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Molecularity and Reaction Order

Learn the difference between molecularity and reaction order, explore elementary steps, rate laws, and how to determine reaction order experimentally.

24 cards~8 min
Molecularity and Reaction Order — Qwi
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1What does molecularity describe in a reaction?
Answer

Number of reacting species that collide simultaneously

It counts molecules, atoms, or ions that must meet in a single elementary step. Molecularity is always a whole number (1, 2, or 3).
2Molecularity is always a whole number ({{1, 2 or 3}}).
Answer

1, 2 or 3

3All reactions have a molecularity equal to their reaction order.
Answer

False

Molecularity applies only to elementary steps and is always an integer, while order is experimentally determined and can be fractional.
4How do unimolecular and bimolecular reactions differ?
Answer

Unimolecular: one reactant collides → molecularity 1 | Bimolecular: two reactants collide → molecularity 2

5What is a trimolecular reaction?
Answer

A reaction where three particles collide simultaneously

Trimolecular reactions have molecularity 3 and are rare because simultaneous three‑particle collisions are unlikely.
6An elementary reaction occurs in {{one single step}}.
Answer

one single step

7Complex reactions proceed through a single elementary step.
Answer

False

Complex reactions involve multiple elementary steps, each with its own molecularity, and the overall rate is controlled by the slowest step.
8What distinguishes a rate‑determining step from a fast step?
Answer

RDS: slowest, controls overall rate | Fast step: occurs quickly, does not limit rate

9How is the order of a reaction defined?
Answer

Sum of the powers of concentration terms in the rate equation

It indicates how the reaction rate depends on reactant concentrations. The order can be zero, fractional, or integer.
10Zero order reaction rate law: r = {{k}}.
Answer

k

11In a zero‑order reaction, increasing reactant concentration increases the rate.
Answer

False

The rate is independent of concentration; it remains equal to the constant k.
12How do zero‑order and first‑order rate‑vs‑concentration graphs differ?
Answer

Zero‑order: horizontal line (rate constant) | First‑order: diagonal line through origin (rate ∝ [A])

13What is the rate law for a first‑order reaction?
Answer

r = k[A]

The rate is directly proportional to the concentration of the single reactant. Doubling [A] doubles the rate.
14First‑order reaction graph is a {{straight diagonal line}} passing through the origin.
Answer

straight diagonal line

15The rate of a first‑order reaction changes linearly with the concentration of the reactant.
Answer

True

If the concentration of A is doubled, the rate also doubles.
16How does a second‑order reaction’s concentration dependence differ from a first‑order reaction?
Answer

Second‑order: rate ∝ [A]^2 or [A][B] (quadratic) | First‑order: rate ∝ [A] (linear)

17Write the rate law for a second‑order reaction involving a single reactant.
Answer

r = k[A]^2

The rate varies with the square of the reactant concentration; doubling [A] increases the rate fourfold.
18Second‑order reaction graph is an {{upward‑curving parabola}}.
Answer

upward‑curving parabola

19A third‑order reaction rate increases eightfold when concentration doubles.
Answer

True

Because rate ∝ [A]^3, doubling [A] gives 2^3 = 8 times the original rate.
20What are the units of the rate constant for zero, first, and second order reactions?
Answer

Zero: mol·L⁻¹·s⁻¹ | First: s⁻¹ | Second: L·mol⁻¹·s⁻¹

Derived from k = rate·[conc]ⁿ; n = 0,1,2 respectively.
21How does the initial concentration method determine reaction order?
Answer

By varying one reactant’s concentration while keeping others constant and comparing initial rates

The method isolates the effect of a single reactant on the rate, allowing the exponent (order) to be calculated from rate ratios.
22In the initial concentration method, other reactant concentrations are kept {{constant}}.
Answer

constant

23If doubling [A] quadruples the rate, the reaction is second order with respect to A.
Answer

True

Rate ∝ [A]^2; 2^2 = 4, confirming second‑order dependence.
24What is the key difference between reaction order and molecularity?
Answer

Order: experimentally determined exponent sum, can be fractional | Molecularity: number of particles colliding in an elementary step, always whole number

Order applies to the overall reaction; molecularity applies only to individual elementary steps.

