Reaction Mechanisms

Introduced to this in AP Chemistry

The overall progress of a chemical reaction can be represented at the molecular level by a series of simple elementary steps or elementary reactions

What is it? §

The sequence of elementary steps that leads to product formation is the reaction mechanism

This comes after experimental data has been collected

Example §

2NO (g) + O${}_2$ (g) $\to$ 2NO${}_2$ (g)

N${}_2$O${}_2$ is detected during the reaction

Elementary step: NO + NO $\to$ $\cancel{N_2{O}_2}$

+ Elementary step: \cancel{N_2O_2} + O$$_2 $\to$ 2NO${}_2$

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Overall reaction: 2NO + O${}_2$ -> 2NO${}_2$

Reaction Mechanisms §

• The balanced chemical equations provides information about the beginning and of rxn
• A rxn mechanism explains how reactants rearrange to form new products
• Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction
• Derived from experimental data

Reaction Intermediates §

Intermediates are species that appear in a reaction mechanism but not in the overall balanced equation

An intermediate is always formed in an early elementary step and consumed in a later elementary step

Shows up in the product then shows up in the reactants where its consumed

Elementary Step: NO + NO $\to$ N${}_2$O${}_2$

+ Elementary Step: $N_2{O}_2+O_2\to 2N{O}_2$

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Overall Reaction: 2NO + O${}_2$ $\to$ 2NO${}_2$

Rate Laws and Determining Steps §

Determining a plausible reaction mechanism

• The sum of the elementary steps must give the overall balanced equation for the reaction
• The rate-determining step should predict the same rate law that is determined experimentally

The rate-determining step is the slowest step in the sequence of steps leading to product formation

Integrated Rate Law

Zero Order: [A]${}_f$ = -kt + [A]${}_0$

First Order: ln[A]${}_f$ = -kt + ln[A]${}_0$

Second Order: $\frac{1}{[A{]}_F}$ = kt + $\frac{1}{[A{]}_0}$

Rate Laws and Elementary Steps §

Slow Elementary Step:

• Unimolecular reaction A $\to$ Products rate = k[A]; rate = k[A]
• Bimolecular reaction A + B $\to$ products; rate = k[A][B]
• Bimolecular reaction A + A $\to$ products; rate = k[A]${}^2$

BE CAREFUL!! We can only equate the coefficient and reaction order for a single elementary step

2 step Potential Energy Diagram §

Endothermic: two step reaction with rate determining 1st step

Exothermic: two step reaction with rate determining 1st step

• When first step has greater energy than 2nd step it means its exothermic