Option A - Material Science
A.1 Materials science introduction
Essential idea: Materials science involves understanding the properties of a material, and then applying those properties to desired structures.
Materials are classified based on their uses, properties, or bonding and structure.
Applications and skills
Use of bond triangle diagrams for binary compounds from electronegativity data.
Evaluation of various ways of classifying materials.
Relating physical characteristics (melting point, permeability, conductivity, elasticity, brittleness) of a material to its bonding and structures (packing arrangements, electron mobility, ability of atoms to slide relative to one another).
Permeability to moisture should be considered with respect to bonding and simple packing arrangements.
Consider properties of metals, polymers and ceramics in terms of metallic, covalent, and ionic bonding.
See section 29 of the data booklet for a triangular bonding diagram.
Bond triangles or van Arkel–Ketelaar triangles (named after Anton Eduard van Arkel and J. A. A. Ketelaar) are triangles used to show different compounds in varying degrees of ionic, metallic and covalent bonding. The bond triangle shows that ionic, metallic and covalent bonds are not just particular bonds of a specific type. Rather, bond types are interconnected and different compounds have varying degrees of different bonding character (for example, covalent bonds with significant ionic character are called polar covalent bonds).
Different compounds can be placed around the triangle. On the right side (from ionic to covalent) should be compounds with varying difference in electronegativity, in the covalent corner compounds with equal electronegativity such as Cl2 (chlorine), in the ionic corner compounds with large electronegativity difference such as NaCl (table salt). The bottom side (from metallic to covalent) is for compounds with varying degree of directionality in the bond. At one extreme is metallic bonds with delocalized bonding and the other are covalent bonds in which the orbitals overlap in a particular direction. The left side (from ionic to metallic) is for delocalized bonds with varying electronegativity difference.
Three species at the vertices of the triangle are: caesium (metallic), fluorine (covalent) and caesium fluoride (ionic).
A.2 Metals and inductively coupled plasma (ICP) spectroscopy
Essential idea: Metals can be extracted from their ores and alloyed for desired characteristics. ICP-MS/OES Spectroscopy ionizes metals and uses mass and emission spectra for analysis.
Reduction by coke (carbon), a more reactive metal, or electrolysis are means of obtaining some metals from their ores.
The relationship between charge and the number of moles of electrons is given by Faraday’s constant, F.
Alloys are homogeneous mixtures of metals with other metals or non-metals.
Diamagnetic and paramagnetic compounds differ in electron spin pairing and their behaviour in magnetic fields.
Trace amounts of metals can be identified and quantified by ionizing them with argon gas plasma in Inductively Coupled Plasma (ICP) Spectroscopy using Mass Spectroscopy ICP-MS and Optical Emission Spectroscopy ICP-OES.
Applications and skills
Deduction of redox equations for the reduction of metals.
Relating the method of extraction to the position of a metal on the activity series.
Explanation of the production of aluminium by the electrolysis of alumina in molten cryolite
Explanation of how alloying alters properties of metals.
Solving stoichiometric problems using Faraday’s constant based on mass deposits in electrolysis.
Discussion of paramagnetism and diamagnetism in relation to electron structure of metals.
Explanation of the plasma state and its production in ICP- MS/OES.
Identify metals and abundances from simple data and calibration curves provided from ICP-MS and ICP-OES.
Explanation of the separation and quantification of metallic ions by MS and OES.
Uses of ICP-MS and ICP-OES
Faraday’s constant is given in the data booklet in section 2.
Details of operating parts of ICP-MS and ICP-OES instruments will not be assessed.
Only analysis of metals should be covered.
The importance of calibration should be covered.
