🧮 Understanding Units in ΔG

Don’t let units wreck your answer.

🎯 1. Why ΔG Units Trip People Up

Plugging into formulas like ΔG = ΔH - TΔS or ΔG = -nFE… and BAM — a unit mismatch ruins everything.

👉 This explainer will help you:

• Know which unit ΔG is usually in
• When to convert J ↔️ kJ
• How to avoid mismatches and save your grade

📚 2. ΔG = ΔH - TΔS (Thermodynamics)

⚠️ The Common Trap:

• ΔH (enthalpy) is usually in kJ/mol
• ΔS (entropy) is often in J/mol·K — watch out!

🧠 Units must match before calculating ΔG. Entropy is usually in joules (J), but enthalpy is in kilojoules (kJ). If you skip converting, you're subtracting apples from oranges — and your final ΔG will be meaningless.

Why the mismatch? Entropy values tend to be smaller and are traditionally recorded in J/mol·K — partly due to historical data conventions. But energy (ΔH and ΔG) is usually reported in kJ/mol to avoid huge numbers.

❌ Mistake Example:

ΔH = -100 kJ/mol
ΔS = 200 J/mol·K
T = 298 K

ΔG = -100 – (298)(200) = 🚫 nonsense

✅ Fixed Version:

Convert ΔS → 0.200 kJ/mol·K
ΔG = -100 – (298)(0.200) =
ΔG = -159.6 kJ/mol

⚡ 3. ΔG = -nFE (Electrochemistry)

🧪 What the symbols mean:

• n = moles of electrons
• F = 96,485 C/mol (Coulombs per mole of e⁻)
• E = Voltage (V = J/C)

All units resolve to Joules (J) by default!
Here's why: Voltage is in volts (V), which means joules per coulomb (J/C). Multiply that by Faraday's constant (C/mol), and the coulombs cancel — leaving you with J/mol.

✅ Example:

ΔG = -(2)(96,485)(1.10 V)
ΔG = -212,267 J
Optional (only if asked): Convert to kJ → -212.3 kJ

🔁 J is correct — only convert if the question asks for kJ!

🔍 4. Quick Reference Table

Formula	Default Unit	Need to Convert Units?
ΔG = ΔH - TΔS	kJ	✅ Yes — match units
ΔG = -nFE	J	❌ No — unless needed

💡 5. Pro Tips & Memory Tricks

🧠 “Entropy is usually in J, Enthalpy in kJ — but ΔG only works if you bring them to the same unit.”

⚡ “If Faraday’s involved, you’re in Joules — because volts = J/C.”

✅ This lesson is flashcard-ready

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