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Collision Theory and Transition State Theory Compared

Physical Chemistry · Kinetics Collision Theory and Transition State Theory Compared Two attempts to explain the same rate constant. One counts collisions; the other treats the transition state as a species with thermodynamic properties. BSc & MSc · Physical Chemistry · Concept The short answer: Collision theory predicts rate from collision frequency, an energy criterion and a steric factor, but the steric factor has to be fitted rather than calculated. Transition state theory instead assumes a quasi-equilibrium with an activated complex, giving the Eyring equation, whose parameters have direct thermodynamic meaning. Collision theory The reasoning is simple: molecules must collide…

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Michaelis–Menten Enzyme Kinetics: Derivation and Interpretation

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…

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The Arrhenius Equation: Getting Activation Energy From Data

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…

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Enzyme Inhibition: Competitive, Non-Competitive and Uncompetitive

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…

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Steady-State Approximation: Three Solved Kinetics Problems

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…

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