Extraction of metals (all common ones)
Trace analysis - ICP (optical emission spectroscopy)
In the Bayer process, bauxite is digested by washing with a hot solution of sodium hydroxide, NaOH, at 175 °C, under pressure. This converts the aluminium oxide in the ore to soluble sodium aluminate, 2NaAlO2, according to the chemical equation:
Al2O3 + 2NaOH → 2NaAlO2 + H2O
This treatment also dissolves silica, but the other components of bauxite do not dissolve. Sometimes lime is added here, to precipitate the silica as calcium silicate. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap, and a flocculant such as starch, to get rid of the fine particles. The mixture of solid impurities is called red mud. Originally, the alkaline solution was cooled and treated by bubbling carbon dioxide into it, through which aluminium hydroxide precipitates:
2NaAlO2 + CO2 → 2 Al(OH)3 + Na2CO3 + H2O
But later, this gave way to seeding the supersaturated solution with high-purity aluminum hydroxide (Al(OH)3) crystal, which eliminated the need for cooling the liquid and was more economically feasible:
2H2O + NaAlO2 → Al(OH)3 + NaOH
Then, when heated to 980°C (calcined), the aluminium hydroxide decomposes to aluminium oxide, giving off water vapor in the process:
2 Al(OH)3 Al2O3 + 3H2O
The left-over NaOH solution is then recycled. This, however, allows gallium and vanadium impurities to build up in the liquors, so these are extracted.
For bauxites having more than 10% silica, Bayer process becomes infeasible due to insoluble sodium aluminum silicate being formed, which reduces yield, and another process must be chosen.
A large amount of the aluminium oxide so produced is then subsequently smelted in the Hall–Héroult process in order to produce aluminium.
Aluminium extraction: https://youtu.be/WaSwimvCGA8
Aluminium extraction: https://youtu.be/jOKMkaqPZvc
Aluminium extraction: https://youtu.be/OvPWvg8XJvE
Glass and Other Ceramics
One of the characteristic properties of a substance is its viscosity, which is a measure of its resistance to flow. Motor oils are more viscous than gasoline, for example, and the maple syrup used on pancakes is more viscous than the vegetable oils used in salad dressings. Viscosity depends on any factor that can influence the ease with which molecules slip past each other. Liquids tend to become more viscous as the molecules become larger, or as the intermolecular forces become stronger. They also become more viscous when cooled.
Imagine what would happen if you cooled a liquid until it became so viscous that it was rigid and yet it lacked any of the long-range order that characterizes the solids discussed in this chapter. You would have something known as a glass. Glasses have three characteristics that make them more closely resemble "frozen liquids" than crystalline solids. First, and foremost, there is no long-range order. Second, there are numerous empty sites or vacancies. Finally, glasses don't contain planes of atoms.
The simplest way to understand the difference between a glass and a crystalline solid is to look at the structure of glassy metals at the atomic scale. By rapidly condensing metal atoms from the gas phase, or by rapidly quenching a molten metal, it is possible to produce glassy metals that have the structure shown in the figure below
The structure of a glassy metal on the atomic scale.
The amorphous structure of glass makes it brittle. Because glass doesn't contain planes of atoms that can slip past each other, there is no way to relieve stress. Excessive stress therefore forms a crack that starts at a point where there is a surface flaw. Particles on the surface of the crack become separated. The stress that formed the crack is now borne by particles that have fewer neighbors over which the stress can be distributed. As the crack grows, the intensity of the stress at its tip increases. This allows more bonds to break, and the crack widens until the glass breaks. Thus, if you want to cut a piece of glass, start by scoring the glass with a file to produce a scratch along which it will break when stressed.
Glass has been made for at least 6000 years, since the Egyptians coated figurines made from sand (SiO2) with sediment from the Nile river, heated these objects until the coating was molten, and then let them cool. Calcium oxide or "lime" (CaO) and sodium oxide or "soda" (Na2O) from the sediment flowed into the sand to form a glass on the surface of the figurines. Trace amounts of copper oxide (CuO) in the sediment gave rise to a random distribution of Cu2+ ions in the glass that produced a characteristic blue color.
