Can you show a worked example?

ΔH = −100 kJ/mol and ΔS = −0.2 kJ/(mol·K). Is it spontaneous at 600 K? ΔG = +20 kJ/mol, so it is not spontaneous at 600 K (it is below 500 K).

Gibbs free energy and spontaneity

Use ΔG = ΔH − TΔS — exothermic alone doesn't make a reaction spontaneous.

Appears onAP Chem

Assuming every exothermic reaction is spontaneous, without checking entropy and temperature.

Instead: Spontaneous means ΔG < 0, where ΔG = ΔH − TΔS. If ΔH is negative but ΔS is also negative, the reaction only goes at low enough temperature.

ΔH = −100 kJ/mol and ΔS = −0.2 kJ/(mol·K). Is it spontaneous at 600 K?

Sugar · What is TΔS at 600 K?
Student · 600 × −0.2 = −120 kJ/mol.
Sugar · Now ΔG = ΔH − TΔS. What do you get?
Student · −100 − (−120) = +20 kJ/mol.
Sugar · Is a positive ΔG spontaneous?
Student · No.

Answer: ΔG = +20 kJ/mol, so it is not spontaneous at 600 K (it is below 500 K).

1. ΔH < 0 and ΔS > 0. Spontaneous at which temperatures?
Your answer
2. ΔH > 0 and ΔS < 0. Spontaneous at which temperatures?
Your answer
3. ΔH = +50 kJ/mol, ΔS = +0.1 kJ/(mol·K). Above what T is it spontaneous?
Your answer

What is the most common mistake with gibbs free energy and spontaneity?: Assuming every exothermic reaction is spontaneous, without checking entropy and temperature.
How do I get gibbs free energy and spontaneity right?: Spontaneous means ΔG < 0, where ΔG = ΔH − TΔS. If ΔH is negative but ΔS is also negative, the reaction only goes at low enough temperature.
Can you show a worked example?: ΔH = −100 kJ/mol and ΔS = −0.2 kJ/(mol·K). Is it spontaneous at 600 K? ΔG = +20 kJ/mol, so it is not spontaneous at 600 K (it is below 500 K).

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