Topic 1 · 6.1 · Free
Topic Notes · Easy summaries with diagrams
Chapter 6 — chapter 6
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📘Easy summary
The system is the part of the universe under study; the rest is the surroundings. Enthalpy (H) is the total heat content of a system at constant pressure (H = E + PV); it cannot be measured absolutely, only its change ΔH = H(products) − H(reactants). Exothermic reactions release heat to the surroundings (ΔH negative, surroundings warm up: combustion, neutralization). Endothermic reactions absorb heat from the surroundings (ΔH positive, surroundings cool: dissolving NH₄Cl, decomposition of CaCO₃, photosynthesis).
⭐Key points
- ΔH = H(products) − H(reactants).
- Exothermic: ΔH < 0.
- Endothermic: ΔH > 0.
🔬Diagram
🌿Real-life example
CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = −890 kJ mol⁻¹.
💡Exam tip
- Always give the sign of ΔH.
✅Quick check
1. Exothermic reaction has ΔH:
2. Enthalpy is heat content at constant:
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