Understanding Molecularity

Molecularity describes the number of reacting species that must collide simultaneously in a single elementary step. It is a structural property of the step and can only be 1, 2, or 3. Because it counts actual particles—molecules, atoms, or ions—it is always an integer.

  • Unimolecular reaction: one reactant undergoes a transformation; molecularity = 1.
  • Bimolecular reaction: two reactants collide; molecularity = 2.
  • Trimolecular reaction: three particles meet at once; molecularity = 3 (rare).

Only elementary reactions have a defined molecularity. Complex reactions, which proceed through multiple elementary steps, do not have a single molecularity value.

Key Points

  • Molecularity is always a whole number (1, 2, or 3).
  • It applies only to elementary steps, not to overall reactions.
  • Complex reactions consist of several elementary steps, each with its own molecularity.

Reaction Order: Definition and Characteristics

The order of a reaction is the sum of the exponents of the concentration terms in the rate law. It reflects how the reaction rate depends on the concentrations of the reactants. Unlike molecularity, order is determined experimentally and can be zero, fractional, or an integer.

Mathematically, for a rate law r = k[A]^a[B]^b, the overall order is a + b. The individual exponents are the orders with respect to each reactant.

Zero‑Order Reactions

Rate law: r = k. The rate is independent of reactant concentration; changing [A] does not affect the rate. Graphically, the rate‑vs‑concentration plot is a horizontal line at the constant value k.

First‑Order Reactions

Rate law: r = k[A]. The rate is directly proportional to the concentration of a single reactant. Doubling [A] doubles the rate. The graph is a straight diagonal line passing through the origin.

Second‑Order Reactions

Two common forms:

  • Rate ∝ [A]^2 (single‑reactant second order) → r = k[A]^2.
  • Rate ∝ [A][B] (two‑reactant second order).

The rate‑vs‑concentration plot is an upward‑curving parabola, reflecting the quadratic dependence.

Higher‑Order Reactions

Third‑order reactions are rare but follow r = k[A]^3. Doubling [A] increases the rate eightfold (2³ = 8). The graph becomes steeper than the second‑order parabola.

Units of the Rate Constant

The units of the rate constant k depend on the overall order n of the reaction, derived from the relationship k = rate·[conc]⁻ⁿ:

  • Zero order: mol·L⁻¹·s⁻¹
  • First order: s⁻¹
  • Second order: L·mol⁻¹·s⁻¹

Rate‑Determining Step vs. Fast Steps

In a multi‑step reaction, the slowest elementary step is the rate‑determining step (RDS). It controls the overall reaction rate, while subsequent fast steps proceed quickly and do not limit the rate. This concept is analogous to traffic flow: the bottleneck dictates the speed of the entire line of cars.

Experimental Determination of Reaction Order

The initial concentration method isolates the effect of a single reactant. By varying the concentration of one reactant while keeping all others constant, the initial rates are measured. Comparing the ratios of rates reveals the exponent for that reactant.

Example: If doubling [A] quadruples the initial rate, the reaction is second order with respect to A because 2² = 4.

Steps of the Initial Concentration Method

  1. Choose a reactant to vary.
  2. Prepare several experiments with different initial concentrations of that reactant, keeping all other concentrations unchanged.
  3. Measure the initial rate for each experiment.
  4. Calculate the ratio of rates and concentrations to determine the order exponent.

Distinguishing Molecularity from Reaction Order

While both concepts involve numbers of particles, they differ fundamentally:

  • Order is an experimentally determined exponent sum that can be fractional or zero; it applies to the overall reaction.
  • Molecularity is the count of particles colliding in an elementary step; it is always an integer (1, 2, or 3) and pertains only to that step.

Therefore, a reaction’s overall order does not necessarily equal the molecularity of any individual step, especially when the reaction proceeds through multiple steps.

Summary

Understanding the distinction between molecularity and reaction order is essential for interpreting kinetic data. Molecularity provides insight into the mechanistic step at the molecular level, whereas reaction order describes how the macroscopic rate depends on reactant concentrations. By mastering elementary rate laws, graph interpretations, and experimental techniques such as the initial concentration method, one can accurately characterize reaction mechanisms and predict how changes in conditions will affect reaction speed.