Q.What is an elementary reaction? Give the differences between order and molecularity of a reaction.

Step 1. An elementary reaction is a single, genuine step of a proposed reaction mechanism -- e.g. in the iodide-catalysed decomposition of $H_2O_2$, $H_2O_2+I^-\rightarrow H_2O+OI^-$ is one elementary step.

Step 2. Molecularity is the total number of reactant species that must come together in ONE elementary step for it to occur; it is always a positive whole number (1, 2, rarely 3), since fractional collisions between partial molecules are not physically meaningful.

Step 3. Order is the sum of the exponents in the experimentally-measured rate law of the OVERALL (possibly multi-step) reaction; it can be zero, a fraction, or any positive integer, and is assigned once to the whole reaction, not per step.

Define an elementary reaction first, then contrast molecularity and order point by point us …

• Treating order and molecularity as interchangeable for a multi-step reaction -- they coincide ONLY when the rate-determining step's molecularity ha …