Paper IV — Chemistry
32 QUESTIONS
Paper IV covers chemistry as the science of bonding, structure, reactivity and change. Its questions examine the bond itself against the electron density, oxidation state, electronegativity and its disagreeing scales, aromaticity, chirality, and the hydrogen bond; the thermodynamics and kinetics of reaction, including the equilibrium constant and free energy, kinetic against thermodynamic control, catalysis, the Arrhenius form, transition states and potential energy surfaces, and Marcus theory with its inverted region; and the quantum-chemical basis of the subject, from Born–Oppenheimer to electron correlation and the principled limits of density functional theory. Biological and material chemistry are represented by enzymes, protein folding, the hydrophobic effect, nitrogen fixation and the metal–semiconductor–insulator distinction, and two later questions treat the oxygen-isotope palaeothermometer and the silicate weathering thermostat.
What is examined is the chemist’s particular habit of mind: the willingness to use a concept that is known to be approximate, together with the ability to say exactly where the approximation fails. Several questions are put in that form directly — whether a bond is a thing or a model, whether a molecular orbital is observable, in what sense a transition state exists, whether the periodic table follows from quantum mechanics alone. A candidate is expected to move fluently between the molecular and the thermodynamic account of the same phenomenon, to know which quantities are measured and which are inferred, and to explain a rule’s success without mistaking success for derivation. The paper ends by asking which single unmeasured quantity would most advance chemical prediction.
Conduct of the response
Answer in the idiom of the college: provocations invite a disciplined essay; technical problems require correct, quantitative reasoning; interdisciplinary questions require two fields to be brought together. A response is an argument, not a survey; where the question admits a position, one is to be taken at the outset and defended. It must be exactly as technical as the question requires, and no more, and its course must remain followable by the educated layman. It may not exceed three thousand words, exclusive of equations, figures, tables, and references; the limit is a ceiling, not a target, and of two answers of equal force the shorter is judged the better. The full statement of the response required is given on the Responses page.
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1.
Is a chemical bond a thing or a model? Answer with reference to the electron density.
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2.
Explain why the concept of oxidation state is useful despite being formally arbitrary.
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3.
Derive the relationship between equilibrium constant and Gibbs free energy, and say what assumptions it hides.
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4.
Why is water anomalous? Give the molecular account.
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5.
What does it mean for a reaction to be "under kinetic" versus "thermodynamic" control?
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6.
Explain aromaticity without invoking the word "resonance."
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7.
Is the periodic table's structure a consequence of quantum mechanics alone, or does chemistry add something?
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8.
What is the physical basis of electronegativity, and why do scales disagree?
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9.
Account for the catalytic power of enzymes in thermodynamic and kinetic terms.
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10.
Why is the Born–Oppenheimer approximation so good, and when does it fail?
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11.
Explain chirality's consequences for reactivity and for biology.
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12.
What limits the accuracy of density functional theory in principle?
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13.
Give a molecular account of why entropy can drive assembly (e.g., the hydrophobic effect).
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14.
What is a transition state, and in what sense does it exist?
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15.
Explain the origin of colour in transition-metal complexes.
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16.
Why can a catalyst change a rate but not an equilibrium?
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17.
What does "electron correlation" mean, and why is it computationally expensive?
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18.
Explain how the Marcus theory of electron transfer predicts an inverted region.
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19.
Is the concept of a molecular orbital observable?
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20.
Account for the strength and directionality of the hydrogen bond.
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21.
Why is nitrogen fixation so difficult, industrially and biologically?
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22.
What determines whether a solid is a metal, semiconductor, or insulator, chemically?
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23.
Explain the thermodynamic driving force for protein folding.
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24.
What is autocatalysis, and why is it relevant to the origin of life?
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25.
Give the basis of the Woodward–Hoffmann rules.
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26.
Why do reaction rates so often follow the Arrhenius form, and when do they not?
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27.
What is measured by a standard electrode potential, and against what?
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28.
Explain the concept of a potential energy surface and its dimensionality.
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29.
Is supramolecular chemistry a distinct science or applied physics?
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30.
Which single unmeasured quantity, if known exactly, would most advance chemical prediction?
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31.
Explain how δ¹⁸O in an ice core records temperature, and state the assumptions the palaeothermometer hides.
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32.
Silicate weathering is said to thermostat the climate over geological time. State the negative feedback precisely, give its characteristic timescale, and say what it cannot regulate.
Questions added by Examination Expansion v5.0, which extended the examination to music and geography. All earlier questions stand verbatim and unrenumbered.