Thermochemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Thermochemistry — Key Facts for WAEC

Definitions:

• Thermochemistry studies heat changes accompanying chemical reactions.
• Enthalpy (H) is the total heat content of a system at constant pressure.
• Standard enthalpy of reaction (ΔH°) is the enthalpy change when moles of reactants in their standard states react to form products in their standard states.
• Exothermic reactions release heat to the surroundings (ΔH = negative, e.g., combustion).
• Endothermic reactions absorb heat from the surroundings (ΔH = positive, e.g., decomposition of calcium carbonate).

Key Equations:

• ΔH = H_products − H_reactants
• ΔH = Σ bonds broken − Σ bonds formed (approximate)
• Heat gained/lost: q = mcΔT (where m = mass, c = specific heat capacity, ΔT = temperature change)

Common ΔH Values to Memorise for WAEC:

• Enthalpy of combustion of methanol: −726 kJ mol⁻¹
• Enthalpy of neutralisation (strong acid + strong alkali): −57.3 kJ mol⁻¹
• Enthalpy of dissolution of ammonium chloride: +15.5 kJ mol⁻¹

⚡ WAEC Exam Tip: In data interpretation questions, always check the sign carefully. A negative ΔH means exothermic. Watch whether the question asks for the enthalpy change of reaction or the enthalpy change of formation. Also, note the state symbols — H₂O(l) and H₂O(g) have different enthalpy values.

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Thermochemistry — Chemistry Study Guide

Enthalpy of Reaction

The enthalpy change for a reaction can be measured experimentally using calorimetry. In a simple school-laboratory setup, a known mass of water is used as the heat sink, and the temperature rise is recorded when a reaction is carried out.

Calorimetry Calculations: A student burns 0.50 g of methanol (M = 32 g mol⁻¹) in a spirit burner and heats 200 cm³ of water from 25°C to 60°C. Calculate the enthalpy of combustion of methanol.

• Moles of methanol = 0.50/32 = 0.0156 mol
• Heat absorbed by water = mcΔT = 200 × 4.2 × 35 = 29,400 J = 29.4 kJ
• ΔH_c = −29.4/0.0156 = −1,885 kJ mol⁻¹ (experimental value — less exothermic than the standard value due to heat losses)

Hess’s Law: Hess’s Law of Constant Heat Summation states that the total enthalpy change for a reaction is independent of the route taken. This allows us to calculate enthalpy changes that cannot be measured directly.

Example — Calculate the enthalpy of formation of methane: C(s) + 2H₂(g) → CH₄(g) ΔH_f = ?

Given:

• C(s) + O₂(g) → CO₂(g) ΔH = −393.5 kJ mol⁻¹
• 2H₂(g) + ½O₂(g) → H₂O(l) ΔH = −285.8 kJ mol⁻¹
• CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = −890.3 kJ mol⁻¹

Applying Hess’s Law (formation enthalpy = sum of combustion enthalpies of reactants minus combustion enthalpy of product): ΔH_f = [−393.5 + 2(−285.8)] − [−890.3] ΔH_f = −965.1 + 890.3 = −74.8 kJ mol⁻¹

Bond Enthalpies: Average bond enthalpy is the energy required to break one mole of bonds in gaseous molecules. Breaking bonds is endothermic (+); forming bonds is exothermic (−).

ΔH ≈ Σ (bond enthalpies of bonds broken) − Σ (bond enthalpies of bonds formed)

⚡ Common Mistakes to Avoid:

• Confusing enthalpy of formation with enthalpy of combustion
• Forgetting that standard enthalpy values are per mole of reaction as written
• Neglecting to include state symbols when comparing enthalpy values
• In Hess’s law cycles, forgetting to multiply enthalpy values when the stoichiometric coefficient is greater than 1

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Thermochemistry — Comprehensive Chemistry Notes

Detailed Treatment of Enthalpy Concepts

Born-Haber Cycles: Born-Haber cycles are a specific application of Hess’s law used to calculate lattice enthalpy (the energy released when one mole of an ionic compound forms from its gaseous ions). The cycle relates lattice enthalpy to enthalpy of formation, atomisation enthalpies, ionisation energies, electron affinity, and bond dissociation enthalpies.

For sodium chloride: ΔH_f = ΔH_atom(Na) + ½ΔH_dissociation(Cl₂) + IE(Na) + EA(Cl) + ΔH_lattice

Enthalpy of Solution and Lattice Energy: The enthalpy of solution has two components: lattice enthalpy (endothermic, energy needed to separate ions) and hydration enthalpy (exothermic, energy released when ions are hydrated). If lattice enthalpy > hydration enthalpy, the overall process is endothermic and the solute may not dissolve readily.

Entropy (S): Entropy is a measure of disorder or randomness. The second law of thermodynamics states that the total entropy of the universe always increases for a spontaneous process. Gibbs free energy (G) combines enthalpy and entropy:

ΔG = ΔH − TΔS

A reaction is spontaneous when ΔG is negative.

Rate vs. Thermodynamic Control: A reaction may be thermodynamically favourable (negative ΔG) but kinetically slow (high activation energy). Thermochemistry tells us about energy changes, not the speed of reaction.

WAEC Past Question Patterns:

• Calculations using q = mcΔT with specific heat capacity of water (4.2 J g⁻¹ K⁻¹)
• Hess’s law cycles presented as energy level diagrams
• Interpreting Born-Haber cycles with labelled arrows
• Explaining endothermic vs. exothermic changes in context (e.g., Instant cold packs, hand warmers)
• Bond energy calculations for enthalpy of reaction
• Predicting sign of ΔS and its effect on spontaneity

⚡ WAEC-Specific Exam Tips:

1. In calculation questions, always write the thermochemical equation alongside your working.
2. Watch for “per mole of reactant” vs. “per mole of product” — read the question carefully.
3. If a value isn’t given in the question, use the standard values from your data booklet.
4. Energy level diagrams: label the axes, mark reactants and products, and indicate ΔH with a double-headed arrow.
5. When answering “explain why” questions about exothermic reactions, mention that bond formation releases more energy than bond breaking, and that the heat released increases the temperature of the surroundings.
6. Lattice enthalpy cannot be measured directly — be ready to construct or interpret a Born-Haber cycle from given data.

---

Content adapted based on your selected roadmap duration. Switch tiers using the pill selector above.