CBSE 2025, Set 1

Question

**Read the passage and answer:**

The rate of a chemical reaction is expressed either in terms of decrease in the concentration of reactants or increase in the concentration of a product per unit time. The sum of powers of the concentration of reactants in the Rate law expression is called **order of reaction** while the number of reacting species taking part in an elementary reaction which must collide simultaneously to bring about a chemical reaction is called **molecularity of the reaction**.

(a) (i) What is a rate determining step?

(a) (ii) Define complex reaction.

(b) What is the effect of temperature on the rate constant of a reaction?

Edvaya · Accepted Answer

The correct answer is Subjective. **(a)(i) Rate Determining Step:** The **slowest elementary step** in a multi-step reaction mechanism. It acts as the bottleneck and controls the overall rate of reaction — the overall reaction cannot proceed faster than its slowest step. **(a)(ii) Complex Reaction:** A reaction that takes place through **two or more elementary steps** (a series of elementary reactions). The overall reaction equation is the algebraic sum of all elementary steps. Example: decomposition of H$_2$O$_2$ via an iodide-catalysed mechanism. **(b) Effect of temperature on rate constant:** The rate constant **increases with increase in temperature**, as described by the **Arrhenius equation**: $$k = A\,e^{-E_a/RT}$$ Higher temperature increases the fraction of molecules possessing energy $\geq E_a$ (activation energy). Typically, a 10°C rise doubles the rate constant ($Q_{10} \approx 2$).

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Chemistry > Physical Chemistry > Chemical Kinetics

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