OCR GCSE Chemistry A Gateway revision notes
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Evaluating Biological Methods of Metal Extraction (Higher) Revision Notes
Introduction to Biological Metal Extraction (Higher)
Biological methods for metal extraction provide environmentally friendly alternatives to traditional, energy-intensive processes like smelting. These methods are particularly useful for low-grade ores, where the concentration of metal is too low to be economically viable for conventional techniques, or for remediating contaminated land. The primary biological methods are bioleaching and phytoextraction, both aiming to recover valuable metals while minimising environmental harm associated with high-temperature, chemical-intensive processes.
Bioleaching Explained (Higher)
Bioleaching is a process that uses microorganisms, typically bacteria, to extract metals from their ores. These bacteria obtain energy by oxidising metal sulfides, converting insoluble metal compounds into soluble metal sulfates. For example, in copper bioleaching, bacteria like Acidithiobacillus ferrooxidans oxidise copper sulfide (CuS) to produce copper sulfate (CuSO₄), which is soluble in water. The copper ions (Cu²⁺) are then leached from the ore using water and can be subsequently recovered, often through displacement reactions with a more reactive metal (e.g., scrap iron) or by electrolysis. This method avoids the need for high temperatures, reducing energy consumption and greenhouse gas emissions.
Phytoextraction Explained (Higher)
Phytoextraction involves using 'hyperaccumulator' plants that absorb metal ions (e.g., copper, nickel) from contaminated soil or low-grade ores through their root systems and concentrate them in their shoots. Once the plants have grown sufficiently, they are harvested and burned (incinerated). The ash produced is rich in the desired metal, which can then be extracted and purified. Phytoextraction is a slower process than bioleaching but is highly effective for large areas of dispersed contamination and offers a visually less intrusive remediation method compared to digging up and treating soil.
Evaluation of Biological Methods (Higher)
When evaluating biological methods, several factors must be considered. Environmental advantages include reduced energy consumption, lower greenhouse gas emissions, and avoidance of toxic gas release (e.g., sulfur dioxide). They are suitable for extracting metals from low-grade ores and for cleaning up contaminated sites. However, they are significantly slower than traditional methods, often taking months or even years. They require large areas of land (especially phytoextraction) and their efficiency can be sensitive to environmental conditions like pH and temperature. The concentration of metal extracted per unit time is typically lower than in smelting, impacting economic viability for high-demand metals from rich ores. The recovery and purification of metals from the leachate or ash can also be complex.
Condensation Polymerisation (Higher) Revision Notes
What is Condensation Polymerisation? (Higher)
Condensation polymerisation is a process where monomers react to form a polymer, releasing a small molecule (like water) in each step. This is different from addition polymerisation where monomers add directly without losing any atoms. For condensation polymerisation to occur, monomers must have at least two different reactive functional groups. These functional groups react with each other to form a new bond and eliminate a small molecule, often water.
Monomers and Functional Groups (Higher)
The monomers used in condensation polymerisation typically have two functional groups. For example, dicarboxylic acids have two -COOH groups, diols have two -OH groups, and diamines have two -NH2 groups. The reaction occurs between these functional groups. For instance, the -OH from a carboxylic acid group can combine with an -H from an alcohol group (in a diol) to form water, leaving an ester linkage. Similarly, the -OH from a carboxylic acid group can combine with an -H from an amine group (in a diamine) to form water, leaving an amide linkage.
Formation of Polyesters and Polyamides (Higher)
Polyesters are formed from dicarboxylic acids and diols. The reaction between -COOH and -OH groups forms an ester link (-COO-) and releases water. For example, terephthalic acid and ethanediol polymerise to form PET (polyethylene terephthalate). Polyamides are formed from dicarboxylic acids and diamines. The reaction between -COOH and -NH2 groups forms an amide link (-CONH-) and releases water. Nylon is a common polyamide formed this way, such as nylon 6,6 from hexanedioic acid and 1,6-diaminohexane.
Key Characteristics (Higher)
A key characteristic of condensation polymerisation is the loss of a small molecule, most commonly water, during the formation of each new bond between monomers. The repeating unit in the polymer does not have the same molecular formula as the original monomer because parts of the functional groups are removed. This distinguishes it from addition polymerisation where the repeating unit has the same atoms as the monomer.
