Use the S ? values in Appendix B in the textbook to calculate ? S ? at 25 ?C for

Question

Use the S? values in Appendix B in the textbook to calculate ?S? at 25 ?C for each of the following reactions. Suggest a reason for the sign of ?S? in each case.

2S(s)+3O2(g)?2SO3(g)

Suggest a reason for the sign of ?S? in this case.

SO3(g)+H2O(l)?H2SO4(aq)

Suggest a reason for the sign of ?S? in this case.

AgCl(s)?Ag+(aq)+Cl?(aq)

Suggest a reason for the sign of ?S? in this case.

NH4NO3(s)?N2O(g)+2H2O(g)

Suggest a reason for the sign of ?S? in this case.

delta S [J/ (Kx mol)] S 167.7 Ag+ 72.7 O2 205.0 SO3 256.6 H2O (l) 69.9 H2SO4 20 AgCl 96.2 Cl- 56.5 NH4NO3 151.1 N2O 219.7 H2O (g) 188.7

Explanation / Answer

You could go through and calculate products-reactants for all equations using the table below or you could look at the number of moles and phase each is in to come up with the sign for S

2S(s)+3O2(g)?2SO3(g)

Would be negative because you are going from 5 moles to 2 moles, thus it is becoming more ordered and entropy will decrease

SO3(g)+H2O(l)?H2SO4(aq)

?S? will be negative becaus 20- (69.9 + 256.6)= a negative number

AgCl(s)?Ag+(aq)+Cl?(aq)

?S will be postive because you are going from a solid to a aqeuous solution, meaning the system is becoming less ordered and thus entropy will increase

NH4NO3(s)?N2O(g)+2H2O(g)

?S will be postive because you are going from a solid to a gas solution, meaning the system is becoming less ordered and thus entropy will increase

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