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The RATE of a chemical reaction is defined as
Athe change in concentration of a reactant or product per unit time
Bthe time taken for the reaction to reach completion
Cthe equilibrium constant of the reaction
Dthe activation energy divided by temperature
Answer & Solution
Correct answer: A. the change in concentration of a reactant or product per unit time
1. NCERT §3.1 defines rate as $\text{rate} = -\dfrac{\Delta [R]}{\Delta t}$ for a reactant (note the negative sign because reactant concentration decreases) or $+\dfrac{\Delta [P]}{\Delta t}$ for a product.
2. The unit is mol L$^{-1}$ s$^{-1}$ (or mol L$^{-1}$ min$^{-1}$ depending on time scale).
3. The instantaneous rate at time $t$ is the derivative $\dfrac{d[\,]}{dt}$.
4. Option A confuses rate with reaction time. Option C is the equilibrium constant — a thermodynamic quantity, not a kinetic one. Option D is unrelated to the definition.
_Source: NCERT Class 12 Chemistry Part 1, Ch 3 "Chemical Kinetics", §3.1 (Rate of a Chemical Reaction), p. 1–2._
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