Reaction P has ΔH<0 and ΔS>0 while reaction Q has ΔH>0 and ΔS>0.
Which of the following is true?Both P and Q are spontaneous at all temperatures because entropy
increases.
P is spontaneous at all temperatures but Q is spontaneous only at
high temperatures.
P is spontaneous only at high temperatures but Q is spontaneous at all
temperatures.
Both P and Q are spontaneous only at high temperatures
Reaction P (ΔH < 0, ΔS > 0) is spontaneous at all temperatures, while reaction Q (ΔH > 0, ΔS > 0) is spontaneous only at high temperatures. The correct answer is: P is spontaneous at all temperatures but Q is spontaneous only at high temperatures.
Gibbs Free Energy Basics
Spontaneity depends on ΔG < 0, where ΔG = ΔH – TΔS. Negative ΔH favors spontaneity by releasing energy, while positive ΔS favors it by increasing disorder. Temperature (T) amplifies the TΔS term, shifting outcomes for certain sign combinations.
Reaction P: ΔH < 0, ΔS > 0
Exothermic (ΔH negative) with entropy increase (ΔS positive) makes ΔG always negative: ΔH pulls it negative, and -TΔS adds more negativity at any T. Thus, P proceeds spontaneously regardless of temperature.
Reaction Q: ΔH > 0, ΔS > 0
Endothermic (ΔH positive) but entropy increases (ΔS positive), so ΔG = positive – T(positive). At low T, ΔH dominates (ΔG > 0, non-spontaneous); at high T, TΔS exceeds ΔH (ΔG < 0, spontaneous). Q needs high temperatures.
Option Analysis
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Both spontaneous at all temperatures because entropy increases: Wrong; ignores ΔH sign—Q’s positive ΔH prevents this.
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P spontaneous at all, Q only high temperatures: Correct, as detailed above.
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P only high temperatures, Q at all: Reversed; P’s negative ΔH ensures all T spontaneity.
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Both only at high temperatures: Incorrect; P works at low T too due to exothermic nature.
