Global challenges Revision Notes

What is Crude Oil?

Crude oil is a complex mixture of hydrocarbon compounds, meaning they are composed primarily of hydrogen and carbon atoms. These hydrocarbons vary greatly in their size and structure, leading to different physical properties, most notably their boiling points. Crude oil is a fossil fuel, formed from the remains of ancient marine organisms over millions of years under high pressure and temperature. It is a vital raw material for producing a wide range of products.

The Principle of Fractional Distillation

Fractional distillation separates the components of crude oil based on differences in their boiling points. Hydrocarbons with shorter carbon chains have weaker intermolecular forces, resulting in lower boiling points. Conversely, longer carbon chains have stronger intermolecular forces and thus higher boiling points. The process relies on heating the crude oil and then allowing the vapours to cool and condense at different temperatures within a fractionating column.

The Fractional Distillation Process

Initially, crude oil is heated to a high temperature (around 350°C) in a furnace, causing most of it to vaporise. This hot mixture then enters the bottom of a tall fractionating column. The column has a temperature gradient: it is hottest at the bottom and coolest at the top. As the vapours rise, they cool. Hydrocarbons condense when they reach a level where the temperature is below their boiling point. Fractions with higher boiling points (longer carbon chains) condense lower down the column, while fractions with lower boiling points (shorter carbon chains) rise higher before condensing. The different liquid fractions are collected at various levels (trays) in the column.

Crude Oil Fractions and Their Uses

Several fractions are obtained from crude oil, each with distinct properties and uses. From top to bottom of the column (lowest to highest boiling points): refinery gases (LPG), gasoline (petrol), naphtha, kerosene (paraffin), diesel (gas oil), fuel oil, and bitumen (asphalt). Refinery gases are used for heating. Gasoline is used as fuel for cars. Naphtha is used to make petrochemicals. Kerosene is jet fuel. Diesel is fuel for diesel engines. Fuel oil is burned in ships and power stations. Bitumen is used for road surfacing.

Why Electrolysis is Used for Some Metals

Electrolysis is employed to extract metals that are high in the reactivity series, such as aluminium, sodium, and magnesium. These metals are more reactive than carbon. This means that they cannot be extracted from their ores using carbon reduction, as carbon is not strong enough to remove oxygen from their compounds. Instead, electrical energy is needed to force the reduction of the metal ions. The process converts their molten ionic compounds into pure metal.

How Electrolysis is Carried Out

For electrolysis, the metal compound (usually an oxide or chloride) must be in a molten state. This is crucial because melting allows the ions to become mobile and conduct electricity. A direct electric current is then passed through this molten electrolyte. The system consists of two electrodes: a cathode (negative electrode) and an anode (positive electrode). At the cathode, positively charged metal ions migrate and gain electrons (reduction) to form the neutral metal atoms. At the anode, negatively charged non-metal ions migrate and lose electrons (oxidation), forming neutral non-metal atoms or molecules, often as a gas.

Aluminium Extraction Example

A classic example is the extraction of aluminium from aluminium oxide, Al₂O₃, dissolved in molten cryolite (to lower its melting point). At the cathode (negative electrode): Al³⁺(l) + 3e⁻ → Al(l). This forms molten aluminium metal. At the anode (positive electrode): 2O²⁻(l) → O₂(g) + 4e⁻. Oxygen gas is produced. The anodes are often made of carbon, and the oxygen produced reacts with the carbon to form carbon dioxide, causing the anodes to burn away and require regular replacement. This reaction is: C(s) + O₂(g) → CO₂(g).

Energy Demands of Electrolysis

Electrolysis is a very energy-intensive process. Significant electrical energy is required to maintain the high temperatures needed to keep the electrolyte molten and to drive the non-spontaneous chemical reactions. The cost of electricity is a major factor in the overall expense of electrolytic metal extraction.

Earth's Early Atmosphere

The atmosphere of early Earth was very different from today's. Scientists believe it was formed by gases released from intense volcanic activity during the planet's early history. The primary gases present were carbon dioxide, water vapour, and nitrogen. There were also smaller amounts of methane and ammonia. Crucially, there was little to no free oxygen in the atmosphere at this stage. Water vapour condensed to form oceans as the Earth cooled.

The Rise of Photosynthesis

The key event that led to an oxygen-rich atmosphere was the evolution of photosynthetic organisms, primarily cyanobacteria, around 2.7 billion years ago. These early life forms developed the ability to carry out photosynthesis, a process where they use sunlight energy to convert carbon dioxide and water into glucose (food) and oxygen gas. This marked the beginning of oxygen production on a large scale.

Oxygen Accumulation and Its Effects

Initially, much of the oxygen produced by photosynthetic organisms reacted with iron dissolved in the oceans, forming iron oxides that precipitated and formed rock layers. Once these 'oxygen sinks' were saturated, free oxygen began to accumulate in the atmosphere over hundreds of millions of years. This increasing oxygen had two major consequences: 1. It enabled the formation of the ozone layer, which filters harmful UV radiation, allowing life to thrive on land. 2. It led to a decrease in atmospheric carbon dioxide as it was used in photosynthesis and locked away in biomass and sedimentary rocks.

What is Crude Oil?

Crude oil is a naturally occurring, dark, viscous liquid found underground. It is a complex mixture of many different organic compounds, primarily hydrocarbons. These hydrocarbons are molecules composed solely of hydrogen and carbon atoms. The exact composition of crude oil can vary significantly depending on its geographical origin, but it always contains a wide range of hydrocarbon molecules of differing sizes and structures. This variation in molecular size leads to different physical properties, such as boiling points.

Crude Oil as a Source of Hydrocarbons

Crude oil is the most significant natural source of hydrocarbons globally. These hydrocarbons are essential for various aspects of modern life. They are separated from crude oil through fractional distillation into different fractions, which then become fuels (like petrol, diesel, and kerosene) or raw materials for further processing. Each fraction contains hydrocarbons with a similar number of carbon atoms and thus similar boiling points, making them suitable for specific applications.

Feedstock for the Petrochemical Industry

A critical use of crude oil, beyond fuels, is its role as a feedstock for the petrochemical industry. A feedstock is a raw material used in an industrial chemical process. In this context, various hydrocarbon fractions obtained from crude oil are chemically processed to synthesise a wide range of valuable products. These include plastics, synthetic rubbers, synthetic fibres, detergents, dyes, pharmaceuticals, fertilisers, and solvents. The petrochemical industry is entirely dependent on crude oil or natural gas as its primary source of these precursor chemicals. Without crude oil as a feedstock, manufacturing many everyday materials would not be possible on a commercial scale.