🔥 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.

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Frequently Asked Questions (FAQs)

• How many hours should I study for the MCAT each day?

  Aim for 4-6 focused hours, ensuring you incorporate breaks to avoid burnout.

• What is the best way to handle test anxiety?

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