🔥 Enthalpy – ΔH, Bond Energies & Reaction Heat Flow
Enthalpy (ΔH) measures the heat content of a system at constant pressure. It’s central to understanding:
Reaction energetics
Bond strength
Phase changes
Thermodynamic equations
The DAT tests your ability to calculate, interpret, and compare ΔH values—both conceptually and numerically.
🔥 Understanding ΔH Values
| ΔH Value | Meaning | Reaction Type |
|---|---|---|
| ΔH < 0 | Heat released | Exothermic |
| ΔH > 0 | Heat absorbed | Endothermic |
| ΔH = 0 | No net heat change | Isoenthalpic |
✅ Exothermic reactions feel hot; endothermic reactions feel cold.
🧮 Enthalpy from Bond Energies
ΔH = ∑ Bonds Broken − ∑ Bonds Formed
Bond breaking = energy input (+)
Bond forming = energy release (−)
✅ Use standard bond enthalpy tables to calculate net ΔH.
🧪 Heat of Formation (ΔHf⁰)
ΔH = ∑ ΔHf∘(products) − ∑ ΔHf∘(reactants)
ΔHf⁰ of elements in standard state = 0
Use for Hess's Law and thermodynamic cycles
✅ Use balanced equations and include coefficients in calculations.
♻️ Enthalpy in Physical Processes
| Process | ΔH Sign | Example |
|---|---|---|
| Melting (fusion) | Positive | Ice → Water |
| Condensation | Negative | Steam → Water |
| Vaporization | Positive | Water → Steam |
| Freezing | Negative | Water → Ice |
🧠 Phase changes at constant T still involve enthalpy changes (latent heat).
📈 DAT-Style Practice Question
Q: Which of the following reactions is exothermic?
A) ΔH = +285 kJ
B) ΔH = 0
✅ C) ΔH = −196 kJ
D) ΔH = +140 kJ
🎯 DAT Strategy Tips
✅ Memorize exothermic vs endothermic ΔH signs
✅ Understand bond breaking = positive ΔH
✅ Know that ΔHf⁰ for elements = 0
✅ Practice Hess’s Law with equation flipping and summing
Review energy diagrams and formulas at kingofthecurve.org/studyscience
🚀 Call-to-Action
Want to bond with better DAT scores?
Download the KOTC App and get:
Bond energy flashcards
Enthalpy problem sets
Reaction pathway diagrams
Curve Coins for heat map mastery
🔥 Bring the heat—master ΔH with KOTC.
Frequently Asked Questions (FAQs)
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Aim for 4-6 focused hours, ensuring you incorporate breaks to avoid burnout.
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Practice mindfulness techniques, take practice exams under realistic conditions, and maintain a balanced lifestyle.
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Set short-term goals, seek support from mentors, and reward yourself for small achievements.
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Regular exercise improves focus, reduces stress, and enhances overall mental clarity.
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KOTC offers personalized learning tools, gamification features, and adaptive question banks to help students stay on track without burnout.
