Crystal Defects and Non-Stoichiometry

September 13, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Solid State Crystal Defects and Non-Stoichiometry A perfect crystal would have zero entropy of disorder, which thermodynamics forbids above absolute zero. Defects are not flaws but a requirement. BSc & MSc · Inorganic Chemistry · Concept The short answer: Point defects exist in every real crystal because they increase entropy enough to lower the free energy despite costing enthalpy. Schottky defects remove ion pairs and reduce density; Frenkel defects relocate an ion and leave density unchanged. Non-stoichiometric compounds arise when defects are accompanied by a change in oxidation state. Why defects must exist Defects are thermodynamically required,…

Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry

September 13, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Aromatic Diazonium Salts: The Most Versatile Intermediate in Aromatic Chemistry One functional group that can be replaced by almost anything, which makes it the standard route to substituents that cannot be installed directly. BSc & MSc · Organic Chemistry · Method The short answer: An aromatic amine treated with nitrous acid at low temperature gives a diazonium salt. Nitrogen is an outstanding leaving group, so the diazonium group can be replaced by halide, hydroxyl, cyano, hydrogen and more — giving access to substitution patterns unreachable by direct electrophilic substitution. Preparation An aromatic primary amine is treated with…

Osmotic Pressure and the van’t Hoff Factor

September 13, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Solutions Osmotic Pressure and the van’t Hoff Factor The most sensitive of the colligative properties, and the only one practical for measuring the molar mass of very large molecules. BSc & MSc · Physical Chemistry · Concept The short answer: Osmotic pressure is the pressure that must be applied to stop solvent flowing through a semipermeable membrane into a solution. It is far larger than the other colligative effects at the same concentration, which is why it is the method of choice for polymers and proteins. The van’t Hoff factor corrects for dissociation or association of the…

Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra

September 13, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Spectra Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra Two diagrams for the same purpose, differing in whether they can handle a spin state change. BSc & MSc · Inorganic Chemistry · Concept The short answer: Both plot the energies of electronic terms against ligand field strength. Orgel diagrams cover weak-field high-spin complexes only. Tanabe–Sugano diagrams cover the whole range including the high-spin to low-spin crossover, and take the ground state as the horizontal axis so transition energies are read directly. What the diagrams show A free ion’s electronic states are described by term symbols. Placing that ion…

Neighbouring Group Participation: When a Reaction Is Faster Than It Should Be

September 12, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Mechanism Neighbouring Group Participation: When a Reaction Is Faster Than It Should Be An unexpected rate enhancement together with retention of configuration is the signature. Both point to the same cause. BSc & MSc · Organic Chemistry · Concept The short answer: A group elsewhere in the molecule can attack the reacting centre internally, forming a cyclic intermediate before the external nucleophile arrives. Because two inversions occur in sequence, the overall configuration is retained — and the reaction is much faster than the substrate structure alone would predict. The two observations that reveal it Neighbouring group participation…

Noble Gas Compounds: Why Xenon Reacts and Helium Does Not

September 12, 2026

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| 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…

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