The Jahn–Teller Effect: Why Some Complexes Distort

September 11, 2026

| 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…

Free Radical Reactions: Chain Mechanisms and Selectivity

September 10, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Radicals Free Radical Reactions: Chain Mechanisms and Selectivity Radical reactions look unruly, but their selectivity follows a clear rule — the more selective reagent is the less reactive one. BSc & MSc · Organic Chemistry · Concept The short answer: A radical chain has initiation, propagation and termination steps, and the propagation steps regenerate the radical so one initiation produces many products. Selectivity depends on how exothermic the abstraction step is: a less reactive radical has a later, more product-like transition state and discriminates more between C–H bonds. The chain Initiation — a bond breaks homolytically, usually…

The Partition Function: Connecting Molecules to Thermodynamics

September 9, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Statistical The Partition Function: Connecting Molecules to Thermodynamics One quantity that, once known, yields every thermodynamic property of the system. Understanding what it counts is more useful than memorising the formulas it generates. BSc & MSc · Physical Chemistry · Concept The short answer: The partition function sums a Boltzmann factor over every accessible state, so it measures how many states are effectively available at a given temperature. Energy, entropy and free energy all follow from it by differentiation, which is why it is the bridge between molecular properties and bulk thermodynamics. The definition and what it…

Metal Carbonyls: Synergic Bonding and What the Spectrum Reveals

September 9, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Organometallics Metal Carbonyls: Synergic Bonding and What the Spectrum Reveals The infrared stretching frequency of bound carbon monoxide is a direct readout of electron density at the metal, which makes it one of the most informative single measurements in the subject. BSc & MSc · Inorganic Chemistry · Concept The short answer: Carbon monoxide donates a lone pair to the metal and simultaneously accepts electron density from filled metal d orbitals into its own antibonding orbital. That back-donation weakens the C–O bond, lowering its stretching frequency — so the observed frequency measures how much back-donation is occurring….

The Distribution Law and Why Extraction in Portions Works Better

September 9, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Equilibria The Distribution Law and Why Extraction in Portions Works Better A short derivation that settles a practical question — is one large extraction better than several small ones, and by how much. BSc & MSc · Physical Chemistry · Concept The short answer: A solute distributes between two immiscible solvents in a fixed ratio at constant temperature. Working through the arithmetic shows that several small extractions remove far more solute than one extraction with the same total volume — which is why laboratory procedure always specifies portions. The law When a solute is shaken with two…

Aromatic Nucleophilic Substitution and the Benzyne Mechanism

September 9, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Aromatic Aromatic Nucleophilic Substitution and the Benzyne Mechanism Two completely different mechanisms produce the same overall transformation, and the conditions tell you which one operated. BSc & MSc · Organic Chemistry · Concept The short answer: The addition–elimination route needs strong electron-withdrawing groups ortho or para to the leaving group, and gives substitution at exactly that position. The benzyne route needs very forcing conditions, has no such requirement, and can give substitution at the adjacent position too — which is the evidence that distinguishes them. Why aromatic rings resist nucleophiles A benzene ring is electron rich, so…

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