SME
Grade 10 Science · Problem type
Does entropy go up or down?
Use this for “Predict the sign of ΔS when a gas forms from a solid.”
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Is ΔS positive or negative?
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Assumptions
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Entropy measures how spread out the particles and their energy are: gas, then liquid, then solid.
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More gas molecules after the arrow than before usually means entropy goes up.
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Related lessons
Entropy, free energy and spontaneity
Main lesson
The temperature where it turns spontaneous
Use this for “Above what temperature is a reaction with ΔH = 60 kJ/mol and ΔS = 150 J/(mol·K) spontaneous?”
ΔS° from a table, then ΔG°
Use this for “The products’ standard entropies add to 214 J/(mol·K) and the reactants’ to 188; ΔH° = −50 kJ/mol. What are ΔS° and ΔG° at 298 K?”
When is it spontaneous?
Use this for “A reaction has ΔH > 0 and ΔS > 0. When is it spontaneous?”