Michaelis–Menten Enzyme Kinetics: Derivation and Interpretation

September 6, 2026

| Chemical Science Exams, Physical Chemistry

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

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| Chemical Science Exams, Physical Chemistry

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

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| 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…

Batteries and Fuel Cells: The Electrochemistry Behind Stored Energy

September 4, 2026

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| 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…

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

September 4, 2026

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| 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

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| 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…

Enzyme Inhibition: Competitive, Non-Competitive and Uncompetitive

September 3, 2026

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| Chemical Science Exams, Physical Chemistry

Biophysical Chemistry · Entrance Exams Enzyme Inhibition: Competitive, Non-Competitive and Uncompetitive Three inhibition types, distinguished not by a definition to memorise but by what happens to Km and Vmax — and by where the lines cross. BSc & MSc · Chemical Kinetics · Concept The short answer: A competitive inhibitor binds the free enzyme at the active site, so more substrate overcomes it — apparent Km rises, Vmax is unchanged. An uncompetitive inhibitor binds only the enzyme–substrate complex, lowering both. A non-competitive inhibitor binds either form equally, lowering Vmax while leaving Km alone. On a Lineweaver–Burk plot each gives a…

Gibbs Free Energy: Why It Is the Criterion for Spontaneity

September 3, 2026

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| 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…

Corrosion and Rusting: The Electrochemistry Behind It, and How It Is Stopped

September 3, 2026

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

Electrochemistry · Class 12 Corrosion and Rusting: The Electrochemistry Behind It, and How It Is Stopped Rusting is a short-circuited galvanic cell on the surface of the metal — which is why the cure is electrochemical, not just a coat of paint. Class 12 · Physical Chemistry · Concept The short answer: Iron rusts because a drop of water on its surface completes a tiny galvanic cell. Iron oxidises at one spot, dissolved oxygen is reduced at another, and the two products meet in solution to precipitate hydrated iron(III) oxide. Everything that slows corrosion works by breaking one part of…

The Langmuir Adsorption Isotherm: Derivation and Its Limits

September 2, 2026

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| 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…