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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)

The Arrhenius Equation HL Only

Specification Reference R2.2.12

Quick Notes:

  • The Arrhenius equation shows how the rate constant (k) depends on temperature (T) and activation energy (Ea).
  • As temperature increases, more particles have energy ≥ Ea meaining k increases (and rate increases).
  • Arrhenius equation:
    IB Chemistry Arrhenius equation showing k = Ae^-Ea/RT
  • Linear form (used for graphing):
    IB Chemistry linear form of Arrhenius equation ln k = -Ea/R (1/T) + ln A
  • A graph of ln k vs 1/T gives a straight line:
    • Slope = –Ea / R
    • Intercept = ln A

Full Notes:

The Arrhenius Equation, how to rearrange it and using Arrhenius plots has been covered in much more detail at this page and this video. What is on this page and here is just the essentials you need to know for IB Chemistry.

Note: You don’t need to remember the Arrhenius equation - it will be provided in the data booklet in your exams.

The Arrhenius equation shows how activation energy (Ea), temperature (T) and the proportion of collisions with correct orientation (A) can be linked together by the rate constant, k.

IB Chemistry Arrhenius equation showing relationship between rate constant, activation energy, temperature, and Arrhenius constant.

Where:

This also shows why k increases with temperature as the e-Ea/RT part of the expression gets bigger as T increases.

Linear Form of the Arrhenius Equation

The Arrhenius equation is exponential because it contains ‘e’. To make it easier to work with, we rearrange it to a straight line form by taking natural logs:

IB Chemistry logarithmic form of the Arrhenius equation ln k = ln A - Ea/RT

This can be rearranged to follow the y = mx + c function of a straight line, where:

IB Chemistry straight line equation form for Arrhenius plots.

Arrhenius Plots

Plotting ln k vs. 1/T gives a straight line, following the y=mx+c form of the Arrhenius Equation.

Because of this, the gradient can be used to calculate activation energy:

IB Chemistry Arrhenius plot showing ln k against 1/T with slope -Ea/R.

Summary