Global challenges Revision Notes

Preparation of Monomer Solutions

To carry out condensation polymerisation in a laboratory, two different monomer solutions are prepared. For example, in the synthesis of nylon, one monomer, a diamine (e.g., hexane-1,6-diamine), is dissolved in an aqueous alkaline solution (e.g., sodium hydroxide solution). The alkali is crucial for neutralising the hydrochloric acid by-product formed during the reaction, which would otherwise inhibit further polymerisation. The second monomer, a diacyl chloride (e.g., decanedioyl dichloride), is dissolved in an immiscible organic solvent like hexane. It is essential that the organic solvent doesn't react with the diacyl chloride and that it forms a distinct layer with the aqueous solution.

Forming the Polymer Interface (Higher)

The two prepared monomer solutions are carefully layered in a beaker. The general practice is to pour the less dense organic solution of the diacyl chloride gently onto the surface of the more dense aqueous solution of the diamine. This creates two distinct, immiscible liquid layers with an interface between them. It is crucial to avoid shaking or mixing the solutions at this stage, as the reaction relies on the controlled interaction at the boundary. The condensation polymerisation reaction occurs precisely at this interface, producing a film of the polymer and a small molecule (like HCl, which then reacts with the NaOH).

Extracting the Polymer (Higher)

Once the distinct layers are formed, a thin film of the polymer starts to form at the interface. This film can be carefully grasped using a pair of forceps or tweezers. By gently and steadily pulling the polymer film upwards from the interface, a continuous 'rope' of the polymer can be drawn out. As the polymer is removed, fresh monomer molecules from the two layers come into contact at the newly exposed interface, allowing the polymerisation reaction to continue. The rate of pulling should be slow and steady to maintain continuous formation. This extraction process can continue until one of the monomers is depleted.

Post-synthesis Treatment and Safety (Higher)

After extraction, the polymer 'rope' is usually washed thoroughly with water to remove any residual unreacted monomers, the organic solvent, and any byproducts (like sodium chloride if NaOH was used). This purification step is important to obtain a clean polymer. The polymer is then dried. Safety considerations are paramount: eye protection must be worn at all times. The sodium hydroxide solution is corrosive. The organic solvent (e.g., hexane) is flammable and its vapours may be harmful, requiring good ventilation (fume cupboard). Diacyl chlorides are irritants and hydrolyse in the presence of moisture to form corrosive acids, so skin contact must be avoided.

Understanding the Greenhouse Effect

The greenhouse effect is a vital natural process. High-frequency, short-wavelength radiation from the Sun (mainly ultraviolet and visible light) passes through the atmosphere and warms the Earth's surface. The warmed surface then emits lower-frequency, longer-wavelength infrared radiation. Certain atmospheric gases, known as greenhouse gases, absorb some of this outgoing infrared radiation, preventing it from escaping directly into space. These gases then re-emit the absorbed energy in all directions, including back towards the Earth's surface, thus trapping heat and warming the planet.

Radiation Interaction with Matter

Solar radiation, primarily visible light, is not significantly absorbed by atmospheric gases like nitrogen and oxygen, allowing it to reach the Earth's surface. However, greenhouse gases such as carbon dioxide and methane are good absorbers of infrared radiation. When infrared radiation interacts with molecules of these gases, the energy causes the bonds within the molecules to vibrate. This vibrational energy is then re-radiated. This absorption and re-emission process is crucial for the greenhouse effect, as it keeps the Earth warm enough to sustain life.

Key Greenhouse Gases

Several gases contribute to the greenhouse effect. Water vapour is the most abundant greenhouse gas and plays a significant role. Carbon dioxide (CO2) is also a major contributor, largely due to human activities like burning fossil fuels. Methane (CH4), released from sources such as agriculture and natural gas leaks, is a potent greenhouse gas. Nitrous oxides (N2O), often from agricultural and industrial processes, also trap heat. While these gases are a small percentage of the atmosphere, their ability to absorb infrared radiation has a large impact on global temperatures.

