Global challenges Study Guide

Organic chemistry focuses on compounds primarily made of carbon and hydrogen, often including oxygen. These compounds are grouped into homologous series, which are families of organic compounds with the same general formula and similar chemical properties. Understanding the general formulas and characteristic functional groups of these series allows us to predict the formulae and structures of their members, including the products of their reactions.

Alkanes are saturated hydrocarbons, meaning they contain only single carbon-carbon bonds and have the general formula C_nH_2n+2. They undergo combustion and substitution reactions. For example, the complete combustion of an alkane produces carbon dioxide and water.

Alkenes are unsaturated hydrocarbons, containing at least one carbon-carbon double bond, and have the general formula C_nH_2n (for non-cyclic alkenes). The double bond makes them more reactive than alkanes, typically undergoing addition reactions where atoms are added across the double bond, breaking it and forming new single bonds. For example, ethene (C₂H₄) can react with hydrogen to form ethane (C₂H₆).

Alcohols are organic compounds containing the hydroxyl (-OH) functional group attached to a carbon atom. Their general formula is C_nH_2n+1OH. Alcohols can undergo combustion, oxidation, and reaction with sodium. For example, ethanol (CH₃CH₂OH) can be oxidised to ethanoic acid (CH₃COOH) or undergo complete combustion to produce carbon dioxide and water.

To predict the products of a reaction, you need to identify the type of homologous series the reactants belong to and the type of reaction occurring. For combustion, the products are usually carbon dioxide and water (for complete combustion) or carbon monoxide/carbon (for incomplete). For addition reactions involving alkenes, the double bond is broken, and two new atoms or groups are added. For substitution reactions of alkanes, one atom (e.g., hydrogen) is replaced by another (e.g., a halogen). By following these general principles and applying the homologous series rules, you can deduce the formulae and structures of the products.

Different materials like glass, clay ceramics, polymers, composites, and metals possess distinct physical properties that make them suitable for various applications. Understanding these properties quantitatively allows us to select the best material for a specific purpose. For instance, metals are generally strong, dense, and excellent conductors of heat and electricity due to their metallic bonding structure with delocalised electrons. This makes them ideal for electrical wiring and structural components. In contrast, glass and clay ceramics are typically brittle, hard, and electrical insulators. Their rigid, ordered or amorphous structures prevent the movement of electrons and resist deformation, but they shatter easily under impact. Polymers exhibit a wide range of properties depending on their structure; some are flexible and lightweight, like polyethylene, making them useful for packaging, while others are strong and rigid, like epoxy resins, used in adhesives. Composites, which combine two or more different materials, often leverage the strengths of their components to create materials with enhanced properties. For example, fibreglass combines strong glass fibres with a flexible polymer matrix, resulting in a material that is both strong and relatively light, used in boat hulls. When comparing materials, we often look at properties such as density, strength (tensile strength, compressive strength), hardness, melting point, electrical conductivity, thermal conductivity, and ductility. Quantitative comparison involves using numerical values and units, rather than just descriptive terms like 'strong' or 'weak', allowing for precise material selection and engineering design.