1. Use standard entropies and heats of formation to calculate delta-G of formation at 25° C for
a) cadmium(II) chloride (s)
b) methyl alcohol, CH3OH (l)
c) copper(I) sulfide (s)
My problem is that I don't know how to calculate delta-G of formation. I know that the Gibbs-Helmholtz equation can take the form
delta-G= delta-H -T(delta-S), but that's not really the same thing, is it? Could I modify the equation so it was all in terms of formation of a substance?
Yes, you can do that but you will need to calculate delta S from Cd, Cl2, and CdCl2. Delta S = S(products)-S(reactants).
Cd + Cl2 ==> CdCl2
I'm still having trouble getting the correct answers. I was able to get the correct one for CdCl2 (344 kJ) but not for the others. The back of my book says CH3OH should be -166.3 kJ, and Cu2S should be -53.6 kJ
Here's my work... maybe you can find where I'm making an error?
CO + 2H2 --> CH3OH
calculating delta-s:
standard molar entropy CH3OH: 126.8 J/K
" " CO: 197.6 J/k
" " H2: 130.6 J/K
-126.8 J/K - (197.6 J/K + 2(130.6 J/K)) = -585.6 J/k
-238.7 kJ (delta-h formation) - 298.2(-.5856 kJ/K) = -154.4 kJ [not correct]
2Cu+1 + S-2 = Cu2S
calculating delta-s
standard molar entropy Cu2S: 120.9 J/k
" " Cu+1: 40.6 J/K
" " S-2: -14.6 J/K
120.9 J/K - (2(20.6 J/K)+ -14.6 J/K)= 54.3 J/K
-79.5 kJ (heat of formation Cu2S) - 298.2(.0543 kJ/K)= -95.69 kJ [not correct]
Where did I go wrong??
My tables don't list CH3OH or inorganic salts so I can't verify any of the other numbers. However, I did verify S for CO and H2. You switched signs from the initial listing of CH3OH delta H as 126.8 from + at the initial listing to - in the calculation. I tried that change and it didn't get 166 either. Have you checked to see if CH3OH is a liquid or a gas? Just as H2O is listed in the tables as a gas and as a liquid, I suspect CH3OH may be too. Look at that. Sorry my tables don't allow me to do any more.
Al(s)+3/2O2(g)-->Al2O3(s)
delta H=-167.6kj
It seems that there might be an error in the sign of the standard molar entropy for CH3OH (methyl alcohol) and there might also be a mistake in the phase of the substance.
To calculate delta-G of formation for methyl alcohol (CH3OH), you will need to use the standard entropies and heats of formation for the reactants (CO and H2) and the product (CH3OH). Make sure you are using the correct phase (gas or liquid) for each substance.
Here's an example of how the calculations should be done:
Reactant:
CO (g): standard molar entropy = 197.6 J/(mol·K)
H2 (g): standard molar entropy = 130.6 J/(mol·K)
Product:
CH3OH (l): standard molar entropy = ?? (check your tables for the correct value)
Delta-S = S(products) - S(reactants)
Delta-S = S(CH3OH) - [S(CO) + S(H2)]
Delta-S = Unknown value - [197.6 J/(mol·K) + 130.6 J/(mol·K)]
Then, calculate delta-G using the equation:
Delta-G = Delta-H - T(Delta-S)
Make sure the temperatures are consistent with the units used for the entropies and enthalpies. If the values from your calculations do not match the provided values in the book, there might be a discrepancy in the given standard entropies or heats of formation.
Furthermore, it might be helpful to verify the phase and reference thermodynamic data for CH3OH and Cu2S from a reliable source to ensure accurate calculations.
If you are still having issues, it might be beneficial to consult with a chemistry professor or review additional resources for guidance on calculating delta-G of formation.