Question

Consider the following reaction: CaCO3(s)→CaO(s)+CO2(g). Estimate ΔG∘ for this reaction at each of the following temperatures. (Assume that ΔH∘ and ΔS∘ do not change too much within the given temperature range.

A) 305 K

B) 1075 K

C) 1465 K

Answer 1

Given:
Hof(CaCO3(s)) = -1206.92 KJ/mol
Hof(CaO(s)) = -635.09 KJ/mol
Hof(CO2(g)) = -393.509 KJ/mol

Balanced chemical equation is:
CaCO3(s) ---> CaO(s) + CO2(g)

ΔHo rxn = 1*Hof(CaO(s)) + 1*Hof(CO2(g)) - 1*Hof( CaCO3(s))
ΔHo rxn = 1*(-635.09) + 1*(-393.509) - 1*(-1206.92)
ΔHo rxn = 178.321 KJ


Given:
Sof(CaCO3(s)) = 92.9 J/mol.K
Sof(CaO(s)) = 39.75 J/mol.K
Sof(CO2(g)) = 213.74 J/mol.K

Balanced chemical equation is:
CaCO3(s) ---> CaO(s) + CO2(g)

ΔSo rxn = 1*Sof(CaO(s)) + 1*Sof(CO2(g)) - 1*Sof( CaCO3(s))
ΔSo rxn = 1*(39.75) + 1*(213.74) - 1*(92.9)
ΔSo rxn = 160.59 J/K

So we have:
ΔHo rxn = 178.321 KJ/mol
ΔSo rxn = 160.59 J/mol.K
= 0.16059 KJ/mol.K

A)
T = 305 K
Now use:
ΔGo rxn = ΔHo rxn - T*ΔSo rxn
= 178.321 - 305.0*0.16059
= 129.341 KJ/mol
Answer: 129.3 KJ/mol

B)
T = 1075 K

Now use:
ΔGo rxn = ΔHo rxn - T*ΔSo rxn
= 178.321 - 1075.0*0.16059
= 5.67 KJ/mol
Answer: 5.67 KJ/mol

C)
T = 1465 K

Now use:
ΔGo rxn = ΔHo rxn - T*ΔSo rxn
= 178.321 - 1465.0*0.16059
= -56.9 KJ/mol
Answer: -56.9 KJ/mol