Question

Given the values of ΔH∘rxn, ΔS∘rxn, and T below, determine ΔSuniv.

Part A

ΔH∘rxn= 117 kJ , ΔS∘rxn=− 263 J/K , T= 291 K .

ΔSuniv=

−1.3•102

J/K

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Part B

ΔH∘rxn=− 117 kJ , ΔS∘rxn= 263 J/K , T= 291 K .

ΔSuniv=

J/K

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Part C

ΔH∘rxn=− 117 kJ , ΔS∘rxn=− 263 J/K , T= 291 K.

ΔSuniv=

J/K

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Part D

ΔH∘rxn=− 117 kJ , ΔS∘rxn=− 263 J/K , T= 557 K.

Express your answer using two significant figures.

ΔSuniv=

J/K

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Part E

Predict whether or not the reaction in part A will be spontaneous. (Assume that all reactants and products are in their standard states.)

Predict whether or not the reaction in part A will be spontaneous. (Assume that all reactants and products are in their standard states.)

spontaneous

nonspontaneous

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Part F

Predict whether or not the reaction in part B will be spontaneous. (Assume that all reactants and products are in their standard states.)

Predict whether or not the reaction in part B will be spontaneous. (Assume that all reactants and products are in their standard states.)

spontaneous

nonspontaneous

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Part G

Predict whether or not the reaction in part C will be spontaneous. (Assume that all reactants and products are in their standard states.)

Predict whether or not the reaction in part C will be spontaneous. (Assume that all reactants and products are in their standard states.)

spontaneous

nonspontaneous

Answer 1

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

dSuniv = -dH*1000/T + dSrxn

A) dSuniv = -117*1000/291 + -263 = -665 J/K

B) dSuniv = --117*1000/291 + 263 = + 665  J/K

C) dSuniv = --117*1000/291 + -263 = +139 J/K

d) dSuniv = --117*1000/557 + -263 = -52.9 J/K

E) since dS negative, nonspontaneous

F) since dS positive, spontaneous

G) since dS positive, spontaneous

H) since dS negative, nonspontaneous