What is Spontaneity and Equilibrium?
A spontaneous reaction occurs naturally without external energy input; it continues until equilibrium is reached, where forward and reverse reactions balance. Understanding spontaneity and equilibrium is central to predicting reaction behaviour.
Spontaneity is the tendency of a reaction to proceed naturally; equilibrium is the state where forward and reverse reaction rates are equal. ΔG < 0 means spontaneous; ΔG = 0 means equilibrium.
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Step-by-step worked examples
At 298 K, a reaction has ΔH = -60 kJ/mol and ΔS = 150 J/(mol·K). Is it spontaneous?
ΔG = ΔH - TΔS ΔG = -60 - 298 × (150/1000) ΔG = -60 - 44.7 = -104.7 kJ/mol ΔG < 0, so the reaction IS spontaneous.
A reaction with ΔH = 80 kJ/mol and ΔS = -50 J/(mol·K) at 300 K. Spontaneous?
ΔG = 80 - 300 × (-50/1000) ΔG = 80 + 15 = 95 kJ/mol ΔG > 0, so the reaction is NOT spontaneous.
Find the temperature at which ΔH = 40 kJ/mol and ΔS = 100 J/(mol·K) reach equilibrium.
At equilibrium, ΔG = 0 0 = 40 - T × (100/1000) T = 40 / 0.1 = 400 K
Flashcards
Quick quiz
Q1.A reaction with ΔG = -50 kJ/mol is…
Q2.At equilibrium, what is ΔG?
Q3.Which increases spontaneity most?
Q4.A reaction is non-spontaneous at 298 K but spontaneous at 600 K. ΔS must be…
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Common mistakes
Spontaneous = fast. — Correct: Spontaneous means thermodynamically favoured; kinetics are separate.
Equilibrium = reaction stops. — Correct: At equilibrium, forward and reverse reactions continue at equal rates.
ΔH < 0 always means spontaneous. — Correct: Only if ΔG < 0; entropy matters too.
Spontaneity doesn't depend on temperature. — Correct: ΔG = ΔH - TΔS shows T affects spontaneity.
FAQ
What is the difference between spontaneity and equilibrium?
Spontaneity (ΔG < 0) is whether a reaction occurs naturally. Equilibrium (ΔG = 0) is when forward and reverse rates balance.
Why does temperature affect spontaneity?
Because ΔG = ΔH - TΔS; the TΔS term grows with T, changing whether ΔG is positive or negative.
Can a spontaneous reaction be reversible?
Yes. A spontaneous reaction continues until equilibrium, then the reverse reaction can also occur.
What is the Gibbs free energy formula?
ΔG = ΔH - TΔS, where ΔG < 0 is spontaneous, ΔG = 0 is equilibrium, and ΔG > 0 is non-spontaneous.