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 ΔS⁰_sys and standard enthalpy of formation values to determine S⁰_surr and ΔS⁰_universe.

Solution

First we should solve for the ΔH⁰_sys using the standard enthalpies of formation values:

ΔH⁰_sys = ΔH⁰_f [CO₂(g)] – ΔH⁰_f [C(s) + O₂(g)]

ΔH⁰_sys = (-393.5 kJ/mol) – (0 kJ/mol + 0 kJ/mol)

ΔH⁰_sys = -393.5 kJ/mol

Now we can convert this to the ΔS⁰_surr:

ΔS⁰_surr =  − ΔHsysT

 = (-393.5 kJ/mol)/(298 K) = -1.32 kJ/mol K

Finally, solve for ΔS⁰_universe:

ΔS⁰_universe = ΔS⁰_sys + ΔS⁰_surr

ΔS⁰_universe = (+2.9 J/mol K) + (-1.32 x 10³ J/mol K)

ΔS⁰_universe = -1.3 x 10³ J/mol K