, releasing more neutrons and more energy. This self-sustaining, multiplying sequence of fission events, where the neutrons from one fission trigger further fissions, is called a chain reaction, e.g. ²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n + energy.

Reprocessing waste (sorting, cleaning and other treatment steps) is expensive and energy-intensive, especially for plastics and electronics. This excessive energy use makes recycling uneconomical when the same raw material is available elsewhere more cheaply, so economic feasibility is a major limitation on recycling.

Alloying mixes a metal with another element to alter its microstructure and enhance properties such as strength. In pure copper the atoms are arranged in uniform layers that can slide over each other easily under stress, making it soft. When zinc atoms (of different size) are introduced into the copper lattice to form brass, they distort the regular arrangement of atoms and make it harder for the layers of atoms to slip past one another. This resistance to layer-slipping makes brass stronger and harder than pure copper while remaining corrosion-resistant.

Energy density is the amount of energy that can be stored per unit volume of a fuel, usually expressed in megajoules per litre (MJ/L). A high energy-density fuel stores a large amount of energy in a small volume. For example, diesel has an energy density of about 35.8 MJ/L, which is higher than gasoline's ~34.2 MJ/L.

Silicon is the most common semiconductor and is used in microchips and solar panels because it is highly effective and abundant. Gallium arsenide (GaAs) is another important semiconductor used in wireless communication devices such as 5G equipment.

X-ray crystallography was used to determine the double-helix structure of DNA and is still used to analyze complex biomolecules such as enzymes and proteins. It is also applied in pharmaceutical drug development, where designers use it to understand how a drug molecule attaches to its target, and in analyzing minerals, synthetic crystals and nanomaterials to improve material performance in electronics, optics and construction.

Advantages (1) Nuclear fission produces a very high amount of energy from a very small quantity of fuel. (2) It is used extensively in nuclear power plants to generate large-scale electricity. Disadvantage: It produces radioactive waste that must be handled with care over a long period, and carries the risk of catastrophic accidents, as seen in the Chernobyl (1986) and Fukushima (2011) disasters.

Being a gas, CO can diffuse freely throughout the furnace and come into intimate contact with the entire surface of the solid iron oxide particles as it rises through the furnace, whereas solid carbon can only react where it physically touches the ore, giving much less contact area. This gives CO a far larger effective reacting surface, allowing it to reduce the oxide more quickly, uniformly and completely at any height of the furnace, making it the more efficient reducing agent on a large industrial scale.

Although fossil fuels (petroleum, natural gas, coal) are efficient energy sources, their combustion releases greenhouse gases, mainly carbon dioxide (CO₂), along with other pollutants. These gases accumulate in the atmosphere and trap heat, causing air pollution, global warming and climate change.

Roasting a sulphide ore such as ZnS releases sulphur dioxide (SO₂) gas: 2ZnS + 3O₂ → 2ZnO + 2SO₂. SO₂ is a toxic gas that causes respiratory problems and combines with atmospheric moisture to form acid rain, damaging buildings, soil and water bodies. Calcination of a carbonate ore such as ZnCO₃, by contrast, releases only CO₂ (ZnCO₃ → ZnO + CO₂), which, although a greenhouse gas, is not directly toxic or acid-rain forming in the same way. Hence roasting of sulphide ores is a greater environmental challenge than calcination of carbonate ores.

For electrical wiring, copper must be 99.99% pure, because even small amounts of impurities significantly increase electrical resistance and reduce conductivity, which can cause energy losses and overheating. A 98%-pure sample is not good enough for this purpose, so the metal must undergo a further refining step (thermal, chemical or electrochemical) to remove the remaining impurities and achieve the very high purity needed for efficient, safe electrical wiring.

Mechanical recycling only sorts, cleans, melts and reforms the plastic, so it is limited by contamination and by degradation of the polymer chains each time it is reprocessed, giving progressively lower-quality material. Chemical recycling (depolymerization), on the other hand, breaks the polymer chains down completely into their original monomers or simpler chemicals using heat, solvents or catalysts. This produces purer reactants that are not affected by prior contamination or chain degradation, so they can be used to make new, virgin-quality plastic, making chemical recycling superior for maintaining material quality.

Gold is a very unreactive metal, placed near the bottom of the reactivity series, so it does not readily combine with oxygen, water or other elements and is found in the free (elemental) state in nature — it requires little more than physical separation to extract. Sodium, in contrast, is highly reactive and lies well above carbon in the reactivity series, so it reacts readily with oxygen and water and is always found combined in ores. Because sodium is more reactive than carbon, carbon cannot reduce its compounds; sodium must instead be extracted by electrolysis, which requires melting the sodium compound and passing a strong electric current through it — a process that demands a great deal of energy. This is why extracting sodium is far more energy-intensive than extracting gold.