Open Question
Which of the following processes are spontaneous?
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19. Chemical Thermodynamics
Spontaneous vs Nonspontaneous Reactions
Multiple Choice
For a reaction to be spontaneous at all temperatures, the signs of ΔH and ΔS must be:
A
ΔH +; ΔS −
B
ΔH −; ΔS −
C
ΔH −; ΔS +
D
ΔH +; ΔS +
Verified step by step guidance1
Understand the concept of spontaneity in chemical reactions: A reaction is spontaneous if it occurs without needing to be driven by an external force. The spontaneity of a reaction is determined by the Gibbs free energy change (ΔG).
Recall the Gibbs free energy equation: ΔG = ΔH - TΔS, where ΔG is the change in Gibbs free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
For a reaction to be spontaneous at all temperatures, ΔG must be negative. Analyze the equation ΔG = ΔH - TΔS: If ΔH is negative (exothermic reaction) and ΔS is positive (increase in disorder), then ΔG will be negative regardless of the temperature.
Consider the implications of ΔH and ΔS: A negative ΔH indicates that the reaction releases energy, making it favorable. A positive ΔS suggests an increase in disorder, which is also favorable for spontaneity.
Conclude that for a reaction to be spontaneous at all temperatures, ΔH should be negative and ΔS should be positive, as this combination ensures that ΔG is negative under any temperature condition.
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