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…

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…

Gibbs Free Energy: Why It Is the Criterion for Spontaneity

September 3, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Thermodynamics Gibbs Free Energy: Why It Is the Criterion for Spontaneity Entropy decides spontaneity, but only for the universe. Gibbs energy repackages that so the system alone is enough. BSc & MSc · Physical Chemistry · Concept The short answer: The second law says the entropy of the universe must increase. Rewriting that condition for a system at constant temperature and pressure gives ΔG < 0. So Gibbs energy is not a new law — it is the second law expressed in variables you can actually measure inside the system. The problem Gibbs energy solves The second…

The Langmuir Adsorption Isotherm: Derivation and Its Limits

September 2, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Surface The Langmuir Adsorption Isotherm: Derivation and Its Limits The derivation is short, the assumptions are strong, and questions test the assumptions at least as often as the equation. BSc & MSc · Physical Chemistry · Concept The short answer: Set the rate of adsorption equal to the rate of desorption at equilibrium and solve for the fraction of surface covered. The result is θ = Kp/(1 + Kp). It assumes a monolayer, identical sites and no interaction between adsorbed molecules — and each of those assumptions is where the model fails. The assumptions, stated first These…

Mathematics for Chemists: The Calculus You Actually Need

September 1, 2026

| No Comments

| Chemical Science Exams, General Aptitude

Foundation · Mathematics Mathematics for Chemists: The Calculus You Actually Need Most physical chemistry difficulty is mathematical, not chemical. A short, targeted refresher removes more obstacles than another chemistry textbook. BSc & MSc · Foundation skills · Guide The short answer: Four areas carry nearly all the load: differentiation including partial derivatives, integration of a small set of standard forms, first-order differential equations, and logarithms. Everything else is occasional. Building these early makes thermodynamics, kinetics and quantum chemistry substantially easier. Why this is worth doing first Students who struggle with physical chemistry often describe the chemistry as hard when the…

Debye–Hückel Theory: Why Concentration Is Not Enough

September 1, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Electrochemistry Debye–Hückel Theory: Why Concentration Is Not Enough Every equation you learned with concentrations is an approximation. Debye–Hückel explains when that approximation breaks and what replaces it. BSc & MSc · Physical Chemistry · Concept The short answer: Ions in solution are surrounded by an atmosphere of oppositely charged ions, which lowers their effective concentration. Activity replaces concentration, and the activity coefficient measures the difference. The limiting law relates that coefficient to ionic strength, and it is reliable only in dilute solution. The problem the theory solves Equilibrium constants, the Nernst equation and rate laws are usually…

Maxwell Relations: Where They Come From and How to Use Them

August 31, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Thermodynamics Maxwell Relations: Where They Come From and How to Use Them Maxwell relations look like four formulas to memorise. They are actually one idea applied four times, and deriving them beats remembering them. BSc & MSc · Physical Chemistry · Concept The short answer: Each thermodynamic potential is an exact differential, so its mixed second derivatives are equal in either order. Applying that to U, H, A and G gives the four Maxwell relations. Their practical value is converting quantities that cannot be measured directly, such as an entropy derivative, into ones that can. The one…

Steady-State Approximation: Three Solved Kinetics Problems

August 29, 2026

| No Comments

| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Solved Problems Steady-State Approximation: Three Solved Kinetics Problems The approximation itself is one line. Knowing which intermediate to apply it to, and being able to defend that choice, is the part that is actually examined. BSc & MSc · Chemical Kinetics · Worked problems The short answer: Set d[intermediate]/dt ≈ 0, solve for the intermediate concentration, and substitute it into the rate expression for a product. The approximation is valid when the intermediate is highly reactive, so its concentration stays small and nearly constant — which is why it is applied to radicals and reactive complexes, never…

CSIR-NET Chemical Sciences Part C: How Physical Chemistry Is Really Asked

August 29, 2026

| No Comments

| Chemical Science Exams, CSIR-NET

CSIR-NET · Exam Analysis CSIR-NET Chemical Sciences Part C: How Physical Chemistry Is Really Asked Part C is not Part B with harder numbers. It is a different kind of question, and preparing for it as though it were simply tougher is why strong candidates still lose marks there. MSc & research aspirants · CSIR-NET · Exam analysis The short answer: Part B rewards recall and single-step calculation. Part C rewards reasoning across topics — a question will hand you a scenario and expect you to decide which principle applies before any arithmetic starts. Negative marking makes selective attempting a…