Session 16503913 Jump to 1:52. 3 4 5 6 7 8 :9) 10 11 12 13 14 15 16 17 matching

Question

Session 16503913 Jump to 1:52. 3 4 5 6 7 8 :9) 10 11 12 13 14 15 16 17 matching question Match the phrase on the left to the statement regarding spontaneity on the right 1st choice spontaneous at all temperatures 2nd choice nonspontaneous at all temperatures 3rd choice spontaneous only at high temperatures 4th choice spontaneous only at low temperatures entropy favors spontaneity, but enthalpy does not enthalpy and entropy both distavor spontaneity enthalpy favors spontaneity, but entropy does not 1st choice enthalpy and entropy both favor spontaneity 2nd choice 3rd choice 4th choice Submit response Send a message to the instructor Join another session ALWAYS LEARNING Copyright © 2017 Pearson or ts affilate(s). Al rights reserved

Explanation / Answer

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

then

entropy favours, enthalpy does not --> T must be high

enthalpy and entropy disfavours --> never

enthalpy favour , entropy does not --> low T must be present

both favours --> this is always possible

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