Higher: Optimising Industrial Chemical Processes: The Rate vs. Equilibrium Trade-Off Revision Notes
Higher: Understanding Reaction Rate and Equilibrium
Reaction rate refers to how quickly reactants are converted into products. It is influenced by temperature, pressure (for gases), concentration, surface area, and the presence of a catalyst. Higher temperatures increase collision frequency and energy, leading to faster rates. Higher pressures (for gases) increase collision frequency. Equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant. The position of equilibrium indicates the relative amounts of reactants and products at equilibrium. Le Chatelier's Principle states that if conditions are changed, the system will counteract the change to re-establish equilibrium.
Higher: Temperature's Impact: A Common Trade-Off
For an exothermic reaction (ΔH < 0, releases heat), increasing the temperature increases the reaction rate but shifts the equilibrium to the left (towards reactants), reducing the yield of products. Decreasing the temperature increases the product yield but slows down the reaction. For an endothermic reaction (ΔH > 0, absorbs heat), increasing temperature increases both the rate and the product yield. In industrial processes, a compromise temperature is chosen. For exothermic reactions, this is usually a moderate temperature that allows for a reasonable rate without excessively reducing the yield.
Higher: Pressure's Role in Optimisation
For reactions involving gases, increasing pressure generally increases the reaction rate because molecules are closer together, leading to more frequent collisions. According to Le Chatelier's Principle, increasing pressure also shifts the equilibrium towards the side with fewer moles of gaseous particles. If the desired product forms on the side with fewer moles of gas, high pressure benefits both rate and yield. However, very high pressures require expensive, robust equipment and pose greater safety risks, leading to a cost-benefit analysis in selecting the optimal operating pressure.
Higher: Catalysts and Economic Viability
Catalysts increase the rate of both the forward and reverse reactions equally, meaning they do not affect the position of equilibrium or the overall percentage yield. Their benefit lies in allowing reactions to proceed at a faster rate under less extreme conditions (e.g., lower temperatures or pressures), which reduces energy consumption and equipment costs, making the process more economically viable. While they don't solve the equilibrium trade-off directly, they significantly improve the overall efficiency and profitability of many industrial processes by speeding up production.
Understanding Corrosion and its Conditions Revision Notes
What is Corrosion?
Corrosion is the destructive chemical reaction of a metal with its surroundings. It typically involves the metal losing electrons to form a more stable compound, often an oxide. This process leads to the gradual deterioration of the metal's properties and structure. A widely known example is rusting, which is the specific term for the corrosion of iron and steel. Rusting damages metallic objects, reducing their strength and lifespan, and making corrosion a significant economic concern.
Conditions for Rusting (Corrosion of Iron)
For iron or steel to rust, two specific substances must be present simultaneously: oxygen and water. Oxygen is typically available from the air, and water can be in the form of liquid water, water vapour, or even dissolved in other substances. If either oxygen or water is absent, rusting will not occur. For example, iron objects stored in a vacuum or submerged in deoxygenated water will not rust. This highlights the importance of controlling these environmental factors to prevent corrosion.
The Chemistry of Rusting
Rusting is an electrochemical process. Iron reacts with oxygen in the presence of water to form hydrated iron(III) oxide, which is commonly known as rust. The overall chemical equation is often simplified, but it represents the iron atoms losing electrons (oxidation) and reacting with oxygen and water molecules. The formula for rust is typically Fe₂O₃·nH₂O, indicating that it is iron(III) oxide with varying amounts of associated water molecules. This specific product is a reddish-brown, flaky substance.
Factors Affecting the Rate of Corrosion
The rate at which corrosion occurs can be influenced by several environmental factors. The presence of impurities, such as dissolved salts (e.g., in seawater), significantly accelerates rusting. Salt water acts as an electrolyte, which increases the electrical conductivity of the water and speeds up the electrochemical reactions involved in corrosion. Similarly, acidic conditions, such as those caused by acid rain, can also increase the rate of corrosion for many metals by providing additional hydrogen ions that react with the metal.
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