Global challenges Study Guide

Electrolysis is a chemical process that uses electrical energy to drive non-spontaneous redox reactions. It's particularly useful for extracting reactive metals that cannot be extracted from their ores using carbon reduction, such as aluminium, sodium, and magnesium. These metals are high in the reactivity series. The reason carbon cannot reduce these metals is that they are more reactive than carbon itself, meaning carbon cannot displace them from their compounds. To overcome this, their molten ionic compounds (typically oxides or chlorides) are subjected to electrolysis. The ionic compound must be molten so that the ions are free to move and conduct electricity. A direct current is passed through the molten electrolyte. At the negative electrode (cathode), the metal ions gain electrons (reduction) and are deposited as the pure metal. At the positive electrode (anode), non-metal ions (often oxide or chloride) lose electrons (oxidation) to form gases or other non-metallic products. For example, in aluminium extraction from molten aluminium oxide, aluminium ions (Al³⁺) gain three electrons to form aluminium metal (Al) at the cathode, and oxide ions (O²⁻) lose two electrons to form oxygen gas (O₂) at the anode. This process is energy-intensive due to the high temperatures required to keep the electrolyte molten and the continuous electrical input needed.

The Earth's atmosphere has undergone significant changes over billions of years, transitioning from an early atmosphere with very little oxygen to the oxygen-rich environment we have today. The initial atmosphere, formed from volcanic outgassing, was primarily composed of carbon dioxide, water vapour, and nitrogen, with trace amounts of methane and ammonia, but crucially, almost no free oxygen.

Approximately 2.7 billion years ago, a pivotal change occurred with the emergence and proliferation of photosynthetic organisms, such as cyanobacteria (often called blue-green algae). These single-celled organisms developed the ability to carry out photosynthesis, a process that uses sunlight to convert carbon dioxide and water into glucose and oxygen. Initially, much of the oxygen produced by these early photosynthetic life forms would have reacted with dissolved iron in the oceans, forming iron oxides that settled out as banded iron formations. This process effectively 'used up' the early oxygen.

Once the dissolved minerals in the oceans were mostly oxidised, oxygen began to accumulate in the atmosphere. Over vast periods, the amount of oxygen slowly but steadily increased. This accumulation had several profound effects. Firstly, it led to the formation of the ozone layer (O₃) in the upper atmosphere. The ozone layer is crucial as it absorbs harmful ultraviolet (UV) radiation from the Sun, protecting life on Earth's surface and allowing more complex life forms to evolve and colonise land. Secondly, the increasing oxygen levels led to a sharp decrease in atmospheric carbon dioxide due to its use in photosynthesis and its subsequent sequestration as organic matter (e.g., fossil fuels) and carbonate rocks. This reduction in greenhouse gases also played a role in temperature regulation. The Great Oxidation Event, a period roughly 2.4 to 2.0 billion years ago, marks a significant rise in atmospheric oxygen, fundamentally changing the planet's habitability and paving the way for the evolution of aerobic life.