Tag: GATE Chemistry

Physical Chemistry · Photochemistry Photochemistry: Quantum Yield and the Jablonski Diagram Absorbing a photon puts a molecule somewhere unusual. The Jablonski diagram maps every route back down, and quantum yield counts which route was taken. BSc & MSc · Physical Chemistry · Concept The short answer: Quantum yield is the number of molecules undergoing a process per photon absorbed. The Jablonski diagram organises the competing routes back to the ground state — fluorescence, internal conversion, intersystem crossing and phosphorescence — and their relative rates decide what is observed. The two basic laws Grotthuss–Draper law: only light that is absorbed can…

Lanthanides: The Contraction and Why Separation Is So Hard

September 7, 2026

Inorganic Chemistry · f Block Lanthanides: The Contraction and Why Separation Is So Hard One structural fact — poor shielding by f electrons — explains the contraction, the similarity of the elements, and the difficulty of separating them. BSc & MSc · Inorganic Chemistry · Concept The short answer: Across the lanthanide series the 4f electrons shield the nuclear charge poorly, so the effective nuclear charge rises steadily and the ionic radius falls. That contraction makes the elements chemically almost identical, which is why they occur together and separating them requires methods exploiting very small differences. What the contraction is…

Vibrational Spectroscopy: Force Constants and Anharmonicity

September 7, 2026

Physical Chemistry · Spectroscopy Vibrational Spectroscopy: Force Constants and Anharmonicity The harmonic oscillator explains where the band is. Anharmonicity explains everything the harmonic model gets wrong, including why molecules can dissociate at all. BSc & MSc · Spectroscopy · Concept The short answer: A vibrating bond behaves approximately as a harmonic oscillator with evenly spaced levels and a selection rule of one quantum. The frequency depends on the force constant and the reduced mass, so a stiffer bond or lighter atoms absorb at higher wavenumber. Real bonds are anharmonic, which produces converging levels and permits overtones. The harmonic oscillator Model…

Rotational Spectroscopy: Measuring a Bond Length From a Spectrum

September 6, 2026

Physical Chemistry · Spectroscopy Rotational Spectroscopy: Measuring a Bond Length From a Spectrum Line spacing gives the rotational constant, the rotational constant gives the moment of inertia, and that gives the bond length. Three steps, no ambiguity. BSc & MSc · Spectroscopy · Method The short answer: For a rigid diatomic rotor the energy levels are E = BJ(J+1), so successive transitions are separated by exactly 2B. Measure that spacing, extract B, convert to the moment of inertia and then to the bond length. The whole chain rests on the selection rule that J changes by one. The rigid rotor…

Boranes and Wade’s Rules: Predicting Cluster Shapes

September 6, 2026

Inorganic Chemistry · Main Group Boranes and Wade’s Rules: Predicting Cluster Shapes Boron clusters look chaotic until you count skeletal electron pairs. Then each structure follows from a single number. BSc & MSc · Inorganic Chemistry · Method The short answer: Count the skeletal electron pairs. For n boron vertices, n+1 pairs gives a closo structure, n+2 gives nido, n+3 gives arachno. Each type is derived from the closo polyhedron by removing vertices, so the shapes are related rather than independent. Why boranes need special treatment Boron has three valence electrons but four valence orbitals, so it cannot form enough…

Michaelis–Menten Enzyme Kinetics: Derivation and Interpretation

September 6, 2026

Physical Chemistry · Kinetics Michaelis–Menten Enzyme Kinetics: Derivation and Interpretation A steady-state derivation applied to a biological catalyst, giving two constants whose meanings are constantly confused. BSc & MSc · Physical Chemistry · Concept The short answer: Apply the steady-state approximation to the enzyme–substrate complex and the rate becomes v = Vmax[S]/(KM + [S]). KM is the substrate concentration at half maximal rate and indicates how tightly the substrate binds; Vmax reflects how fast the enzyme turns over once saturated. The mechanism E + S ⇌ ES (k1 forward, k−1 reverse) ES → E + P (k2) The enzyme…

The Phase Rule and Reading Phase Diagrams

September 5, 2026

Physical Chemistry · Equilibria The Phase Rule and Reading Phase Diagrams One short equation that tells you how many variables you are free to change, and a diagram that shows the consequence. BSc & MSc · Physical Chemistry · Concept The short answer: The phase rule states F = C − P + 2, where F is the degrees of freedom, C the number of components and P the number of phases in equilibrium. Applied to a one-component diagram it explains why an area has two degrees of freedom, a line one, and the triple point none. The rule F…

Hückel Molecular Orbital Theory for Conjugated Systems

September 5, 2026

Physical Chemistry · Quantum Hückel Molecular Orbital Theory for Conjugated Systems A drastically simplified model that nonetheless predicts aromaticity, reactivity and spectra correctly for planar conjugated molecules. BSc & MSc · Physical Chemistry · Concept The short answer: Treat only the π electrons, assume every carbon contributes one p orbital, and set all Coulomb integrals equal to α and all resonance integrals between neighbours equal to β. Solving the resulting determinant gives orbital energies of the form α + mβ, from which delocalisation energy and aromaticity follow. The assumptions Hückel theory works because it throws almost everything away and keeps…

Supramolecular Chemistry: Crown Ethers, Cryptands and Host–Guest Binding

September 4, 2026

Inorganic Chemistry · Entrance Exams Supramolecular Chemistry: Crown Ethers, Cryptands and Host–Guest Binding Chemistry beyond the covalent bond — where selectivity comes from the size of a hole and the number of contacts, not from making or breaking bonds. BSc & MSc · Inorganic Chemistry · Concept The short answer: Supramolecular chemistry studies assemblies held together by non-covalent forces. A crown ether binds an alkali metal ion whose radius matches its cavity, which is how selectivity is achieved without any covalent chemistry. Cryptands enclose the ion in three dimensions and bind far more strongly still. The extra stability of a…

Term Symbols and Russell–Saunders Coupling, Step by Step

September 4, 2026