We can use Hess’s Law to determine the ΔG°f for the formation of phosphoric acid (H3PO4):
1. 6 H2O(g) + P4O10(s) ⟶ 4 H3 PO4(l) ΔG°298 = −428.66 kJ/mol
2. 2 H2(g) + O2(g) ⟶ 2 H2O(g) ΔG° 298 = −457.18 kJ/mol
3. P4(s) + 5 O2(g) ⟶ P4O10(s) ΔG° 298 = −2697.0 kJ/mol
Adding the current sequence:
P4(s) + 5O2(g)⟶P4O10(s) ΔG° 298 = −2697.0 kJ/mol
2H2(g) + O2(g)⟶2H2O(g) ΔG° 298 = −457.18 kJ/mol
6H2O(g) + P4O10(s)⟶4H3 PO4(l) ΔG°298 = −428.66 kJ/mol
(a) Determine the standard free energy of formation, ΔG f ° , for phosphoric acid.
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