Example 1
For the previous example, the change in the standard entropy, ΔS⁰, for the synthesis of carbon dioxide from graphite and oxygen, use the previously calculated ΔS⁰sys and standard enthalpy of formation values to determine S⁰surr and ΔS⁰universe.
Solution
First we should solve for the ΔH⁰sys using the standard enthalpies of formation values:
ΔH⁰sys = ΔH⁰f [CO2(g)] – ΔH⁰f [C(s) + O2(g)]
ΔH⁰sys = (-393.5 kJ/mol) – (0 kJ/mol + 0 kJ/mol)
ΔH⁰sys = -393.5 kJ/mol
Now we can convert this to the ΔS⁰surr:
ΔS⁰surr = − ΔHsysT
= (-393.5 kJ/mol)/(298 K) = -1.32 kJ/mol K
Finally, solve for ΔS⁰universe:
ΔS⁰universe = ΔS⁰sys + ΔS⁰surr
ΔS⁰universe = (+2.9 J/mol K) + (-1.32 x 103 J/mol K)
ΔS⁰universe = -1.3 x 103 J/mol K
