S(g) + O2(g) → SO2(9) ∆H1 = -137,32 kcal /x(-1)
S(s) + O2(g) → SO2(9) ∆H2 = -70,98 kcal
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S(s) ------------> S(g) ∆Hr = ?
legea lui Hess spune ca ∆Hr = ∆H1 + ∆H2
=> ∆Hr = (-1)∆H1 + ∆H2
= (-1)(-137,32) + (-70,98)
= 66,34 kJ/mol