What is the difference between order and molecularity of a reaction?

Order: experimentally determined exponent sum in rate law (r = k[A]ˣ[B]ʸ); can be 0, 1, 2, fraction, negative; refers to overall reaction. Molecularity: number of molecules actually participating in an elementary step; always a positive integer (1, 2, or 3); applies to elementary reactions only. Order = molecularity only for simple one-step reactions.

Why is half-life of a first-order reaction independent of initial concentration?

For first-order: t½ = 0.693/k. The rate constant k depends only on temperature, not concentration. Therefore t½ is constant regardless of initial concentration. This is a unique property of first-order reactions. Example: radioactive decay is first-order — each isotope has a characteristic constant half-life.

How does a catalyst work?

A catalyst provides an alternate reaction pathway with lower activation energy (Ea). With lower Ea, more molecules in the Boltzmann distribution have sufficient energy to react → reaction rate increases. The catalyst is not consumed overall (though it participates in intermediate steps). It affects both forward and reverse rates equally, so equilibrium position remains unchanged.

Rate of Reaction and Rate Laws

Chemical Kinetics • Class 12 Chemistry • NCERT • CBSE

Rate of reaction = change in concentration/time. Rate law: r = k[A]ˣ[B]ʸ. Zero-order: t½ = [A₀]/2k. First-order: t½ = 0.693/k (constant). Arrhenius equation: k = Ae^(−Ea/RT). Activation energy Ea from slope of ln k vs 1/T graph.

Key Formulas

r = k[A]ˣ[B]ʸ (rate law)
First-order: k = (2.303/t)log([A₀]/[A])
t½ = 0.693/k (first order)
k = Ae^(−Ea/RT) (Arrhenius)
log(k₂/k₁) = Ea/(2.303R) × (1/T₁ − 1/T₂)

Frequently Asked Questions

What is the difference between order and molecularity of a reaction?: Order: experimentally determined exponent sum in rate law (r = k[A]ˣ[B]ʸ); can be 0, 1, 2, fraction, negative; refers to overall reaction. Molecularity: number of molecules actually participating in an elementary step; always a positive integer (1, 2, or 3); applies to elementary reactions only. Order = molecularity only for simple one-step reactions.
Why is half-life of a first-order reaction independent of initial concentration?: For first-order: t½ = 0.693/k. The rate constant k depends only on temperature, not concentration. Therefore t½ is constant regardless of initial concentration. This is a unique property of first-order reactions. Example: radioactive decay is first-order — each isotope has a characteristic constant half-life.
How does a catalyst work?: A catalyst provides an alternate reaction pathway with lower activation energy (Ea). With lower Ea, more molecules in the Boltzmann distribution have sufficient energy to react → reaction rate increases. The catalyst is not consumed overall (though it participates in intermediate steps). It affects both forward and reverse rates equally, so equilibrium position remains unchanged.

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