Reactivity 1.4 Flashcards

Active Recall for Entropy & Spontaneity

Flashcards: read all 10 questions and answers

Define entropy (S).
A measure of the number of possible arrangements (microstates) of particles and energy in a system. Units: J K⁻¹ mol⁻¹.
State the Third Law of Thermodynamics.
The entropy of a perfect crystal at absolute zero (0 K) is zero.
State the Second Law of Thermodynamics.
The entropy of the universe tends to increase. For a spontaneous process, ΔS(universe) > 0.
How do you calculate ΔS for a reaction?
ΔS = Σ S(products) − Σ S(reactants), using standard entropy values from the data booklet.
State the Gibbs energy equation.
ΔG = ΔH − TΔS, where T is in Kelvin.
What does ΔG < 0 mean?
The reaction is thermodynamically spontaneous. Note: this says nothing about the reaction rate.
When is a reaction spontaneous at ALL temperatures?
When ΔH < 0 (exothermic) and ΔS > 0 (entropy increase). ΔG is always negative.
When is a reaction NEVER spontaneous?
When ΔH > 0 (endothermic) and ΔS < 0 (entropy decrease). ΔG is always positive.
How do you find the crossover temperature?
Set ΔG = 0: T = ΔH / ΔS. Ensure both are in the same units (kJ or J).
Why is the unit conversion ΔS important?
ΔH is usually in kJ mol⁻¹ but ΔS is in J K⁻¹ mol⁻¹. You must divide ΔS by 1000 before substituting into ΔG = ΔH − TΔS.