Multiple Choice
The process of iron being oxidized to make iron(III) oxide (rust) is spontaneous. Which of these statements about this process is true?
19. Chemical Thermodynamics
Spontaneous vs Nonspontaneous Reactions
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What can be said about an endothermic reaction with a negative entropy change in terms of spontaneity?
A
The reaction is nonspontaneous at all temperatures.
B
The reaction is spontaneous at low temperatures.
C
The reaction is spontaneous at all temperatures.
D
The reaction is spontaneous at high temperatures.
Verified step by step guidance1
Understand the concepts: An endothermic reaction absorbs heat, meaning the change in enthalpy (ΔH) is positive. A negative entropy change (ΔS) indicates a decrease in disorder.
Recall the Gibbs free energy equation: ΔG = ΔH - TΔS, where ΔG is the change in free energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
Determine spontaneity: A reaction is spontaneous if ΔG is negative. For an endothermic reaction (ΔH > 0) with a negative entropy change (ΔS < 0), both terms in the Gibbs free energy equation contribute positively to ΔG.
Analyze temperature effects: Since both ΔH and -TΔS are positive, increasing temperature (T) will increase the positive contribution of -TΔS, making ΔG more positive, thus less likely to be spontaneous.
Conclude the spontaneity: Given that both ΔH and -TΔS are positive, the reaction is nonspontaneous at all temperatures because ΔG will always be positive.
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