Enzyme Inhibition: Competitive, Non-Competitive and Uncompetitive

September 3, 2026

| 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…

Carbenes and Nitrenes: Structure, Spin State and Reactions

September 3, 2026

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| Chemical Science Exams, Organic Chemistry

Reactive Intermediates · Entrance Exams Carbenes and Nitrenes: Structure, Spin State and Reactions Whether a carbene adds to an alkene with retention or scrambles the stereochemistry is decided by one thing: singlet or triplet. BSc & MSc · Organic Chemistry · Concept The short answer: A carbene has six valence electrons on carbon and two non-bonding electrons that can be paired in one orbital or unpaired in two. Paired means singlet, and singlet carbenes add to alkenes in one step with complete retention of alkene geometry. Unpaired means triplet, which must react in two steps through a diradical, and rotation…

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…

Molecular Orbital Theory for Diatomics: Bond Order and Magnetism

September 3, 2026

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| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Bonding Molecular Orbital Theory for Diatomics: Bond Order and Magnetism MO diagrams answer three questions at once — is the molecule stable, how strong is the bond, and is it paramagnetic. BSc & MSc · Inorganic Chemistry · Concept The short answer: Atomic orbitals combine to give bonding and antibonding molecular orbitals. Fill them by the aufbau principle, then bond order is half the difference between bonding and antibonding electrons. Unpaired electrons mean paramagnetism — which is where MO theory succeeds and valence bond theory fails. The core idea When two atomic orbitals of comparable energy and…

Cross-Coupling Reactions: Suzuki, Heck, Sonogashira and Stille

September 3, 2026

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| Chemical Science Exams, Inorganic Chemistry, Organic Chemistry

Organometallics · Entrance Exams Cross-Coupling Reactions: Suzuki, Heck, Sonogashira and Stille Four named couplings that share one palladium cycle — and differ only in what supplies the second carbon fragment. BSc & MSc · Organometallic Chemistry · Concept The short answer: All four couplings run on the same three-step palladium cycle. Oxidative addition and reductive elimination are common to every one of them; what distinguishes Suzuki, Sonogashira and Stille is only which organometallic delivers the partner group. The Heck is the exception — it has no transmetalation at all, and that single difference explains its different substrate scope and its…

Electrophilic Addition to Alkenes: Regiochemistry and Stereochemistry

September 3, 2026

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

Organic Chemistry · Class 11 Electrophilic Addition to Alkenes: Regiochemistry and Stereochemistry Markovnikov’s rule is not a rule to memorise — it is a consequence of which carbocation forms, and it fails exactly where that carbocation does not form. Class 11 · Organic Chemistry · Mechanism The short answer: An alkene is electron rich, so it attacks an electrophile first. Whichever intermediate that step produces controls everything afterwards — a free carbocation gives Markovnikov orientation and mixed stereochemistry, a bridged bromonium ion gives anti addition, and a radical chain reverses the orientation entirely. Learn the intermediate and the product follows….

E1 and E2 Elimination: Saytzeff, Hofmann and Which Alkene Forms

September 3, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Mechanism E1 and E2 Elimination: Saytzeff, Hofmann and Which Alkene Forms Elimination questions almost always come down to one thing: which of two possible alkenes is the major product, and why. BSc & MSc · Organic Chemistry · Concept The short answer: E2 is concerted and requires the leaving group and the beta hydrogen to be anti-periplanar. E1 goes through a carbocation. Saytzeff orientation gives the more substituted alkene and is normal; Hofmann orientation gives the less substituted one and arises with bulky bases or charged leaving groups. The two mechanisms E2 is a single concerted step….

Hydrogen Bonding: One Interaction, Many Consequences

September 3, 2026

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

Chemical Bonding · Class 11 Hydrogen Bonding: One Interaction, Many Consequences Why water boils at 100°C, why ice floats, why ortho-nitrophenol is more volatile than the para isomer — all the same explanation. Class 11 & 12 · Chemical Bonding · Concept The short answer: A hydrogen bond needs hydrogen attached to nitrogen, oxygen or fluorine, and a lone pair on another such atom to accept it. It is about ten times stronger than an ordinary dipole interaction and roughly a tenth the strength of a covalent bond. Almost every anomaly in Class 11 and 12 physical property data traces…

SN1 versus SN2: The Complete Comparison

September 2, 2026

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| Chemical Science Exams, Organic Chemistry

Organic Chemistry · Mechanism SN1 versus SN2: The Complete Comparison Four factors decide which pathway operates. Check them in a fixed order and the prediction is reliable every time. BSc & MSc · Organic Chemistry · Concept The short answer: SN2 is one concerted step with backside attack, giving inversion and second-order kinetics. SN1 goes through a carbocation, giving racemisation and first-order kinetics. Substrate structure is the dominant factor: methyl and primary favour SN2, tertiary favours SN1, and secondary depends on everything else. The two mechanisms side by side Feature SN2 SN1 Steps One, concerted Two, via carbocation Kinetics Second…

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