What is Addition Polymerisation?

Addition polymerisation is a process where many small molecules, called monomers, join together to form a very long chain molecule called a polymer. This type of reaction occurs without the loss of any atoms; all atoms from the monomers are incorporated into the polymer structure. It is a key method for creating synthetic polymers like plastics used in everyday life. The process is initiated by breaking the double bonds present in the monomer molecules, allowing them to form new single bonds with adjacent monomers.

Monomers and their Functional Group

For addition polymerisation to occur, the monomer molecules must contain a carbon-carbon double bond (C=C). This double bond is the functional group that enables the polymerisation process. Examples of such monomers include ethene (CH₂=CH₂), propene (CH₂=CHCH₃), and chloroethene (CH₂=CHCl). The presence of this double bond makes the monomer 'unsaturated', meaning it has the potential to add more atoms without replacing existing ones.

From Monomer to Repeating Unit

The repeating unit in an addition polymer is derived directly from the monomer. When the monomers link together, the carbon-carbon double bond within each monomer breaks. Free electrons from this broken double bond then form new single bonds with adjacent monomer units. The repeating unit is essentially the original monomer without its double bond, now shown with single bonds extending from its ends to indicate its connection within the polymer chain. For instance, the ethene monomer (CH₂=CH₂) becomes the repeating unit (-CH₂-CH₂-) in poly(ethene).

Example: Ethene to Poly(ethene)

Consider ethene (CH₂=CH₂) as a monomer. Each individual ethene molecule has a carbon-carbon double bond. During addition polymerisation, this double bond breaks in many ethene molecules. These molecules then link up end-to-end to form the long polymer chain, poly(ethene). The functional group (C=C) in ethene is crucial. The repeating unit for poly(ethene) is -CH₂-CH₂-, which is simply the ethene molecule with the double bond opened up to form single bonds extending outwards to join other units. This highlights the direct relationship between the monomer structure and its repeating unit.

What is Crude Oil?

Crude oil is a fossil fuel formed over millions of years from the buried remains of marine organisms. It is a dark, viscous liquid found in reservoirs beneath the Earth's surface. Chemically, it is a complex mixture of many different hydrocarbons, which are compounds made up solely of carbon and hydrogen atoms. The hydrocarbons in crude oil vary greatly in the number of carbon atoms they contain, from very short chains to very long chains. This variation in chain length is why crude oil has a range of boiling points and why it is not useful in its raw form.

Crude Oil as a Source of Hydrocarbons

The primary significance of crude oil lies in its abundant supply of hydrocarbons. These hydrocarbons are separated into different fractions based on their boiling points through a process called fractional distillation. Each fraction consists of hydrocarbons with similar numbers of carbon atoms, and therefore similar properties. Examples of these fractions include refinery gases, gasoline (petrol), naphtha, kerosene, diesel (gas oil), fuel oil, and bitumen. Each fraction has specific uses; for example, gasoline is used as fuel for cars, and bitumen is used for road surfacing.

Feedstock for the Petrochemical Industry

Beyond its use as fuel, crude oil is a vital 'feedstock' for the petrochemical industry. A feedstock is essentially a raw material used to create other products. The fractions obtained from crude oil, particularly naphtha and gas oil, are transformed into a massive range of useful substances. The petrochemical industry uses these fractions to produce plastics, synthetic fibres, detergents, solvents, fertilisers, and many other chemicals that are essential in modern life. Without crude oil, the production of these everyday materials would be significantly different or impossible.

Importance of Hydrocarbons

Hydrocarbons are fundamental to many aspects of modern life. As fuels, they power transportation, generate electricity, and provide heating for homes and industries. As feedstocks, they are the building blocks for countless organic chemicals. The varying properties of different hydrocarbons (e.g., flammability, viscosity, boiling point) mean they can be tailored for diverse applications. The ability to separate and chemically transform these hydrocarbons makes crude oil an exceptionally valuable natural resource.