Lesson 6.6: Reaction Mechanisms in Chemical Kinetics

Reaction Mechanisms

Introduction to Reaction Mechanisms

Chemical reactions often proceed through a series of simpler steps rather than occurring in a single event. The overall balanced chemical equation summarizes the starting materials and products, but does not reveal the detailed pathway or the sequence of events that occur during the reaction. Understanding these steps is crucial for a complete description of how a reaction proceeds.

• Elementary step: A single step in a reaction mechanism involving one, two, or three chemical entities colliding and reacting. It cannot be broken down into simpler reactions.

• Reaction mechanism: The sequence of elementary steps by which a chemical reaction occurs.

• Reaction intermediate: A species that is produced in one step of a reaction mechanism and consumed in a subsequent step. It does not appear in the overall balanced equation.

• Rate-determining step: The slowest step in a reaction mechanism, which determines the rate of the overall reaction.

Requirements for a Plausible Reaction Mechanism

For a proposed reaction mechanism to be considered plausible, it must satisfy two main requirements:

1. The sum of the elementary steps must yield the overall balanced chemical equation for the reaction.

2. The mechanism must agree with the experimentally determined rate law equation.

Experimental evidence, such as the observed rate law, is used to confirm or refute proposed mechanisms.

Example: Reaction of Nitrogen Dioxide and Carbon Monoxide

Consider the reaction:

$\mathrm{NO_2(g) + CO(g) \rightarrow NO(g) + CO_2(g)}$

The experimentally determined rate law is:

$\text{rate} = k[\mathrm{NO_2}]^2$

The proposed mechanism is:

• Step 1 (slow): $\mathrm{NO_2(g) + NO_2(g) \xrightarrow{k_1} NO_3(g) + NO(g)}$

• Step 2 (fast): $\mathrm{NO_3(g) + CO(g) \xrightarrow{k_2} NO_2(g) + CO_2(g)}$

Here, $\mathrm{NO_3(g)}$ is a reaction intermediate. The first step is the rate-determining step, so the overall rate law is determined by this step and matches the experimental rate law.

Writing Rate Laws for Elementary Steps

For elementary steps, the rate law can be written directly from the balanced equation:

Elementary Step	Rate Law Equation
$A \rightarrow$ products	$\text{rate} = k[A]$
$A + A \rightarrow$ products	$\text{rate} = k[A]^2$
$A + B \rightarrow$ products	$\text{rate} = k[A][B]$

Elementary steps involving three reactant molecules are rare due to the low probability of simultaneous collisions among three entities.

Sample Problem: Nitryl Fluoride Synthesis

Consider the reaction:

$2\mathrm{NO_2(g)} + \mathrm{F_2(g)} \rightarrow 2\mathrm{NO_2F(g)}$

Experimental rate law: $\text{rate} = k[\mathrm{NO_2}]^2[\mathrm{F_2}]$

Proposed mechanism:

• Step 1 (slow): $\mathrm{NO_2(g) + F_2(g) \rightarrow NO_2F(g) + F(g)}$

• Step 2 (fast): $\mathrm{NO_2(g) + F(g) \rightarrow NO_2F(g)}$

This mechanism satisfies both requirements: the sum of the steps gives the overall equation, and the rate law for the slow step matches the experimental rate law.

Identifying the Rate-Determining Step

Given a reaction with the rate law $\text{rate} = k[\mathrm{NO}][\mathrm{Cl_2}]$, and the following steps:

• $\mathrm{NO(g) + Cl_2(g) \rightarrow NOCl_2(g)}$

• $\mathrm{NOCl_2(g) + NO(g) \rightarrow 2NOCl(g)}$

The first step is the rate-determining step, as its rate law matches the experimental rate law. The intermediate is $\mathrm{NOCl_2(g)}$.

Practice Problems

• Write rate law equations for given elementary reactions.

• Determine overall reactions, intermediates, and rate laws for proposed mechanisms.

• Evaluate the plausibility of mechanisms based on experimental rate laws.

Summary Table: Key Terms

Term	Definition
Elementary step	A single reaction event involving one, two, or three entities
Reaction mechanism	Sequence of elementary steps for a reaction
Reaction intermediate	Species formed and consumed during the mechanism
Rate-determining step	Slowest step, controls overall reaction rate

Key Points

• Most reactions occur in multiple steps (reaction mechanisms).

• Each elementary step has its own rate law; the slowest step determines the overall rate law.

• Plausible mechanisms must sum to the overall equation and match the experimental rate law.