Question 1

What is the difference between reaction order and molecularity?

Accepted Answer

Molecularity counts the reacting species—atoms, ions or molecules—taking part in one elementary reaction step. Reaction order is obtained by adding the powers of reactant concentrations in the experimental rate law. They are equal for an elementary reaction but need not be equal for a complex reaction.

Question 2

Can the order of a reaction be fractional or negative?

Accepted Answer

Yes. The order of a reaction may be zero, a positive integer, fractional, or negative. The value comes from the experimentally determined rate law and is not necessarily connected to the stoichiometric coefficients.

Question 3

Why is molecularity not assigned to a complex reaction?

Accepted Answer

Molecularity applies to a single elementary step because it counts the reacting species in that step. A complex reaction consists of multiple steps, so molecularity is assigned to its individual elementary steps, not to the overall balanced equation.

Question 4

How are the units of the rate constant calculated?

Accepted Answer

If rate has units mol L^-1 s^-1 and the reaction order is n, then k has units (mol L^-1)^(1-n) s^-1. Therefore the units of k change with the order of the reaction; for example, a first-order rate constant has units s^-1.

Question 5

Do stoichiometric coefficients determine reaction order?

Accepted Answer

No. Stoichiometric coefficients describe atom conservation in a balanced equation, while the reaction order is calculated from the powers in the experimental rate law. A complex reaction can therefore have an order different from the sum of its overall coefficients.

Mechanism, Reaction Order & Molecularity 3D Lab

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