Understandings

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.
Energy density = energy released from fuel/ volume of fuel consumed.
Specific energy = energy released from fuel/ mass of fuel consumed.
The efficiency of an energy transfer = useful output energy/total input energy x 100%.

Applications and skills

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.

Understandings

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.

Applications and skills

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.

Guidance:

The cost of production and availability (reserves) of fossil fuels and their impact on the environment should be considered.

Understandings

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:

235U92 + 1n0 ? 236U92 ? X + Y + neutrons.

The critical mass is the mass of fuel needed for the reaction to be self-sustaining.
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.

Applications and skills

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.

Guidance:

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.
The equations, are given in section 1 of the data booklet.

Understandings

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.
Trans-esterification 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.

Applications and skills

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.

Guidance:

Only a conjugated system with alternating double bonds needs to be covered.

Understandings

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.

Applications and skills

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.

Guidance:

Greenhouse gases to be considered are CH₄, H₂O and CO₂.

Understandings

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.

Applications and skills

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 (?G/?H) of a fuel cell.
Explanation of the workings of rechargeable and fuel cells including diagrams and relevant half-equations.

Guidance:

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 leadacid storage battery, the nickel cadmium (NiCad) battery and the lithiumion battery should be considered.
Students should be familiar with the anode and cathode half-equations and uses of the different cells.

Understandings

Nuclear fusion:
The mass defect (?m) 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 massenergy 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 (Grahams 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.

Applications and skills

Nuclear fusion:
Calculation of the mass defect and binding energy of a nucleus.
Application of the Einstein massenergy equivalence relationship, E=mc², to
determine the energy produced in a fusion reaction.
Nuclear fission:
Application of the Einstein massenergy equivalence relationship to determine the energy produced in a fission reaction.
Discussion of the different properties of UO2 and UF6 in terms of bonding and
structure.
Solution of problems involving radioactive half-life.
Explanation of the relationship between Grahams law of effusion and the kinetic theory.
Solution of problems on the relative rate of effusion using Grahams law.

Guidance:

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 the possibility that nuclear fuels may be used in nuclear weapons.
Grahams law of effusion is given in the data booklet in section 1.
Decay relationships are given in the data booklet in section 1.
A binding energy curve is given in the data booklet in section 36.

Understandings

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 p-type
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.

Applications and skills

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 photovoltaic cell.

Guidance:

The relative conductivity of metals and semiconductors should be related to ionization energies.
Only a simple treatment of the operation of the cells is needed. In p-type semiconductors, electron holes in the crystal are created by introducing a small percentage of a group 3 element. In n-type semiconductors inclusion of a group 5 element provides extra electrons.
In a photovoltaic cell the light is absorbed and the charges separated in the silicon semiconductor. The processes of absorption and charge separation are
separated in a dye-sensitized solar cell.
Specific redox and electrode reactions in the newer Gratzel DSSC should be covered. An example is the reduction of I₂/I₃^- ions to I^-.