Global challenges Revision Notes

What is Crude Oil?

Crude oil is a fossil fuel and a finite resource. It is a complex mixture of hydrocarbon molecules, which are compounds containing only hydrogen and carbon atoms. These hydrocarbons vary significantly in size, from small molecules with low boiling points to very large molecules with high boiling points. Because it is a mixture, crude oil cannot be used directly and must be separated into useful fractions.

Principles of Fractional Distillation

Fractional distillation separates crude oil into fractions based on differences in their boiling points. The process relies on the fact that smaller hydrocarbon molecules have lower boiling points and evaporate more easily, while larger molecules have higher boiling points and are less volatile. This difference in volatility is exploited in a fractionating column to separate the different components.

The Fractional Distillation Process

Crude oil is heated, causing most of it to turn into a hot mixture of liquid and vapour. This mixture then enters the bottom of a tall fractionating column, which has a temperature gradient (hottest at the bottom, coolest at the top). As vapours rise, they cool. Hydrocarbons condense into liquid fractions at different levels according to their specific boiling points. Fractions with higher boiling points (larger molecules) condense lower down the column, while fractions with lower boiling points (smaller molecules) continue to rise before condensing at higher levels. The collected liquid fractions are useful products.

Metals: Strength, Conductivity, and Malleability

Metals are typically dense, strong, and hard, exhibiting high tensile strength, meaning they can withstand significant pulling forces before breaking. They are excellent electrical and thermal conductors due to their delocalised electrons. Most metals have high melting points. They are also ductile (can be drawn into wires) and malleable (can be hammered into sheets), which allows for shaping. Examples include iron, copper, and aluminium. These properties make them suitable for construction, wiring, and cooking utensils. Quantitatively, copper's electrical conductivity is around 5.96 x 10^7 S/m, significantly higher than many other material classes.

Glass and Clay Ceramics: Hardness and Brittleness

Glass and clay ceramics are generally very hard but brittle. They have high melting points and are good thermal and electrical insulators. Unlike metals, they are not ductile or malleable. Glass is amorphous, while clay ceramics typically have a crystalline structure. Their hardness makes them resistant to scratching, but their brittleness means they shatter easily under impact or sudden temperature changes. Common examples include windows (glass) and pottery (clay ceramics). Quantitatively, the hardness of ceramics can be much higher than metals, but their fracture toughness (resistance to crack propagation) is usually much lower.

Polymers: Variability in Properties

Polymers (plastics) demonstrate a wide range of physical properties, from soft and flexible to hard and rigid, depending on their molecular structure and degree of cross-linking. They are generally lightweight, poor electrical and thermal conductors (insulators), and have relatively low melting points compared to metals and ceramics. Some polymers are transparent, others opaque. Their properties can be tailored for specific applications. For example, polyethylene is flexible and low density, while epoxy resins are rigid and strong after curing. Quantitatively, densities of polymers typically range from 0.9 g/cm³ to 1.5 g/cm³, much lower than most metals.

Composites: Combining Strengths for Enhanced Performance

Composites are materials made from two or more constituent materials with significantly different physical or chemical properties, which remain separate and distinct at the macroscopic or microscopic level within the finished structure. They combine the best properties of their components. For instance, fibreglass (glass fibres in a polymer matrix) is strong and lightweight, ideal for boat hulls and sports equipment. Carbon fibre reinforced polymers (CFRP) are exceptionally strong and light, used in aerospace and high-performance vehicles. Quantitatively, composites can achieve specific strengths (strength-to-weight ratio) far exceeding individual component materials.

Purpose and Scale Differences

Industrial production of fertilisers focuses on manufacturing large quantities continuously, efficiently, and at low cost, often using automated plants. The primary goal is profit and meeting market demand. Laboratory synthesis, however, aims to produce small, pure samples for research, education, or analysis, where control over experimental conditions and purity are paramount, and cost efficiency is a secondary concern. Batch processes are common in laboratories, while continuous processes dominate industry.

Reactants and Conditions

Industrially, raw materials are often sourced from abundant and relatively cheap supplies, even if they are less pure, and large-scale purification steps may be integrated into the process. Reactions frequently occur under extreme conditions of high temperature, high pressure, sometimes with catalysts, to maximise reaction rates and yields. In a laboratory, reagents are typically of higher purity, used in smaller quantities, and reactions usually occur under milder conditions (e.g., room temperature, atmospheric pressure), using simple heating or cooling if needed.

Apparatus and Control

Industrial plants feature large, robust reaction vessels, complex piping systems, and advanced instrumentation for continuous monitoring and automated control of temperature, pressure, flow rates, and product composition. Safety systems are elaborate due to the scale and conditions. Laboratory setups use smaller, simpler glassware (beakers, flasks, burettes), often with manual control and observation. Safety procedures are still crucial but are typically simpler due to the smaller scale.

Environmental and Economic Considerations

For industrial processes, minimising waste, recycling by-products, conserving energy, and managing environmental impact (e.g., emissions, effluent treatment) are essential for economic viability and regulatory compliance. The economic aspect drives decisions on raw material choice, process optimisation, and energy use. In the laboratory, while waste disposal is important, the environmental and economic impact of a single small-scale process is far less significant compared to continuous industrial operations.

Reactivity Series and Extraction Methods

The reactivity series ranks metals by their tendency to react, particularly with oxygen. This series dictates the industrial method for extracting a metal from its ore. Metals above carbon in the reactivity series (e.g., potassium, sodium, calcium, magnesium, aluminium) are more difficult to extract from their compounds. They require electrolysis, a process that uses electricity to separate the metal from its ore, which is very energy-intensive and expensive. Metals below carbon in the reactivity series (e.g., zinc, iron, lead, copper) can be extracted by reduction with carbon, a less energetically demanding process carried out in a furnace.

Extraction by Reduction with Carbon

For metals less reactive than carbon, such as iron, carbon acts as a reducing agent. This means carbon removes oxygen from the metal oxide, leaving the pure metal. A common example is the extraction of iron in a blast furnace, where iron(III) oxide (Fe₂O₃) reacts with carbon monoxide (produced from carbon) at high temperatures to yield iron (Fe) and carbon dioxide (CO₂). This process is represented by the equation: 2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g) or Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g). This method is economically viable for these metals due to lower energy costs compared to electrolysis.

Extraction by Electrolysis

Metals more reactive than carbon require electrolysis for extraction. Aluminium, for example, is extracted from its ore, bauxite (aluminium oxide), by electrolysis. Aluminium oxide has a very high melting point, so it is dissolved in molten cryolite to lower the melting point and improve conductivity. The molten mixture is then electrolysed. At the cathode (negative electrode), aluminium ions gain electrons to form molten aluminium metal (Al³⁺ + 3e⁻ → Al). At the anode (positive electrode), oxide ions lose electrons to form oxygen gas (2O²⁻ → O₂ + 4e⁻). The oxygen then reacts with the carbon anodes, which need frequent replacement. This process consumes a significant amount of electricity, making it costly.

Extraction of Unreactive Metals

Very unreactive metals, such as gold and silver, are often found as uncombined elements in the Earth's crust. They do not require chemical extraction using carbon or electrolysis. They are typically separated from other materials by physical methods, such as panning or filtration, after mining. This is the simplest and least expensive extraction method due to their inherent chemical stability and low reactivity.