Thermochemistry and Energetics

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Thermochemistry and Energetics — Key Facts First law of thermodynamics: $\Delta U = q + w$; energy is conserved, not created or destroyed Enthalpy change $\Delta H = \Delta U + P\Delta V$; for reactions at constant pressure Hess’s law: $\Delta H_{rxn} = \sum \Delta H_f°(products) - \sum \Delta H_f°(reactants)$ Standard enthalpy of formation $\Delta H_f°$: enthalpy change when 1 mole forms from elements in standard states ⚡ Exam tip: Bond dissociation energy is always positive (endothermic to break bond); bond formation is always negative (exothermic)

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Thermochemistry and Energetics — JAMB Chemistry Study Guide

Types of enthalpy:

• $\Delta H_f°$: standard enthalpy of formation (elements → compound)
• $\Delta H_c°$: standard enthalpy of combustion (compound + O₂ → CO₂ + H₂O)
• $\Delta H_{diss}$: bond dissociation energy (breaking bonds)
• $\Delta H_{solution}$: enthalpy of solution
• $\Delta H_{atomisation}$: solid → gaseous atoms

Calorimetry: Heat $q = mc\Delta T$ where $m$ = mass, $c$ = specific heat capacity, $\Delta T$ = temperature change. For constant pressure calorimetry (coffee cup): $q_{rxn} = -q_{solution}$ For constant volume calorimetry (bomb calorimeter): $q_{rxn} = -\Delta U$ (no work done, so $\Delta U = q_v$)

Born-Haber cycle: Lattice energy = energy required to separate 1 mole of ionic solid into gaseous ions. $\Delta H_f° = \Delta H_{atomisation} + IE_1 + IE_2 + … + EA + U$ (with signs appropriate) Used to calculate lattice energy from measurable quantities.

Average bond enthalpy: Average energy to break a bond (per mole); can estimate $\Delta H_{rxn}$: $$\Delta H_{rxn} = \sum \Delta H_{bonds\ broken} - \sum \Delta H_{bonds\ formed}$$

Common student mistakes: Confusing $\Delta H$ and $\Delta U$ (for gases, $\Delta H = \Delta U + \Delta n_g \cdot RT$); forgetting signs for endothermic (+ΔH) vs exothermic (-ΔH); not using proper state symbols.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Thermochemistry and Energetics — Comprehensive Chemistry Notes

Born-Haber cycle for NaCl: $\Delta H_f°(NaCl) = \Delta H_{sub}(Na) + IE_1(Na) + \frac{1}{2}\Delta H_{diss}(Cl_2) + EA(Cl) + U$ Where:

• $\Delta H_{sub} = 107$ kJ/mol (Na solid → Na gas)
• $IE_1 = 496$ kJ/mol (Na → Na⁺ + e⁻)
• $\frac{1}{2}\Delta H_{diss} = \frac{1}{2}(243) = 122$ kJ/mol ($Cl_2 \rightarrow 2Cl$)
• $EA = -349$ kJ/mol ($Cl + e^- \rightarrow Cl^-$)
• $U = ?$ (lattice energy, calculated to be -788 kJ/mol) $\Delta H_f° = 107 + 496 + 122 - 349 - 788 = -412$ kJ/mol (observed: -411 kJ/mol)

Entropy: $\Delta S° = \sum S°(products) - \sum S°(reactants)$ Second law: $\Delta S_{universe} = \Delta S_{system} + \Delta S_{surroundings} > 0$ for spontaneous process. $\Delta S_{surroundings} = -\Delta H_{system}/T$ (at constant pressure and temperature)

Gibbs free energy: $\Delta G° = \Delta H° - T\Delta S°$

• $\Delta G° < 0$: spontaneous
• $\Delta G° = 0$: at equilibrium
• $\Delta G° > 0$: non-spontaneous

At equilibrium: $\Delta G = 0$, so $\Delta H = T\Delta S$ Also: $\Delta G° = -RT\ln K$ where $K$ is equilibrium constant.

Thermodynamic vs kinetic control: A reaction may be thermodynamically favoured ($\Delta G < 0$) but kinetically slow ($E_a$ high). A catalyst affects kinetics but not thermodynamics.

JAMB exam patterns:

• 2023 JAMB: Calculate $\Delta H$ for reaction $N_2 + 3H_2 \rightarrow 2NH_3$ given bond enthalpies
• 2022 JAMB: State Hess’s law and calculate $\Delta H°$ using standard enthalpies of formation
• 2021 JAMB: Predict whether $\Delta S$ is positive or negative for the reaction $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$
• 2020 JAMB: Why does spontaneity not depend only on enthalpy?

Bond dissociation energies (kJ/mol):

Bond	Energy	Bond	Energy
C-H	413	H-H	436
C-C	347	O=O	496
C=O	799	O-H	463
C≡O	1072	N≡N	945
C-Cl	339	Cl-Cl	243

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📊 JAMB Exam Essentials

Detail	Value
Questions	180 MCQs (UTME)
Subjects	4 subjects (language + 3 for course)
Time	2 hours
Marking	+1 per correct answer
Score	400 max (used for university admission)
Registration	January – February each year

🎯 High-Yield Topics for JAMB

• Use of English (Grammar + Comprehension) — 60 marks
• Biology for Science students — 40 marks
• Chemistry (Organic + Physical) — 40 marks
• Physics (Mechanics + Optics) — 35 marks
• Mathematics (Algebra + Geometry) — 40 marks

📝 Previous Year Question Patterns

• Q: “The process of photosynthesis requires…” [2024 Biology]
• Q: “The electronic configuration of Fe is…” [2024 Chemistry]
• Q: “Find the value of x if 2x + 5 = 15…” [2024 Mathematics]

💡 Pro Tips

• Use of English carries the most weight — master grammar rules and comprehension strategies
• JAMB syllabus is your Bible — questions come directly from it. Download and use it.
• Past questions are highly predictive — repeat patterns appear every year
• For Science students, Biology and Chemistry are high-scoring if you study NCERT-level content

🔗 Official Resources

• JAMB Official
• JAMB Syllabus

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