6.2 Energy Diagrams

Energy Diagrams

A two-level energy diagram redraws for 8 physical and chemical processes as the reactant and product bars snap to their kJ/mol levels. Zoom or reverse to watch ΔH flip sign.

2-Level Diagram8 ProcessesΔH SignPhysical + Chemical
Topic 6.2

Energy Diagrams

Represent a chemical or physical transformation with an energy diagram.

A physical or chemical process can be described with an energy diagram that shows the endothermic or exothermic nature of that process.

The form used in Unit 6 is simpler than the reaction energy profile of Unit 5. It shows only two horizontal levels — reactants and products — on an energy axis, with an arrow for ΔH:

  • Products below reactants → the system lost energy → exothermic, ΔH < 0.
  • Products above reactants → the system gained energy → endothermic, ΔH > 0.

Requirements for a correct diagram: a labeled vertical axis (energy or enthalpy, with units), labeled reactant and product levels, and an arrow with the magnitude and sign of ΔH indicated. Some questions also ask for chemical formulas on each level.

Relation to Unit 5. A Unit 5 reaction energy profile adds the activation-energy hump between the two levels. Both diagrams show the same ΔH; only the profile shows the barrier. Be careful not to add an unrequested activation hump when the question asks only for an energy diagram — and be equally careful not to omit it when the question is about kinetics.

Phase changes work identically. H₂O(l) → H₂O(g) is endothermic, so the gas level sits above the liquid level, with ΔH = ΔHvap.

Key points

  • Products lower than reactants = exothermic; higher = endothermic.
  • ΔH is a difference between levels, always products minus reactants.
  • An energy diagram shows no information about rate — that requires an activation-energy barrier.
  • Label the axis, both levels, and the ΔH arrow to earn full credit.

Equations

  • not on the sheet

Common mistakes

  • Reversing the subtraction. Always products minus reactants.
  • Adding an activation hump when it was not asked for, or omitting it when it was.
  • Unlabeled axes. A common source of lost points on sketch questions.

Worked example

Sketch and describe an energy diagram for the combustion of methane, ΔH = −890 kJ/mol, and for the vaporization of water, ΔH = +40.7 kJ/mol.

Combustion of methane. Vertical axis labeled "Enthalpy (kJ/mol)". Upper level labeled CH₄(g) + 2 O₂(g). Lower level, 890 kJ/mol below it, labeled CO₂(g) + 2 H₂O(l). A downward arrow between them labeled ΔH = −890 kJ/mol. Products are lower, so energy was released to the surroundings: exothermic.

Vaporization of water. Lower level labeled H₂O(l); upper level 40.7 kJ/mol above it labeled H₂O(g). An upward arrow labeled ΔH = +40.7 kJ/mol. Products are higher, so energy was absorbed from the surroundings: endothermic.

Note the very different magnitudes: 890 vs 40.7 kJ/mol. Breaking and forming covalent bonds (chemical change) costs far more than overcoming hydrogen bonds (physical change).

Full notes for topic 6.2 →