🔥 DAT Thermodynamics Simplified: ΔH, ΔS, and ΔG Explained

You’ll be asked to:

• Predict whether a reaction is spontaneous

• Use the ΔG = ΔH – TΔS equation

• Interpret signs of ΔH and ΔS

• Compare heat flow, disorder, and energy change

🔬 Key Thermodynamic Definitions

Symbol	Term	Meaning
ΔH	Enthalpy	Heat change at constant pressure
ΔS	Entropy	Measure of disorder or randomness
ΔG	Gibbs Free Energy	Energy available to do work

📈 The Gibbs Free Energy Equation

ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta SΔG=ΔH−TΔS

• T = temperature in Kelvin

• ΔH and ΔS must be in consistent units (usually kJ or J)

• Sign of ΔG tells you if the process is spontaneous

✅ Spontaneity Rules (DAT Gold)

ΔH	ΔS	ΔG at all T	Spontaneity
−	+	−	Always Spontaneous
+	−	+	Never Spontaneous
−	−	Depends on T	Spontaneous at Low T
+	+	Depends on T	Spontaneous at High T

📌 Tip: When ΔH and ΔS have the same sign, temperature becomes the deciding factor!

🧪 Enthalpy Trends to Know

• Exothermic: ΔH < 0 → Heat released (e.g. combustion)

• Endothermic: ΔH > 0 → Heat absorbed (e.g. melting ice)

🌀 Entropy Trends to Know

• Increases when:

  • Solids → liquids → gases

  • Particles increase (more moles on product side)

  • Systems become more disordered

🔁 DAT Tips & Common Questions

• Always convert °C → K for the equation (K = °C + 273)

• Watch for units mismatch (convert J ↔ kJ as needed)

• Look for physical change clues (e.g. melting, freezing, vaporizing)

• Choose “spontaneous” only if ΔG < 0

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✅ Summary

• Use ΔG = ΔH – TΔS to predict spontaneity

• Know sign relationships and how temperature affects ΔG

• Master unit conversions and thermodynamic trends

• Thermo appears in reaction prediction, phase changes, and bio/chem crossover Qs

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