• A useful energy source releases energy at a reasonable rate and produces minimal pollution.
• The quality of energy is degraded as heat is transferred to the surroundings. Energy and materials go from a concentrated into a dispersed form. The quantity of the energy available for doing work decreases.
• Renewable energy sources are naturally replenished. Non-renewable energy
sources are finite.
• Discussion of the use of different sources of renewable and non-renewable energy.
• Determination of the energy density and specific energy of a fuel from the enthalpies of combustion, densities and the molar mass of fuel.
• Discussion of how the choice of fuel is influenced by its energy density or specific energy.
• Determination of the efficiency of an energy transfer process from appropriate data.
• Discussion of the advantages and disadvantages of the different energy sources in C.2 through to C.8.
• Fossil fuels were formed by the reduction of biological compounds that contain carbon, hydrogen, nitrogen, sulfur and oxygen.
• Petroleum is a complex mixture of hydrocarbons that can be split into different component parts called fractions by fractional distillation.
• Crude oil needs to be refined before use. The different fractions are separated by a physical process in fractional distillation.
• The tendency of a fuel to auto-ignite, which leads to “knocking” in a car engine, is related to molecular structure and measured by the octane number.
• The performance of hydrocarbons as fuels is improved by the cracking and catalytic reforming reactions.
• Coal gasification and liquefaction are chemical processes that convert coal to gaseous and liquid hydrocarbons.
• A carbon footprint is the total amount of greenhouse gases produced during human activities. It is generally expressed in equivalent tons of carbon dioxide.
• Discussion of the effect of chain length and chain branching on the octane number.
• Discussion of the reforming and cracking reactions of hydrocarbons and explanation how these processes improve the octane number.
• Deduction of equations for cracking and reforming reactions, coal gasification and liquefaction.
• Discussion of the advantages and disadvantages of the different fossil fuels.
• Identification of the various fractions of petroleum, their relative volatility and their uses.
• Calculations of the carbon dioxide added to the atmosphere, when different fuels burn and determination of carbon footprints for different activities.
• The cost of production and availability (reserves) of fossil fuels and their impact on the environment should be considered.
Nuclear fusion
• Light nuclei can undergo fusion reactions as this increases the binding energy per nucleon.
• Fusion reactions are a promising energy source as the fuel is inexpensive and abundant, and no radioactive waste is produced.
• Absorption spectra are used to analyse the composition of stars.
Nuclear fission
• Heavy nuclei can undergo fission reactions as this increases the binding energy
per nucleon.
• 235U undergoes a fission chain reaction:
• The critical mass is the mass of fuel needed for the reaction to be selfsustaining.
• 239Pu, used as a fuel in “breeder reactors”, is produced from 238U by neutron capture.
• Radioactive waste may contain isotopes with long and short half-lives.
• Half-life is the time it takes for half the number of atoms to decay.
Nuclear fusion
• Construction of nuclear equations for fusion reactions.
• Explanation of fusion reactions in terms of binding energy per nucleon.
• Explanation of the atomic absorption spectra of hydrogen and helium, including
the relationships between the lines and electron transitions.
Nuclear fission
• Deduction of nuclear equations for fission reactions.
• Explanation of fission reactions in terms of binding energy per nucleon.
• Discussion of the storage and disposal of nuclear waste.
• Solution of radioactive decay problems involving integral numbers of half-lives.
• Students are not expected to recall specific fission reactions.
• The workings of a nuclear power plant are not required.
• Safety and risk issues include: health, problems associated with nuclear waste and core meltdown, and the possibility that nuclear fuels may be used in nuclear weapons.
• Light can be absorbed by chlorophyll and other pigments with a conjugated electronic structure.
• Photosynthesis converts light energy into chemical energy:
6CO2 + 6H2O --> C6H12O6 + 6O2
• Fermentation of glucose produces ethanol which can be used as a biofuel:
C6H12O6 --> 2C2H5OH + 2CO2
• Energy content of vegetable oils is similar to that of diesel fuel but they are not used in internal combustion engines as they are too viscous.
• Transesterification between an ester and an alcohol with a strong acid or base catalyst produces a different ester:
RCOOR1 + R2OH --> RCOOR2 + R1OH
• In the transesterification process, involving a reaction with an alcohol in the presence of a strong acid or base, the triglyceride vegetable oils are converted to a mixture mainly comprising of alkyl esters and glycerol, but with some fatty acids.
• Transesterification with ethanol or methanol produces oils with lower viscosity that can be used in diesel engines.
• Identification of features of the molecules that allow them to absorb visible light.
• Explanation of the reduced viscosity of esters produced with methanol and ethanol.
• Evaluation of the advantages and disadvantages of the use of biofuels.
• Deduction of equations for transesterification reactions.
• Only a conjugated system with alternating double bonds needs to be covered.
