QUESTION 4 The following reaction is spontaneous at all temperatures: CaC2(s) +

Question

QUESTION 4 The following reaction is spontaneous at all temperatures: CaC2(s) + 2H20(/) Ca(OH)2(s) + C2H2(g) . Which of the following statements is true? e A. AH is negative and AS is positive. B·dH is positive and as is positive. C AH is positive and AS is negative. D.AH is negative and AS is negative. O E. AG is positive at all temperatures. QUESTION S In which reaction is Ase expected to be negative? A. 2C2H 6(g) + 702(g) 4CO2(g) + 6H20(/) e B. Gal)-Ga(s) C. H2O( ) + 2SO2(g) H2SO4() e D.CO2(g) CO2(s) E. all ofabove Click Save and Submin to sove and súb

Explanation / Answer

Q4

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

therefore, if it is always spontaneous

this must be case 1..

dH = negtive, dS = positive

Q5

dS negative = ordering

all go form gases to liquid/soldis which are very ordered

therefore

choose ALL

Q6

dG = Gproducts- Greactnas

dG = CO2 + 2H2O - (CH4 + 2O2)

dG = -394.4 + 2*-237.4 - (-50.72 + 2*0)

dG = -818.5

choose A

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