Sand is still the most common ingredient from which glass is made. (More than 90% of the sand consumed each year is used by the glass industry.) Sand consists of an irregular network of silicon atoms held together by Si-O-Si bonds. If the network was perfectly regular, each silicon atom would be surrounded by four oxygen atoms arranged toward the corner of a tetrahedron. Because each oxygen atom in this network is shared by two silicon atoms, the empirical formula of this solid would be SiO2 and the material would have the structure of quartz. In sand, however, some of the Si-O-Si bridges are broken, in a random fashion.
Modifiers (or fluxes) such as Na2O and CaO are added to the sand to alter the network structure by replacing Si-O-Si bonds with Si-O- Na+ or Si-O- Ca2+ bonds. This separates the SiO2 tetrahedral from each other, which makes the mixture more fluid and therefore more likely to form a glass after it has been melted and then cooled. These so-called "soda-lime" glasses account for 90% of the glass produced.
Al2O3 is added to some glasses to increase their durability; MgO is added to slow down the rate at which the glass crystallizes. Replacing Na2O with B2O3 produces a borosilicate glass that expands less on heating. Adding PbO produces lead glasses that are ideally suited for high-quality optical glass.
The most common way of preparing a glass is to heat the mixture of sand and modifiers until it melts, and then cool it quickly so that it solidifies to produce a glass. If the cooling is rapid enough, the particles in the liquid state can't return to the original crystalline arrangement of the starting materials. Instead, they occupy randomly arranged lattice sites in which no planes of atoms can be identified. The result is an amorphous (literally: "without shape") material.
Essential idea: Catalysts work by providing an alternate reaction pathway for the reaction. Catalysts always increase the rate of the reaction and are left unchanged at the end of the reaction.
Reactants adsorb onto heterogeneous catalysts at active sites and the products desorb.
Homogeneous catalysts chemically combine with the reactants to form a temporary activated complex or a reaction intermediate.
Transition metal catalytic properties depend on the adsorption/absorption properties of the metal and the variable oxidation states.
Zeolites act as selective catalysts because of their cage structure.
Catalytic particles are nearly always nanoparticles that have large surface areas per unit mass.
Applications and skills
Explanation of factors involved in choosing a catalyst for a process.
Description of how metals work as heterogeneous catalysts.
Description of the benefits of nanocatalysts in industry
Consider catalytic properties such as selectivity for only the desired product, efficiency, ability to work in mild/severe conditions, environmental impact and impurities.
The use of carbon nanocatalysts should be covered.
A.4 Liquid crystals
Essential idea: Liquid crystals are fluids that have physical properties which are dependent on molecular orientation relative to some fixed axis in the material
Liquid crystals are fluids that have physical properties (electrical, optical and elasticity) that are dependent on molecular orientation to some fixed axis in the material.
Thermotropic liquid-crystal materials are pure substances that show liquid-crystal behaviour over a temperature range.
Lyotropic liquid crystals are solutions that show the liquid-crystal state over a (certain) range of concentrations.
Nematic liquid crystal phase is characterized by rod shaped molecules which are randomly distributed but on average align in the same direction.
Applications and skills
Discussion of the properties needed for a substance to be used in liquid-crystal displays (LCD).
Explanation of liquid-crystal behaviour on a molecular level
Properties needed for liquid crystals include: chemically stable, a phase which is stable over a suitable temperature range, polar so they can change orientation when an electric field is applied, and rapid switching speed.
Soap and water is an example of lyotropic liquid crystals and the biphenyl nitriles are examples of thermotropic liquid crystals.
Liquid crystal behaviour should be limited to the biphenyl nitriles.
Smectics and other liquid crystals types need not be discussed.
Essential idea: Polymers are made up of repeating monomer units which can be manipulated in various ways to give structures with desired properties
Thermoplastics soften when heated and harden when cooled.
A thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into a hardened thermoset by curing.
Elastomers are flexible and can be deformed under force but will return to nearly their original shape once the stress is released.
