← Bonding, structure, properties and energy changes

Energy changes: enthalpies & profiles

ΔH = broken − formed; label profiles fully. Exam-style H₂ + Cl₂ → 2HCl H–H 436, Cl–Cl 243, H–Cl 431 kJ mol⁻¹ Broken: 436 + 243 = 679 Formed: 2 × 431 = 862 ΔH = 679 − 862 = −183 kJ mol⁻¹ ✓ Exothermic — products sit lower on the profile. ℹ

Profile marks

Axes (energy vs progress), reactants, products, ΔH arrow, E_a to the peak.

What a labelled diagram must show, feature by feature

• Both axes labelled (energy on the vertical, reaction progress on the horizontal) — an unlabelled sketch of the right shape earns fewer marks than it looks like it should.
• A horizontal line or point for the reactants' energy level, and a separate one for the products' energy level.
• An arrow (usually vertical) between the two levels, labelled ΔrH, with its direction matching the sign (down for exothermic, up for endothermic).
• For a rate-of-reaction context specifically, the activation energy Eₐ is a SEPARATE arrow from the reactants' level up to the peak of the curve — do not confuse it with ΔrH.

Practice for this standard covers the whole topic — flashcards and quiz.