IP Chemistry Upper Sec 09: Chemical Energetics

These notes align with SEAB GCE O-Level Chemistry (6092) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Chemistry 6092 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for this note.

What you must know

• Enthalpy change is heat change at constant pressure; exothermic releases heat (ΔH < 0), endothermic absorbs heat (ΔH > 0).
• Energy profile diagrams must show relative energy of reactants/products, activation energy peak, and ΔH arrow.
• Bond breaking is endothermic; bond making is exothermic; overall ΔH is “energy to break − energy released making.”
• Qualitative bond-energy comparisons explain why combustion is strongly exothermic and why thermal decompositions need heating.

Detailed notes

• Sign convention: exothermic transfers heat to surroundings (temperature rises), ΔH negative; endothermic absorbs heat (temperature falls), ΔH positive.
• Calorimetry (qualitative at IP): measure mass of solution, temperature change; assume density 1 g/cm³ and specific heat capacity $4.18\ \text{J g}^{-1}\text{K}^{-1}$. Identify limiting reagent when expressing ΔH per mole.
• Hess’ idea (qualitative): total enthalpy change is path independent; can use known combustion/formation data to deduce unknown ΔH.