C. Energy

C. Energy

Core

C.1 Energy sources

Nature of science:

• Use theories to explain natural phenomena - energy changes in the world around us result from potential and kinetic energy changes at the molecular level.

• Energy has both quantity and quality.

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.

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.

• 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.

C.2 Fossil fuels

Nature of science:

• Scientific community and collaboration - the use of fossil fuels has had a key role in the development of science and technology.

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.

C.3 Nuclear fusion and fission

Nature of science:

• Assessing the ethics of scientific research - widespread use of nuclear fission for energy production would lead to a reduction in greenhouse gas emissions. Nuclear fission is the process taking place in the atomic bomb and nuclear fusion that in the hydrogen bomb.

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

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

• h4~: 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.

C.4 Solar energy

Nature of science:

• Public understanding - harnessing the sun’s energy is a current area of research and challenges still remain. However, consumers and energy companies are being encouraged to make use of solar energy as an alternative energy source.

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.

• 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.

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.

C.5 Environmental impact - global warming

Nature of science:

• Transdisciplinary - the study of global warming encompasses a broad range of concepts and ideas and is transdisciplinary.

• Collaboration and significance of science explanations to the public - reports of the Intergovernmental Panel on Climate Change (IPCC).

• Correlation and cause and understanding of science - CO₂ levels and Earth average temperature show clear correlation but wide variations in the surface temperature of the Earth have occurred frequently in the past.

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.

• Discussion of pH changes in the ocean due to increased concentration of carbon dioxide in the atmosphere.

Additional higher level

C.6 Electrochemistry, rechargeable batteries and fuel cells

Nature of science:

• Environmental problems - redox reactions can be used as a source of electricity but disposal of batteries has environmental consequences.

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, E=E0−(RTnF)lnQ 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.

C.7 Nuclear fusion and nuclear fission

Nature of science:

• Trends and discrepancies - our understanding of nuclear processes came from both theoretical and experimental advances. Intermolecular forces in UF₆ are anomalous and do not follow the normal trends.

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 (△E) 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 E=mc2. 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 λ=ln2t12. 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 mass–energy equivalence relationship, E=mc2, to determine the energy produced in a fusion reaction.

• Nuclear fission: Application of the Einstein mass–energy 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 Graham’s law of effusion and the kinetic theory. Solution of problems on the relative rate of effusion using Graham’s law.

C.8 Photovoltaic cells and dye-sensitized solar cells (DSSC)

Nature of science:

• Transdisciplinary - a dye-sensitized solar cell, whose operation mimics photosynthesis and makes use of TiO₂ nanoparticles, illustrates the transdisciplinary nature of science and the link between chemistry and biology.

• Funding - the level of funding and the source of the funding is crucial in decisions regarding the type of research to be conducted. The first voltaic cells were produced by NASA for space probes and were only later used on Earth.

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.