Full question:
This question is incomplete, here it is completed:
An endergonic reaction with a ______ ∆H and a ______ ∆S can be changed into an exergonic reaction by decreasing the temperature.
Option A: negative, positive
Option B: negative, negative
Option C: positive, positive
Option D: positive, negative
Answer:
Option B: negative, negative
Step-by-step explanation:
The change in free energy (ΔG) of a system for a constant-temperature process is
ΔG = ΔH - TΔS
free energy is the energy available to do work. Thus, if a particular reaction is accompanied by a release of usable energy (that is, ΔG is negative), it is said to be exergonic. On the other hand, if a reaction consumes energy (that is, ΔG is positive), it is said to be endergonic.
Looking at the equation, we can see that if ΔH is negative and ΔS is negative, then ΔG will be negative only when TΔS is smaller in magnitude than ΔH. This condition is met when T is small.
ΔG = ΔH - TΔS
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This means that the reaction proceeds spontaneously at low temperatures. At high temperatures, the reverse reaction becomes spontaneous. An example of that would be the following reaction:
NH₃(g) + HCl(g) → NH₄Cl(s)