High density polyethene (HDPE) has no branching allowing chains to be packed together.
Low density polyethene (LDPE) has some branching and is more flexible.
Plasticizers added to a polymer increase the flexibility by weakening the intermolecular forces between the polymer chains.
Atom economy is a measure of efficiency applied in green chemistry.
Isotactic addition polymers have substituents on the same side.
Atactic addition polymers have the substituents randomly placed.
Applications and skills
Description of the use of plasticizers in polyvinyl chloride and volatile hydrocarbons in the formation of expanded polystyrene.
Solving problems and evaluating atom economy in synthesis reactions.
Description of how the properties of polymers depend on their structural features.
Description of ways of modifying the properties of polymers, including LDPE and HDPE.
Deduction of structures of polymers formed from polymerizing 2-methylpropene
The equation for percent atom economy is provided in the data booklet in section 1.
Consider only polystyrene foams as examples of polymer property manipulation.
Essential idea: Chemical techniques position atoms in molecules using chemical reactions whilst physical techniques allow atoms/molecules to be manipulated and positioned to specific requirements
Molecular self-assembly is the bottom-up assembly of nanoparticles and can occur by selectively attaching molecules to specific surfaces. Self-assembly can also occur spontaneously in solution.
Possible methods of producing nanotubes are arc discharge, chemical vapour deposition (CVD) and high pressure carbon monoxide (HIPCO).
Arc discharge involves either vaporizing the surface of one of the carbon electrodes, or discharging an arc through metal electrodes submersed in a hydrocarbon solvent, which forms a small rod-shaped deposit on the anode.
Applications and skills
Distinguishing between physical and chemical techniques in manipulating atoms to form molecules.
Description of the structure and properties of carbon nanotubes.
Explanation of why an inert gas, and not oxygen, is necessary for CVD preparation of carbon nanotubes.
Explanation of the production of carbon from hydrocarbon solvents in arc discharge by oxidation at the anode.
Deduction of equations for the production of carbon atoms from HIPCO.
Discussion of some implications and applications of nanotechnology.
Explanation of why nanotubes are strong and good conductors of electricity.
Possible implications of nanotechnology include uncertainty as to toxicity levels on a nanoscale, unknown health risks with new materials, concern that human defence systems are not effective against particles on the nanoscale, responsibilities of the industries and governments involved in this research.
Conductivity of graphene and fullerenes can be explained in terms of delocalization of electrons. An explanation based on hybridization is not required.
A.7 Environmental impact—plastics
Essential idea: Although materials science generates many useful new products there are challenges associated with recycling of and high levels of toxicity of some of these materials.
Plastics do not degrade easily because of their strong covalent bonds.
Burning of polyvinyl chloride releases dioxins, HCl gas and incomplete hydrocarbon combustion products.
Dioxins contain unsaturated six-member heterocyclic rings with two oxygen atoms, usually in positions 1 and 4.
Chlorinated dioxins are hormone disrupting, leading to cellular and genetic damage.
Plastics require more processing to be recycled than other materials.
Plastics are recycled based on different resin types.
Applications and skills
Deduction of the equation for any given combustion reaction.
Discussion of why the recycling of polymers is an energy intensive process.
Discussion of the environmental impact of the use of plastics.
Comparison of the structures of polychlorinated biphenyls (PCBs) and dioxins.
Discussion of the health concerns of using volatile plasticizers in polymer production.
Distinguish possible Resin Identification Codes (RICs) of plastics from an IR spectrum.
Dioxins do not decompose in the environment and can be passed on in the food chain.
Consider polychlorinated dibenzodioxins (PCDD) and PCBs as examples of carcinogenic chlorinated dioxins or dioxin-like substances.
Consider phthalate esters as examples of plasticizers.
House fires can release many toxins due to plastics (shower curtains, etc). Low smoke zero halogen cabling is often used in wiring to prevent these hazards.