• Greenhouse gases allow the passage of incoming solar short wavelength radiation but absorb the longer wavelength radiation from the Earth. Some of the absorbed radiation is re-radiated back to Earth.
• There is a heterogeneous equilibrium between concentration of atmospheric carbon dioxide and aqueous carbon dioxide in the oceans.
• Greenhouse gases absorb IR radiation as there is a change in dipole moment as the bonds in the molecule stretch and bend.
• Particulates such as smoke and dust cause global dimming as they reflect sunlight, as do clouds.
• Explanation of the molecular mechanisms by which greenhouse gases absorb
infrared radiation.
• Discussion of the evidence for the relationship between the increased concentration of gases and global warming.
• Discussion of the sources, relative abundance and effects of different greenhouse gases.
• Discussion of the different approaches to the control of carbon dioxide emissions.
• Discussion of pH changes in the ocean due to increased concentration of carbon dioxide in the atmosphere.
• Greenhouse gases to be considered are CH4, H2O and CO2.
• An electrochemical cell has internal resistance due to the finite time it takes for ions to diffuse. The maximum current of a cell is limited by its internal resistance.
• The voltage of a battery depends primarily on the nature of the materials used while the total work that can be obtained from it depends on their quantity.
• In a primary cell the electrochemical reaction is not reversible. Rechargeable cells involve redox reactions that can be reversed using electricity.
• A fuel cell can be used to convert chemical energy, contained in a fuel that is consumed, directly to electrical energy.
• Microbial fuel cells (MFCs) are a possible sustainable energy source using different carbohydrates or substrates present in waste waters as the fuel.
• The Nernst equation can be used to calculate the potential of a half-cell in an electrochemical cell, under non-standard conditions.
• The electrodes in a concentration cell are the same but the concentration of the electrolyte solutions at the cathode and anode are different.
• Distinction between fuel cells and primary cells.
• Deduction of half equations for the electrode reactions in a fuel cell.
• Comparison between fuel cells and rechargeable batteries.
• Discussion of the advantages of different types of cells in terms of size, mass and voltage.
• Solution of problems using the Nernst equation.
• Calculation of the thermodynamic efficiency (ΔG/ΔH) of a fuel cell.
• Explanation of the workings of rechargeable and fuel cells including diagrams and relevant half-equations.
• A battery should be considered as a portable electrochemical source made up of one or more voltaic (galvanic) cells connected in series.
• The Nernst equation is given in the data booklet in section 1.
• Hydrogen and methanol should be considered as fuels for fuel cells. The operation of the cells under acid and alkaline conditions should be considered. Students should be familiar with proton-exchange membrane (PEM) fuel cells.
• The Geobacter species of bacteria, for example, can be used in some cells to oxidize the ethanoate ions (CH3COO-) under anaerobic conditions.
• The lead–acid storage battery, the nickel–cadmium (NiCad) battery and the lithium–ion battery should be considered.
• Students should be familiar with the anode and cathode half-equations and uses of the different cells.
Nuclear fusion:
• The mass defect (Δm) is the difference between the mass of the nucleus and the sum of the masses of its individual nucleons.
• The nuclear binding energy (ΔE) is the energy required to separate a nucleus into protons and neutrons.
Nuclear fission:
• The energy produced in a fission reaction can be calculated from the mass difference between the products and reactants using the Einstein mass–energy equivalence relationship
• The different isotopes of uranium in uranium hexafluoride can be separated, using diffusion or centrifugation causing fuel enrichment.
• The effusion rate of a gas is inversely proportional to the square root of the molar mass (Graham’s Law).
• Radioactive decay is kinetically a first order process with the half-life related to the decay constant by the equation
• The dangers of nuclear energy are due to the ionizing nature of the radiation it produces which leads to the production of oxygen free radicals such as superoxide (O2-), and hydroxyl (HO·). These free radicals can initiate chain reactions that can damage DNA and enzymes in living cells.
• Molecules with longer conjugated systems absorb light of longer wavelength.
• The electrical conductivity of a semiconductor increases with an increase in temperature whereas the conductivity of metals decreases.
• The conductivity of silicon can be increased by doping to produce n-type and ptype semiconductors.
• Solar energy can be converted to electricity in a photovoltaic cell.
• DSSCs imitate the way in which plants harness solar energy. Electrons are "injected" from an excited molecule directly into the TiO2 semiconductor.
• The use of nanoparticles coated with light-absorbing dye increases the effective surface area and allows more light over a wider range of the visible spectrum to be absorbed.
• Relation between the degree of conjugation in the molecular structure and the wavelength of the light absorbed.
• Explanation of the operation of the photovoltaic and dye-sensitized solar cell.
• Explanation of how nanoparticles increase the efficiency of DSSCs.
• Discussion of the advantages of the DSSC compared to the silicon-based