Calculate delta G rxn and delta S theoretical for the reaction. Using delta G rx

Question

Calculate delta G rxn and delta S theoretical for the reaction. Using delta G rxn and the average of the experimentally determined value for delta H in experiment, report a semi-experimental value for delta S (assume standard temp). Compare value to the theoretical value and calculate percent error. Does the sign on the experimental value agree with the type of change?

Found average of the experimental delta H to be 3.29 x 105

I keep finding my percent error to be extremely drastic. I don't know if I am doing the steps wrong, or if my experimental delta H is wrong. Appreciate the help.

Component delta G (kJ/mol) S (J/mol x K) Ca (s) 0 41.6 H20 (l) -237.13 69.9 Ca(OH)2(s) -897.5 83.4 H2(g) 0 131

Explanation / Answer

We have reaction:

Ca(s) + 2H2O(l) ------>Ca(OH)2(s) + H2(g)

overall Grxn = G of products - G of reactants

-897.5 kJ/mol x 1 mol + 0 x 1 mol -(2 mol x -237.13 kJ/mol + 0 x 1mol)

= -423.24 kJ = -423240 J

S = S of products - s of reactants = 83.4 J/mol.K x 1mol+ 131J/mol.K x 1mol - (2 mol x 69.9 J/mol.K + 41.6 J/mol.K x 1 mol)

= 34 J/.K

G = H -TS

Standard temperature is defined as zero degrees Celsius (0 0C) or 273.15 K.

-423240 J = H - 273.15 K x 34 J/mol.K

H = -413952.9 J = -4.139 x 105 J

% error = {-4.139 x 105 J -(-3.29 x 105J)}/-4.139 x 105 J x 100 = 20.52 %

you might have forgot to convert the units from kJ to J

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