Resin Identification Codes (RICs) are in the data booklet in section 30.
Structures of various materials molecules are in the data booklet in section 31.
A.8 Superconducting metals and X-ray crystallography - AHL
Essential idea: Superconductivity is zero electrical resistance and expulsion of magnetic fields. X-ray crystallography can be used to analyse structures.
Superconductors are materials that offer no resistance to electric currents below a critical temperature.
The Meissner effect is the ability of a superconductor to create a mirror image magnetic field of an external field, thus expelling it.
Resistance in metallic conductors is caused by collisions between electrons and positive ions of the lattice.
The Bardeen–Cooper–Schrieffer (BCS) theory explains that below the critical temperature electrons in superconductors form Cooper pairs which move freely through the superconductor.
Type 1 superconductors have sharp transitions to superconductivity whereas Type 2 superconductors have more gradual transitions.
X-ray diffraction can be used to analyse structures of metallic and ionic compounds.
Crystal lattices contain simple repeating unit cells.
Atoms on faces and edges of unit cells are shared.
The number of nearest neighbours of an atom/ion is its coordination number.
Applications and skills
Analysis of resistance versus temperature data for Type 1 and Type 2 superconductors.
Explanation of superconductivity in terms of Cooper pairs moving through a positive ion lattice
Deduction or construction of unit cell structures from crystal structure information.
Application of the Bragg equation, nl= 2dsin0 in metallic structures.
Determination of the density of a pure metal from its atomic radii and crystal packing structure.
Only a simple explanation of BCS theory with Cooper pairs is required. At low temperatures the positive ions in the lattice are distorted slightly by a passing electron. A second electron is attracted to this slight positive deformation and a coupling of these two electrons occurs.
Operating principles of X-ray crystallography are not required.
Only pure metals with simple cubic cells, body centred cubic cells (BCC) and face centred cubic cells (FCC) should be covered.
Perovskite crystalline structures of many superconductors can be analysed by X-ray crystallography but these will not be assessed.
Bragg's equation will only be applied to simple cubic structures.
A.9 Condensation polymers
Essential idea: Condensation polymers are formed by the loss of small molecules as functional groups from monomers join.
Condensation polymers require two functional groups on each monomer.
NH3, HCl and H2O are possible products of condensation reactions.
Kevlar® is a polyamide with a strong and ordered structure. The hydrogen bonds between O and N can be broken with the use of concentrated sulfuric acid.
Applications and skills
Distinguishing between addition and condensation polymers.
Completion and descriptions of equations to show how condensation polymers are formed.
Deduction of the structures of polyamides and polyesters from their respective monomers.
Explanation of Kevlar®’s strength and its solubility in concentrated sulfuric acid
Consider green chemistry polymers.
A.10 Environmental impact—heavy metals
Essential idea: Toxicity and carcinogenic properties of heavy metals are the result of their ability to form coordinated compounds, have various oxidation states and act as catalysts in the human body.
Toxic doses of transition metals can disturb the normal oxidation/reduction balance in cells through various mechanisms.
Some methods of removing heavy metals are precipitation, adsorption, and chelation.
Polydentate ligands form more stable complexes than similar monodentate ligands due to the chelate effect, which can be explained by considering entropy changes.
Applications and skills
Explanation of how chelating substances can be used to remove heavy metals
Deduction of the number of coordinate bonds a ligand can form with a central metal ion.
Calculations involving Ksp as an application of removing metals in solution.
Compare and contrast the Fenton and Haber–Weiss reaction mechanism
Ethane-1,2-diamine acts as a bidentate ligand and EDTA4- acts as hexadentate ligand.
The Haber–Weiss reaction generates free radicals naturally in biological processes. Transition metals can catalyse the reaction with the iron-catalysed (Fenton) reaction being the mechanism for generating reactive hydroxyl radicals.
Ksp values are in the data booklet in section 32.