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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

R1.1 - Measuring enthalpy changes

1.1.1 Energy Transfer 1.1.2 Endothermic and Exothermic 1.1.3 Energy Profile 1.1.4 Enthalpy Change

Energy Profiles for Endothermic and Exothermic Reactions

Specification Reference R1.1.3

Quick Notes:

  • Exothermic reactions: products are more stable (lower potential energy) than reactants.
  • Endothermic reactions: products are less stable (higher potential energy) than reactants.
  • Energy profile diagrams:
    • Y-axis: potential energy
    • X-axis: reaction coordinate (progress of reaction)
  • The difference in height between reactants and products = enthalpy change (ΔH).
  • The activation energy is the energy required to start the reaction — shown as the peak of the curve.

Full Notes:

Sketching Energy Profile Diagrams

Energy profiles are graphs used to visualise energy changes during a reaction.

Axes:
Y-axis: potential energy (units often in kJ)
X-axis: reaction coordinate (represents progress of reaction)

Exothermic Reaction

IB Chemistry energy profile diagram for an exothermic reaction showing reactants at higher potential energy than products, with ΔH negative and a peak indicating activation energy.

Reactants start high, products end lower.

ΔH is negative.

A downward slope from reactants to products.

Endothermic Reaction

IB Chemistry energy profile diagram for an endothermic reaction showing reactants at lower potential energy than products, with ΔH positive and a peak indicating activation energy.

Reactants start low, products end higher.

ΔH is positive.

An upward slope from reactants to products.

Both profiles include a hump representing the activation energy (Ea) – the minimum energy required to initiate the reaction.

Key Labels on the Diagram

Label Meaning Where it appears on the diagram
Reactants Starting potential energy level Left plateau at the beginning
Products Final potential energy level Right plateau at the end
ΔH Enthalpy change (products − reactants) Vertical gap between product and reactant levels
Ea Activation energy Vertical rise from reactants to the peak
Reaction coordinate Progress of reaction X-axis
Potential energy Relative energy of system Y-axis

Summary Table

Reaction type Relative energies Sign of ΔH Profile trend Activation energy (Ea)
Exothermic Products < Reactants Negative Overall downward Peak above reactants
Endothermic Products > Reactants Positive Overall upward Peak above reactants
Structure 2.2 – Linked Course Question

Why is the combustion of N2 endothermic, unlike most exothermic combustion reactions?

Most combustion reactions are exothermic because they form strong bonds in the products. However, nitrogen gas (N2) has an extremely strong triple bond, which requires a large amount of energy to break. In nitrogen combustion, more energy is needed to break N≡N than is released when new bonds form (e.g. in NO or NO2), making the overall process endothermic.

Summary