Debye–Hückel Theory: Why Concentration Is Not Enough

September 1, 2026

| 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

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| 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

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| 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…

Solid State (Class 12): Unit Cells, Packing and Density Numericals

August 29, 2026

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

Class 12 · Physical Chemistry Solid State (Class 12): Unit Cells, Packing and Density Numericals A chapter where a handful of formulas cover almost every question — provided the student can picture the cell. Class 12 · Chemistry · Physical · Published 29 August 2026 In short: Solid State is one of the most scoring chapters in Class 12 Chemistry, because the question types are limited and the formulas are few. The difficulty is spatial rather than mathematical: a student who cannot picture where the atoms sit in the cube will struggle to count them, and everything else follows from…

Particle in a Box: The Derivation IIT-JAM and CSIR-NET Actually Ask

August 29, 2026

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| Chemical Science Exams, Physical Chemistry

Physical Chemistry · Quantum Particle in a Box: The Derivation IIT-JAM and CSIR-NET Actually Ask Almost every quantum question at entrance level is this one model wearing a different hat. Learn the derivation properly once and a whole unit becomes routine. BSc & MSc · Physical Chemistry · Concept · IIT-JAM, CSIR-NET, GATE The short answer: A particle confined to a one-dimensional box of length L has quantised energy En = n²h²/8mL², with n = 1, 2, 3… The quantisation is not assumed — it falls out of forcing the wavefunction to vanish at both walls. Understanding why that boundary…