Global challenges Revision Notes

Alkanes: Properties and Reactions

Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2. This means they only contain single carbon-carbon bonds. The first four alkanes are methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). They are relatively unreactive due to their strong C-C and C-H single bonds. Their main reactions are complete combustion, producing carbon dioxide and water, and substitution reactions with halogens under UV light, where a hydrogen atom is replaced by a halogen atom. For example, methane reacting with chlorine can produce chloromethane and hydrogen chloride.

Alkenes: Properties and Reactions

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond, with the general formula C_nH_2n. The first four alkenes are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈), and pentene (C₅H₁₀). The presence of the double bond makes them much more reactive than alkanes. Alkenes typically undergo addition reactions across the double bond. This means that an atom (like hydrogen, a halogen, or water) adds to each carbon of the double bond, breaking the double bond and forming a saturated product. For example, ethene reacts with bromine water, decolorising it, to form 1,2-dibromoethane.

Alcohols: Properties and Reactions

Alcohols are organic compounds characterised by the hydroxyl (-OH) functional group. Their general formula is C_nH_2n+1OH. The first four alcohols are methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH), and butanol (C₄H₉OH). Like alkanes, alcohols undergo complete combustion, producing carbon dioxide and water. They can also be oxidised; for instance, ethanol can be oxidised to ethanoic acid. Another characteristic reaction is with reactive metals like sodium, where hydrogen gas and a metal alkoxide are produced, e.g., sodium ethoxide from ethanol.

Predicting Products from Reactant Structures

To predict the products of a reaction involving alkanes, alkenes, or alcohols, first identify the homologous series of the reactant(s) and understand the characteristic reactions of that series. For alkanes, expect combustion (CO₂ + H₂O) or substitution (e.g., with halogens). For alkenes, expect addition reactions across the double bond, leading to a single-bonded product. For alcohols, expect combustion, oxidation, or reactions with reactive metals. By knowing these patterns and general formulas, you can reliably deduce the structure and formula of the product molecules. For example, if ethene (C₂H₄) reacts with hydrogen (H₂) in an addition reaction, the product will be ethane (C₂H₆).

What is Addiction Polymerisation?

Addition polymerisation is a chemical reaction where many small, unsaturated molecules called monomers, typically alkenes, join together to form a very large saturated molecule called a polymer. During this process, the carbon-carbon double bond (C=C) in each alkene monomer breaks, allowing the monomer units to link up end-to-end to form a long chain. No small molecules are eliminated during this reaction, meaning all atoms from the original monomers are incorporated into the polymer structure.

Deducing Polymer Structure from Monomer

When given an alkene monomer, you can deduce the repeating unit of its addition polymer. The key step is to 'break' the carbon-carbon double bond and extend single bonds outwards from the two carbons that were part of the double bond. These extended single bonds represent the points where the monomer unit connects to other monomer units in the polymer chain. For example, if the monomer is propene (CH₂=CH-CH₃), the double bond between the first two carbons breaks. The repeating unit for poly(propene) would be -[CH₂-CH(CH₃)]-, with the square brackets and 'n' indicating it's a repeating unit in a long chain.

Deducing Monomer Structure from Polymer

To work backwards from an addition polymer to its monomer, you need to identify the repeating unit within the polymer chain. This is often represented by a section enclosed in square brackets with an 'n' subscript. Once the repeating unit is identified, you then 'reform' the carbon-carbon double bond by placing it between the two carbons that formed the backbone of the repeating unit. For example, if the repeating unit is -[CH₂-CF₂]-, the monomer would be tetrafluoroethene (CF₂=CF₂). Remember that the monomer must be an alkene.

Crude Oil Fractions are Mixtures

Crude oil is a naturally occurring fossil fuel formed from the remains of ancient organisms. It is a complex mixture of many different hydrocarbons. When crude oil is heated and separated into different fractions in a fractional distillation column, each resulting fraction (such as petrol, diesel, or kerosene) is not a single compound. Instead, each fraction is a mixture of hydrocarbons that have similar boiling points. These hydrocarbons are primarily members of the alkane homologous series, alongside other types of hydrocarbons.

Alkanes and their General Formula

Alkanes are a specific family of hydrocarbons within the fractions of crude oil. They are characterised by having only carbon-carbon single bonds and carbon-hydrogen single bonds, making them saturated hydrocarbons. The general formula for alkanes is CnH2n+2, where 'n' represents the number of carbon atoms in the molecule. For example, if an alkane has 4 carbon atoms (n=4), its formula would be C4H(2*4)+2 = C4H10 (butane). This formula allows chemists to determine the number of hydrogen atoms for any given number of carbon atoms in an alkane molecule.

Homologous Series Characteristics

Alkanes belong to a homologous series, which means they form a series of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties as the number of carbon atoms increases. For alkanes, as the carbon chain length increases, physical properties such as boiling point, density, and viscosity tend to rise. Conversely, their flammability decreases with increasing molecular size. This consistent pattern helps explain why different crude oil fractions, consisting of alkanes with different carbon chain lengths, have distinct uses.

What is a Life Cycle Assessment (LCA)?

A Life Cycle Assessment (LCA) is a 'cradle-to-grave' analysis of the environmental impact of a product, from the extraction of raw materials to its final disposal. It's a comprehensive method to understand the environmental footprint throughout a product's entire existence. The goal is to identify points in the product's lifespan where environmental damage is highest, allowing for improvements to be made.

Stages of an LCA

An LCA typically covers several key stages:

1. Raw Material Extraction: Assessing the environmental impact of obtaining natural resources (e.g., mining, drilling, logging).
2. Manufacturing and Processing: Evaluating the energy and resources used, and pollutants released during the production of materials and the product itself.
3. Distribution and Transport: Considering the impact of moving raw materials, components, and finished products.
4. Use, Reuse, and Maintenance: Accounting for energy consumption and waste generated during the product's active life, including repairs and replacements.
5. Disposal or Recycling: Analysing the impact of discarding the product, considering landfill, incineration, or recycling processes.

Importance and Limitations of LCA

LCAs are important for making informed environmental decisions and comparing products. They help identify hidden environmental costs. However, they have limitations:

1. Subjectivity: It can be difficult to quantify all environmental impacts precisely, and assumptions must be made. Different assessors may use different values or focus on different aspects, leading to varied results.
2. Complexity: Conducting a full LCA is complex, time-consuming, and can be expensive due to the vast amount of data required.
3. Bias: The way an LCA is conducted and presented can sometimes be influenced by the interests of the organisation commissioning it.
4. Balancing Factors: LCAs often need to balance environmental impacts with economic cost and the product's overall benefit to society.

Key Considerations in an LCA

When carrying out an LCA, it's crucial to consider all types of environmental impacts, such as energy consumption, water use, greenhouse gas emissions, pollution (air, water, soil), and waste generation. The relative importance of these impacts can vary, and assigning numerical values to them can be challenging. For example, the impact of releasing carbon dioxide is often weighed against the impact of local water pollution. The assumptions made about the product's lifespan, use patterns, and disposal methods significantly influence the assessment's outcome. It's also vital to compare the environmental impact with the social and economic benefits a product provides.