Question

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

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?

 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

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.

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 -T∆S

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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