Noble Gas Compounds: Why Xenon Reacts and Helium Does Not

September 12, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Main Group Noble Gas Compounds: Why Xenon Reacts and Helium Does Not The elements once believed inert form a well-defined set of compounds, and which ones form is entirely predictable from ionisation energy. BSc & MSc · Inorganic Chemistry · Concept The short answer: Xenon has a low enough ionisation energy to be oxidised by fluorine and oxygen. Its fluorides have structures predicted correctly by VSEPR once the lone pairs are counted, and the fluorides hydrolyse to oxides and oxofluorides. Helium and neon do not react, because their ionisation energies are far too high. Why xenon and…

Birch Reduction and Dissolving Metal Reactions

September 12, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Reduction Birch Reduction and Dissolving Metal Reactions A reduction that stops exactly where you want it, and whose regiochemistry is decided by the substituent already on the ring. BSc & MSc · Organic Chemistry · Concept The short answer: A metal dissolved in liquid ammonia supplies solvated electrons that reduce an aromatic ring to a non-conjugated cyclohexadiene. Electron-donating substituents end up on a double bond carbon; electron-withdrawing substituents end up on a saturated carbon — and that opposite outcome is the standard question. The reagent An alkali metal dissolved in liquid ammonia gives a deep blue solution…

Ionic Conductance, Kohlrausch’s Law and Transport Numbers

September 12, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Electrochemistry Ionic Conductance, Kohlrausch’s Law and Transport Numbers Conductance measurements answer questions that look inaccessible — the degree of dissociation of a weak acid, and the solubility of a nearly insoluble salt. BSc & MSc · Physical Chemistry · Concept The short answer: Molar conductivity rises as a solution is diluted, but for different reasons in strong and weak electrolytes. Kohlrausch’s law of independent migration lets the limiting conductivity of a weak electrolyte be constructed from ionic values, which then gives its degree of dissociation and dissociation constant. The quantities Conductivity is conductance per unit dimension of…

Mass Spectrometry: Reading Fragmentation Patterns

September 11, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Spectroscopy Mass Spectrometry: Reading Fragmentation Patterns The molecular ion gives the mass. The fragments give the structure, and they fragment in predictable ways. BSc & MSc · Spectroscopy · Method The short answer: The molecular ion peak gives the molecular mass, and isotope peaks reveal certain elements immediately. Fragmentation follows rules based on which cation is most stable, so the losses observed identify the groups present. The nitrogen rule links an odd molecular mass to an odd number of nitrogens. What the spectrum shows The sample is ionised, and the resulting ions are separated by mass-to-charge ratio….

Ideal and Non-Ideal Solutions: Raoult, Henry and Azeotropes

September 11, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Solutions Ideal and Non-Ideal Solutions: Raoult, Henry and Azeotropes Deviations from ideality are not an inconvenience — they are the reason azeotropes exist, and the sign of the deviation predicts which kind forms. BSc & MSc · Physical Chemistry · Concept The short answer: An ideal solution obeys Raoult’s law across the whole composition range, which requires the interactions between unlike molecules to match those between like ones. Where unlike interactions are weaker, the solution shows positive deviation and can form a minimum boiling azeotrope; where stronger, negative deviation and a maximum boiling azeotrope. Raoult’s law and…

The Jahn–Teller Effect: Why Some Complexes Distort

September 11, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Structure The Jahn–Teller Effect: Why Some Complexes Distort A degenerate electronic state is unstable against distortion. Knowing which configurations are degenerate tells you immediately which complexes distort and how strongly. BSc & MSc · Inorganic Chemistry · Concept The short answer: A non-linear molecule in a degenerate electronic state will distort to remove that degeneracy, because doing so lowers the energy. In octahedral complexes the effect is strong for unevenly occupied eg orbitals and weak for uneven t2g occupation, because eg orbitals point directly at ligands. The statement Any non-linear molecule in a degenerate electronic state is…

The Wittig Reaction: Making Alkenes Where You Want Them

September 11, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Synthesis The Wittig Reaction: Making Alkenes Where You Want Them Its value is not that it makes alkenes but that it puts the double bond in an unambiguous position, which elimination cannot guarantee. BSc & MSc · Organic Chemistry · Method The short answer: A phosphorus ylide adds to a carbonyl, and the resulting four-membered intermediate collapses to give an alkene and a phosphine oxide. The double bond forms exactly where the carbonyl was, with no possibility of migration — which is what makes it superior to elimination for regiochemical control. Why it matters Elimination reactions make…

Collision Theory and Transition State Theory Compared

September 10, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Kinetics Collision Theory and Transition State Theory Compared Two attempts to explain the same rate constant. One counts collisions; the other treats the transition state as a species with thermodynamic properties. BSc & MSc · Physical Chemistry · Concept The short answer: Collision theory predicts rate from collision frequency, an energy criterion and a steric factor, but the steric factor has to be fitted rather than calculated. Transition state theory instead assumes a quasi-equilibrium with an activated complex, giving the Eyring equation, whose parameters have direct thermodynamic meaning. Collision theory The reasoning is simple: molecules must collide…

Chromatography: The Principles Behind Every Variant

September 10, 2026

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| Chemical Science Exams, Physical Chemistry

Analytical Chemistry · Separation Chromatography: The Principles Behind Every Variant A dozen named techniques, one underlying idea — components separate because they spend different fractions of their time in a moving phase. BSc & MSc · Analytical Chemistry · Concept The short answer: Every form of chromatography has a stationary and a mobile phase. Components that interact more strongly with the stationary phase move more slowly. The retention factor quantifies this, resolution measures how well two peaks are separated, and the van Deemter equation explains what controls band broadening. The single underlying idea A mixture is carried by a mobile…

Latimer and Frost Diagrams: Reading Redox Chemistry From a Picture

September 10, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Redox Latimer and Frost Diagrams: Reading Redox Chemistry From a Picture Two ways of displaying the same data, each answering a different question — one about individual couples, one about which species survive. BSc & MSc · Inorganic Chemistry · Method The short answer: A Latimer diagram lists reduction potentials between adjacent oxidation states. A Frost diagram plots nE° against oxidation state, so stability becomes visual: the lowest point is the most stable species, and any species lying above the line joining its neighbours will disproportionate. Latimer diagrams A Latimer diagram writes the oxidation states of an…

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