Global challenges Revision Notes

What is a Chemical Cell?

A chemical cell is a device that generates electricity from a chemical reaction. It converts chemical energy into electrical energy. This process involves a spontaneous redox reaction where electrons are transferred between different chemical species. The flow of these electrons through an external circuit is what constitutes an electrical current, resulting in a potential difference across the cell.

How Chemical Cells Work

Chemical cells typically consist of two different metals (or a metal and a non-metal) immersed in an electrolyte. The difference in reactivity between these materials drives the reaction. The more reactive metal loses electrons (oxidation) and acts as the negative electrode (anode). These electrons travel through an external wire to the less reactive material, which gains electrons (reduction) and acts as the positive electrode (cathode). The electrolyte allows ions to move, completing the circuit internally and maintaining charge balance.

Producing a Potential Difference

The movement of electrons from the more reactive metal to the less reactive metal through the external circuit generates a potential difference, also known as voltage. This potential difference is the driving force that pushes the electrons around the circuit, enabling the cell to power electrical devices. The magnitude of the potential difference depends on the difference in reactivity between the two materials used in the cell.

When a Chemical Cell Stops Working

A chemical cell continues to produce a potential difference and generate current as long as the chemical reaction can proceed. However, the reactants are consumed during this process. Once one or both of the reactants are completely used up, or if the electrolyte becomes depleted or changes composition significantly, the chemical reaction stops. At this point, the flow of electrons ceases, and the cell is no longer able to produce a potential difference.

What is a Polymer?

A polymer is a large molecule (macromolecule) composed of many repeated smaller identical or similar units called monomers, which are linked together through chemical reactions. This process is known as polymerisation. Naturally occurring polymers are abundant in biological systems and are fundamental to life.

DNA: A Polymer of Nucleotides

Deoxyribonucleic Acid, or DNA, is a vital natural polymer. It is made up of four different types of monomer units called nucleotides. Each nucleotide consists of three main components: a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases (adenine, guanine, cytosine, or thymine). The sequence of these nucleotides encodes genetic information.

Polymers from Sugars

Sugars, specifically simple sugars called monosaccharides, can act as monomers to form larger polymer structures known as polysaccharides. Examples of these include starch (for energy storage in plants), glycogen (for energy storage in animals), and cellulose (a structural component of plant cell walls).

Polymers from Amino Acids

Amino acids are the monomer units that link together to form proteins. Proteins are highly diverse natural polymers with crucial roles in almost all biological processes. Their specific three-dimensional structure, determined by the sequence of amino acids, dictates their function within an organism.

Potable Water from Fresh Water Sources

Potable water is safe to drink and contains low levels of dissolved salts and microbes. Fresh water sources like rivers and lakes can be treated to become potable. The main steps involve screening to remove large debris, sedimentation to allow larger solids to settle, and filtration through sand and gravel beds to remove smaller insoluble particles. Chemical sterilisation, usually with chlorine, is then used to kill harmful bacteria and other microorganisms, making the water safe for consumption.

Desalination of Salty Water

Salty water, such as seawater, needs to have its high concentration of dissolved salts removed to become potable. This process is called desalination. One common method is distillation. In distillation, the salty water is boiled, and the resulting steam is collected and condensed back into liquid water. The dissolved salts are left behind in the original container, as they do not evaporate. This effectively separates the water from the salts. Distillation requires a significant amount of energy to heat and boil the water.

Reverse Osmosis as a Desalination Method

Another important desalination method is reverse osmosis. This process involves forcing salty water through a semi-permeable membrane under high pressure. The membrane has tiny pores that allow water molecules to pass through but block the larger dissolved salt ions. This separates the fresh water from the concentrated salt solution. Reverse osmosis is often more energy-efficient than distillation for large-scale desalination, though it still requires energy for the pumps needed to create the high pressure.

Importance of Separation Techniques

The various separation techniques are fundamental to producing potable water. Sedimentation and filtration are physical separation methods that remove insoluble solids. Distillation and reverse osmosis are more complex separation methods designed to remove dissolved solids, primarily salts. Chemical sterilisation, while not a separation technique in itself, is a vital final step to ensure the water is free from dangerous pathogens after physical and chemical impurities have been removed. These combined processes ensure water meets safety standards for drinking.

What are Hydrocarbons and Their Primary Source?

Hydrocarbons are chemical compounds composed solely of hydrogen and carbon atoms. They are fundamental to organic chemistry and exist in various forms, from simple gases like methane to complex liquids and solids found in crude oil. The primary source of the vast majority of hydrocarbons used by modern society is crude oil, a naturally occurring fossil fuel. Crude oil formed over millions of years from the decomposition of organic matter, primarily ancient marine organisms, under intense heat and pressure deep within the Earth's crust.

How Modern Life Relies on Hydrocarbons

Modern life is critically dependent on hydrocarbons due to their versatile applications. They are essential as fuels for transportation (petrol, diesel, jet fuel), heating (kerosene, fuel oil), and electricity generation. Beyond energy, crude oil provides the raw materials, known as feedstocks, for the petrochemical industry. This industry processes hydrocarbons to create a vast array of products, including plastics for packaging, construction, and electronics; synthetic fibres for clothing; detergents for cleaning; lubricants for machinery; and pharmaceuticals. Without these hydrocarbon-derived products, many aspects of daily life, from communication and healthcare to food preservation and transport, would be severely impacted.

Crude Oil as a Finite Resource

Crude oil is a finite resource, meaning there is a limited, non-renewable supply on Earth. Its formation process takes millions of years, far longer than the rate at which humanity consumes it. Once extracted and used, it cannot be replaced within a human timeframe. This finiteness means that at some point, the world's accessible reserves will diminish significantly, leading to potential shortages and increased costs. The recognition of crude oil as a finite resource drives the search for alternative energy sources and sustainable materials to reduce our reliance on it, mitigating future resource scarcity and environmental impacts.