Global challenges Study Guide

The method used to extract a metal from its ore depends significantly on the metal's position in the reactivity series relative to carbon. The reactivity series is a list of common metals arranged in order of decreasing reactivity. Metals that are very reactive, such as potassium, sodium, calcium, magnesium, and aluminium, are found above carbon in the reactivity series. These metals have a strong affinity for oxygen and other non-metals, meaning they are difficult to reduce. Consequently, they cannot be extracted from their ores using carbon reduction and require more powerful methods, primarily electrolysis, which is an energy-intensive process.

Metals that are less reactive than carbon, including zinc, iron, lead, and copper, can be extracted from their oxides by heating with carbon (e.g., in a blast furnace for iron). Carbon is more reactive than these metals, so it can displace them from their compounds, acting as a reducing agent. This process is generally more cost-effective than electrolysis. For example, iron ore (iron oxide) is reduced to iron by carbon monoxide (formed from carbon) in a blast furnace. The carbon 'takes' the oxygen from the metal oxide, leaving the pure metal.

Some very unreactive metals, like gold and silver, are often found in their native, uncombined form in the Earth's crust, meaning they do not require chemical extraction processes; they are simply mined and purified. Copper is sometimes found native, but more often in compounds, and can be extracted using carbon reduction if it's in an oxide form, or by other methods if it's a sulfide.

Understanding carbon's position is crucial: if a metal is above carbon, electrolysis is usually needed; if below, carbon reduction is often viable. This principle helps determine the industrial process required, influencing economic and environmental considerations.

Crude oil is a complex mixture of many different compounds, predominantly hydrocarbons. When crude oil is separated into fractions through fractional distillation, each fraction is still a mixture, but the compounds within that fraction share similar boiling points and molecular sizes. A key characteristic of these compounds is that they are largely alkanes. Alkanes are a type of hydrocarbon, meaning they are organic compounds made up only of carbon and hydrogen atoms, joined together by single bonds. They follow a general formula of CnH2n+2, where 'n' represents the number of carbon atoms in the molecule. For instance, if n=1, the alkane is methane (CH4); if n=2, it's ethane (C2H6). This general formula means that as the number of carbon atoms increases, the number of hydrogen atoms also increases in a predictable way. Compounds that can be described by a general formula and have gradually changing properties are part of a homologous series. The alkanes form such a series. Key features of alkanes, therefore, include being saturated hydrocarbons (only single bonds), having the general formula CnH2n+2, and belonging to a homologous series where each successive member differs by a CH2 unit. Understanding these features helps explain the chemical properties and physical characteristics of crude oil fractions, such as their boiling points and flammability.