Hückel Molecular Orbital Theory for Conjugated Systems

September 5, 2026

| Chemical Science Exams, Physical Chemistry

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…

Aldol Condensation and Enolate Chemistry

September 4, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Carbonyl Aldol Condensation and Enolate Chemistry Almost every carbon–carbon bond formed at a carbonyl runs through an enolate. Learn the enolate and a dozen named reactions collapse into one idea. BSc & MSc · Organic Chemistry · Concept The short answer: A hydrogen alpha to a carbonyl is acidic because the resulting anion is resonance stabilised as an enolate. That enolate is a nucleophile, and it attacks another carbonyl to give a beta-hydroxy carbonyl. Heating then eliminates water to give the conjugated enone, which is the condensation step. Why the alpha hydrogen is acidic A hydrogen on…

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

September 4, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

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

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Spectra Term Symbols and Russell–Saunders Coupling, Step by Step A mechanical procedure that looks abstract. Follow the five steps in order and any ground-state term symbol takes about a minute. BSc & MSc · Inorganic Chemistry · Method The short answer: Combine the individual orbital angular momenta into L and the spins into S, then couple them into J. The term symbol is written 2S+1LJ. Hund’s rules then pick the ground state: maximum multiplicity first, then maximum L, then J by whether the shell is less or more than half filled. What a term symbol encodes 2S+1LJ…

Batteries and Fuel Cells: The Electrochemistry Behind Stored Energy

September 4, 2026

| No Comments

| class 11-12 chemistry, Class 12, Physical Chemistry

Electrochemistry · Class 12 Batteries and Fuel Cells: The Electrochemistry Behind Stored Energy A battery carries its reactants inside it and eventually runs down. A fuel cell is fed from outside and does not. That single difference explains most of their properties. Class 12 · Physical Chemistry · Concept The short answer: Every cell in this chapter is a galvanic cell, so oxidation always happens at the anode and reduction at the cathode, and the anode is negative. Primary cells cannot be recharged because their electrode reactions are not practically reversible. Secondary cells can. Fuel cells never run down because…

Asymmetric Synthesis: How Enantioselectivity Is Actually Achieved

September 4, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Stereochemistry · Entrance Exams Asymmetric Synthesis: How Enantioselectivity Is Actually Achieved Two enantiomers have identical energies, so no achiral reagent can ever prefer one. Every method in this area is a way of breaking that symmetry. BSc & MSc · Organic Chemistry · Concept The short answer: Enantiomeric products come from enantiomeric transition states, which have identical energies, so an achiral system must give a racemate. Introducing something chiral makes the two competing transition states diastereomeric instead, and diastereomers differ in energy. Whether the chirality comes from an auxiliary, a reagent or a catalyst, that is the single mechanism behind…

The Trans Effect: Predicting the Product of Square Planar Substitution

September 4, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Reaction Mechanism The Trans Effect: Predicting the Product of Square Planar Substitution A synthesis question in coordination chemistry usually reduces to one thing — knowing which ligand directs the incoming group to the position opposite itself. BSc & MSc · Inorganic Chemistry · Concept The short answer: In square planar complexes, certain ligands strongly labilise the position trans to themselves. Arranging synthesis steps so the right ligand is present at the right time is how specific geometric isomers are made deliberately rather than as a mixture. What the effect is In a square planar complex, a ligand…

Grignard Reagents: What They Make and What Destroys Them

September 4, 2026

| No Comments

| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Reagents Grignard Reagents: What They Make and What Destroys Them One reagent class that builds carbon skeletons, and one failure mode that ruins more exam answers than any other. BSc & MSc · Organic Chemistry · Method The short answer: A Grignard reagent is a carbon nucleophile, effectively a carbanion. It attacks carbonyl carbons to build new carbon–carbon bonds, and the product depends entirely on which carbonyl it attacks. It is destroyed instantly by any acidic hydrogen, which is the constraint that governs how it can be used. What it is An alkyl or aryl magnesium halide,…

X-Ray Diffraction and Bragg’s Law: Reading a Powder Pattern

September 4, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Solid State · Entrance Exams X-Ray Diffraction and Bragg’s Law: Reading a Powder Pattern From a row of peaks on a diffractogram to a lattice type and a unit cell edge — the calculation examiners actually set. BSc & MSc · Physical & Solid State Chemistry · Method The short answer: Bragg’s law relates the angle at which a crystal diffracts to the spacing between its planes. For a cubic crystal the plane spacing depends on the Miller indices in a simple way, so the ratio of sin²θ values across the peaks reveals the lattice type directly — and the…

The Arrhenius Equation: Getting Activation Energy From Data

September 3, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Kinetics The Arrhenius Equation: Getting Activation Energy From Data Two forms of one equation, and knowing which to use is the difference between a clean answer and an arithmetic mess. BSc & MSc · Physical Chemistry · Method The short answer: The rate constant varies with temperature as k = A exp(−Ea/RT). Taking logarithms gives a straight line of slope −Ea/R against 1/T, which is how activation energy is measured. With only two temperatures, the two-point form avoids plotting altogether. The equation and what each part means k = A e−Ea/RT The exponential term is the fraction…

8 of 12
« Previous
1
…
6
7
8
9
10
…
12
Next »