IB Chemistry Topic R2.2.6 Reaction Mechanisms and Rate-determining Steps Revision Notes (HL)

IB Chemistry Topic R2.2.6 Reaction Mechanisms and Rate-determining Steps Revision Notes (HL) | RevisionDojo

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Reactions Happen in a Series of Elementary Steps

Definition

Elementary step

An elementary step is a single molecular event in a reaction mechanism where reactants are converted into products or intermediates.

An elementary step is a single collision or transformation of particles. It cannot be broken down into simpler steps.

Example

Consider the reaction: $$2NO_2(g) + F_2(g) \to 2NO_2F(g)$$
It occurs in two elementary steps: $$NO_2(g) + F_2(g) \to NO_2F(g) + F(g) $$ $$ NO_2(g) + F(g) \to NO_2F(g)$$
Adding the two steps gives the overall equation, so the mechanism is consistent with it.

Definition

Intermediate

An intermediate is a species that is formed in one elementary step and consumed in another. It is not present in the overall reaction equation because it does not appear in the final products or reactants.

Note

In the mechanism above, $F(g)$ is an intermediate: it is made in step 1 and used up in step 2, so it does not appear in the overall equation.
The number of particles reacting in a single step is its molecularity, covered in R2.2.8.
The energy changes across the steps are shown on an energy profile, covered in R2.2.7.

The Rate-Determining Step Controls the Overall Rate

In a multistep reaction, the steps rarely happen at the same speed.

Definition

Rate-determining step

The slowest step in the mechanism is called the rate-determining step (RDS).

The rate-determining step has the highest activation energy of all the steps.

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Reactions Happen in a Series of Elementary Steps

Definition

Elementary step

An elementary step is a single molecular event in a reaction mechanism where reactants are converted into products or intermediates.

An elementary step is a single collision or transformation of particles. It cannot be broken down into simpler steps.

Example

Consider the reaction: $$2NO_2(g) + F_2(g) \to 2NO_2F(g)$$
It occurs in two elementary steps: $$NO_2(g) + F_2(g) \to NO_2F(g) + F(g) $$ $$ NO_2(g) + F(g) \to NO_2F(g)$$
Adding the two steps gives the overall equation, so the mechanism is consistent with it.

Definition

Intermediate

Note

In the mechanism above, $F(g)$ is an intermediate: it is made in step 1 and used up in step 2, so it does not appear in the overall equation.
The number of particles reacting in a single step is its molecularity, covered in R2.2.8.
The energy changes across the steps are shown on an energy profile, covered in R2.2.7.

The Rate-Determining Step Controls the Overall Rate

In a multistep reaction, the steps rarely happen at the same speed.

Definition

Rate-determining step

The slowest step in the mechanism is called the rate-determining step (RDS).

The rate-determining step has the highest activation energy of all the steps.

Definition

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R2.2.6 Reaction Mechanisms and Rate-determining Steps Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Reactions Happen in a Series of Elementary Steps

The Rate-Determining Step Controls the Overall Rate

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Reactions Happen in a Series of Elementary Steps

The Rate-Determining Step Controls the Overall Rate

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