In: Chemistry
using the equation for delta G, what can be said about a system with a large negative enthalpy and a large positive entropy:
a. delta G is negative.
b. delta G is positive.
c. delta G is zero.
d. the reaction is favorable.
e. both a and d.
could you explain why? Thank you.
In order to compare equilibirum vs. spontaneous/nonspontaneous reactions, we better use a criteria.
Recall that if dSuniverse > 0, this is spontaneous, if dSuniverse = 0, this is inequilbirium and if dSuniverse < 0 this is never possible.
Then, recall that
dSuniverse = dSsurroundings + dSsystem
dSsystem = Sproducts - Sreactants
dSsurroundings = Qsurroundings/T = -dHsystem/T
therefore
dSystem = -dHsystem/T + dSsystem
If we multiply by -T
dGrxn = dHrxn - T*dSrxn
Now, analysis of dG value... which is the "free energy" available for a process to follow
if dG <0 , this will be spontaneous
if dG = 0 , this is in equilibrium
if dG > 0, this will not be spontaneous
Now...
dG = dH - T*dS
Possible values are, dH = +/- and dS = +/-; T is always positive ( absolute value)
Analysis of cases:
Case 1.
if dH is positive (-) and dS is positive (+) --> this favours always a negative value of dG; spontaneous
Case 2.
if dH is positive (+) and dS is positive (-) --> this favours always a positive value of dG; not spontaneous
Case 3.
if dH is positive (+) and dS is positive (+) --> dG = dH - T*dS --> analysis must be done
if T is very low... then dH > T*dS; then this will be Positive value in dG; i.e. not spontaneous
if T is very high... then dH < T*dS; then this will be Negative value in dG; i.e. spontaneous
Case 4.
if dH is positive (-) and dS is positive (-) --> dG = dH - T*dS --> analysis must be done
if T is very low... then dH > T*dS; then this will be Negative value in dG; i.e. spontaneous
if T is very high... then dH < T*dS; then this will be Positive value in dG; i.e. non spontaneous
we are talking about:
Case 1.
recall that in Case 1. = if dH is positive (-) and dS is positive (+) --> this favours always a negative value of dG; spontaneous
choose "E", since it is dG < 0, and it faovurs products