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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

5.1.1 How fast?

Effect of temperature on rate constants, Arrhenius EquationOrders, rate equations and rate constantsRate-determining stepRate graphs and orders

Rate-determining step

Specification Reference 5.1.1 (i)

Quick Notes

  • The rate equation is determined experimentally (found using experimental data).
  • General form:
    Rate = k [A]m [B]n
    where m and n (orders of reaction) must be found experimentally.
  • If we know the orders with respect to each reactant, we may be able to predict how the reaction occurs via a proposed mechanism.
  • Rate-determining step (RDS): The slowest step in a mechanism. Only reactants involved this step appear in the rate equation.

Full Notes

See rates and rate equations for essential background theory to this page.

Rate-Determining Step (RDS) and Reaction Mechanisms

Most reactions happen in multiple steps, with each step occuring at different rates.

The slowest step is the Rate-Determining Step (RDS). Only species involved the RDS appear in the rate equation.

Worked Example

Reaction: NO2 + CO → NO + CO2

The rate equation is Rate = k [NO2]2. Since CO does not appear in the rate equation, it is not involved in the rate-determining step (RDS).

Possible Mechanism:

  • NO2 + NO2 → NO3 + NO (slow)
  • NO3 + CO → NO2 + CO2 (fast)

Step 1 is the slow step (RDS), explaining why CO does not appear in the rate equation.


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Matt’s exam tip

Remember that mechanisms are proposed using rate equations. They are only predictions — there may be more than one possible mechanism for a given rate equation.

Summary