Which of the following statements about spontaneity are true? A decrease in the

Question

Which of the following statements about spontaneity are true?

A decrease in the entropy of the reaction ensures that only reactants will be formed during the course of a reaction.

A reaction with an increase in enthalpy and entropy will be spontaneous at all temperatures.

A reaction with a negative value for Gnonstand will result in only products being present at the end of the reaction.

The enthalpy change in a reaction can be used to predict if a reaction will be spontaneous or not.

A reaction with a positive value for Gnonstand will result in some amount of products being present at the end of the reaction.

Explain

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

knowing all this....

ANSWER:

A decrease in the entropy of the reaction ensures that only reactants will be formed during the course of a reaction. --> false, we need entropy of universe

A reaction with an increase in enthalpy and entropy will be spontaneous at all temperatures. --> false for H, we need decrease of H

A reaction with a negative value for Gnonstand will result in only products being present at the end of the reaction. --> most likely, specially if dG is large (negative)

The enthalpy change in a reaction can be used to predict if a reaction will be spontaneous or not. --> false, enthalpy is just energy of products/Reactants, no spontanoeus relationship

A reaction with a positive value for Gnonstand will result in some amount of products being present at the end of the reaction. --> False, most likely only reactants, since dG has the highest level of "free energy"

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