Question 19

[Guest]

How do you know that delta G is -? How do you know this is an exothermic rxn when heat is not present on either side of the rxn?

[admin]

By definition delta G is -ve for a sponataneous rxn. The problem has suggested that the forward rxn is preferred thus it is sponatneous. To deduce that delta H must be -ve, follow the rest of the Explanation.

[dnpgr16513]

HUH? I've never seen anything like this before. Can someone PLEASE clarify this further............

thanks.

[jellywing_2058]

Entropy is a state of disorder within a system.
If you look at the equation at equilibrium,

N2(g) + 3H2(g) <--> 2NH3(g),

there is 1 nitrogen gas molecule reacting with 3 hydrogen gas molecules to yield 2 ammonia.
Thus, the state of disorder has decreased; there are less products formed than there were reactants to begin with; the entropy change becomes more negative (ΔS < 0).

As the question says in the exam, the reaction is spontaneous (ΔG < 0); therefore,

ΔG = ΔH - T(ΔS)
ΔH = T(ΔS)+ ΔG
ΔH = (negative value) + (negative value)
ΔH < 0: The reaction is exothermic (answer choice B)