# Energy changes in a selection of thermodynamics questions

hey guys,

i am having a lil trouble with 2 of my homework problems. they deal with enthlapy and entropy.

my first question is below.
(this is are the reactions)
Rxn (1): 2 H2(g) + O2(g)  2 H2O(l)
Rxn (2): C(s, graphite) + 2 H2(g)  CH4(g)
Rxn (3): C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(l)
Rxn (4): Fe2O3(s) + 2 Al(s)  Al2O3(s) + 2 Fe(s)

1a). What is ?Go for Rxn (1)? i got the answer as, this which is right - 474.2
when i solve for the second part is where i get stuck.

b)What is ?G at 298 K if P = 0.25 bar for each gaseous substance in Rxn (1)?
i keep getting large numbers.

for delta H i got this : (2)(285.83)-(0)+(2)(0)= 571.66
for delta S i got this : (2)(69.95)-(250.152)+(2)(130.62)= -371.4
i solve for delta G doing the following: G= H-TS.

my answers are really big and they are wrong.

2. Solid CO2 (dry ice) sublimes with ?Hsubl = 25.2 kJ/mol at 195 K. A block of dry ice whose mass is 39.9 g sublimes in a room whose temperature is 26.5 oC.

a)What is the entropy change (?S) of the CO2?
b)What is ?S of the room?
c)What is the overall ?S?

how would i go about solving this, i know i have to use ?S=q/T, but it seems to be wrong.

thanks everyone

what equation are you using to solve 1b?

?S=q/T

For 1b, I believe that they are looking for you to use this equation:

delta G =  delta Go + RT ln Q

where Q is the reaction quotient (the reaction quotient has the same form as the Keq - products over reactants raised to the power of their coefficients, but it can be evaluated when the system is not at equilibrium.
Q = 1 / (pH2 2 x pO2)  =  1/(.252 x .25) = 64
(solids and liquids are not included in equilibrium expressions so pH20 does not appear in the equation for Q or Keq)

delta G = (-472.4E3 J/mol) + 8.31 J/mol K x 298K x ln (64)      (You didn't label your delta Go but I'm assuming it's in kJ/mol)
= -462.1E3 J/mol or -462.1 kJ/mol

For Question 2, in order for the CO2 to sublime delta G must be negative so

delta H - T delta S must be negative

39.9 grams of CO2 / 44 g/mol  = .907 mol of CO2

delta H = 25.2 kJ/mol x .907 mol =  22.9 kJ

22.9 kJ - 195K delta S   .117 kJ

i am still having a lil bit of difficulty answering number 2, it seems like it is simple but i keep getting the wrong answers for .

a)What is the entropy change (?S) of the CO2?
b)What is ?S of the room?
c)What is the overall ?S?

also does anyone know the thermodynamic standard amount of ?Go for C2H5OH(l), i cant find it in my textbook or online.

The entropy change of the room is - entropy change of the CO2.

The overall entropy change is zero.