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?

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.

Answer 1

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