AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R2.2 - How fast? The rate of chemical change

2.2.1 Rate of Reaction 2.2.2 Collision Theory 2.2.3 Factors Affecting Reaction Rate 2.2.4 Activation Energy and Temperature 2.2.5 Catalyst and Activation Energy 2.2.6 Reaction Mechanism and Intermediates (AHL) 2.2.7 Energy Profile and Rate Determining Step (AHL) 2.2.8 Molecularity in Reaction Mechanism (AHL) 2.2.9 Rate Equations and Experimental Data (AHL) 2.2.10 Reaction Orders and Graphs (AHL) 2.2.11 Rate Constant, K (AHL) 2.2.12 Arrhenius Reaction and Temperature (AHL) 2.2.13 Arrhenius Factor and Activation Energy (AHL)

Reaction Mechanisms and Rate-Determining Step HL Only

Specification Reference R2.2.6

Quick Notes

  • Reactions often occur in a series of elementary steps, not all at once.
  • The slowest step is the rate-determining step (RDS).
  • Reaction intermediates are formed in one step and used in a later one.
  • Transition states are high-energy states between reactants and products that don't exist for a measurable amount of time.
  • Energy profiles can show multiple steps, each with a peak (transition state).
  • Mechanisms must be consistent with:
    • Kinetic data (rate equation)
    • Stoichiometry (overall balanced equation)
  • The RDS is not always the first step.

Full Notes

Many chemical reactions don’t happen in one step, they occur through a sequence of elementary steps:

IB Chemistry schematic showing a reaction mechanism as a sequence of elementary steps that sum to the overall reaction.

Each step involves the breaking or forming of a small number of bonds. The overall reaction is the sum of all elementary steps that occur.

How the steps link together is referred to as the ‘reaction mechanism’.

Rate-Determining Step (RDS)

Each step in a multi-step reaction will have its own rate. As a result, it is the slowest elementary step in a mechanism that limits the overall reaction rate.

This slowest step is called the rate determining step (RDS).

IB Chemistry diagram highlighting the slowest elementary step as the rate-determining step in a mechanism.

A reaction can only go as fast as the rate determining step in its mechanism.

The rate determining step also determines the rate equation (see 2.2.9 for more detail).

A species will only appear in the rate equation if it is part of (or influences) the RDS.

Intermediates vs. Transition States

Understanding the difference between intermediates and transition states is crucial for interpreting reaction mechanisms and energy profiles.

Intermediates:

Intermediates are formed during a reaction and exist for a measurable amount of time before turning into products. They do not appear in the overall equation.

IB Chemistry example of a reaction intermediate that forms in one step and is consumed in a subsequent step.

Transition states:

Transition states are high-energy, unstable configurations that arise during bond breaking/forming. They don’t actually exist for any measurable amount of time - a bit like imagining a ball as its thrown up in the air, at its maximum height.

IB Chemistry SN2 transition state diagram showing the high-energy configuration at the top of the energy barrier.

Transition states are represented as peaks on an energy profile (see below).

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Matt’s exam tip

Key distinction: Intermediates are real, though short-lived whereas transition states are theoretical high energy configurations - they can't be isolated.

Energy Profile with Multiple Steps

A multi-step reaction will have multiple peaks in its energy profile diagram:

IB Chemistry multi-step energy profile with multiple peaks for transition states and valleys for intermediates; highest peak corresponds to the RDS.

The highest peak corresponds to the rate determining step (RDS), as it has the largest energy barrier (Ea).

Matching Mechanisms to Data

Mechanisms are predicted based on experimental data and a proposed mechanism must:

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Matt’s exam tip

Remember that mechanisms are proposed using experimental kinetic and stoichiometric data. They are only predictions, there may be more than one possible mechanism for a given reaction.

Rate-Determining Step (RDS) Beyond Step One

In many mechanisms, the slowest step (rate-determining step) occurs after the first step, often involving an intermediate formed earlier.

Example: Reaction of NO2 and CO

Overall equation:

NO2 + CO → NO + CO2

Mechanism:

The second step is the RDS, even though it's not first. The intermediate NO3 is formed in step 1 and consumed in step 2. Since step 2 is the slow step, the rate depends on the concentrations of the reactants involved in (or before) that step.

Summary

Reactivity 3.4 — Linked Course Question

Which mechanism in the hydrolysis of halogenoalkanes involves an intermediate?

The SN1 mechanism (unimolecular nucleophilic substitution) involves a carbocation intermediate.

IB Chemistry SN1 mechanism showing stepwise formation of a carbocation intermediate followed by nucleophilic attack.

In this mechanism, the halogenoalkane first loses the halide ion in a slow step, forming a positively charged carbocation. This is followed by a fast attack by a nucleophile (e.g. water or OH) on the intermediate. SN1 is typical for tertiary halogenoalkanes, where the carbocation is stabilized by surrounding alkyl groups.

The SN2 mechanism (bimolecular) occurs in only one step, with no intermediate formed.

IB Chemistry SN2 mechanism showing single-step backside attack with a pentacoordinate transition state and no intermediate.