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# Reactions: Enthalpy and Entropy

APCHEM-2A6IBN

Consider the table below, showing the formation of the hydrogen halides, with their accompanying $\Delta {H}^{º}$ and $\Delta {S}^{º}$ values. Assume each reaction occurs at constant pressure.

Reaction $\Delta { H }^{ \circ}$ $\Delta { S }^{ \circ}$
A ${ H }_{ 2 }(g)+{ F }_{ 2 }(g)\rightarrow 2HF(g) \quad \quad$ –273 kJ/mol +174 J/mol K
B ${H}_{2}(g)+{Cl}_{2}(g)\rightarrow 2HCl(g) \quad \quad \quad$ – 92 kJ/mol +187 J/mol K
C ${ H }_{ 2 }(g)+{Br }_{ 2 }(l)\rightarrow 2HBr(g) \quad$ – 36 kJ/mol +199 J/mol K
D ${ H }_{ 2 }(g)+{ I }_{ 2 }(s)\rightarrow 2HI(g) \quad$ + 27 kJ/mol +207 J/mol K

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Which of the four statements is TRUE, based on the information in the table?

A

Of the four reactions, only Reaction A is spontaneous at all temperatures.

B

Reaction A will have the greatest rate of reaction.

C

Hydrogen fluoride, $HF$, has the greatest bond dissociation energy of the four hydrogen halides.

D

Of the four, only Reaction D is spontaneous at all temperatures.