Heat of reaction (_H) & Spontaneous Change
All combustion reactions are spontaneous and exothermic:
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O(g)
_Horxn = -802 kJ
Iron rusts spontaneously and exothermically:
2 Fe(s) + O2 (g) Fe2O3 (s)
_ Horxn = -826 kJ
Ionic compounds form spontaneously from their elements with a large
release of heat:
Na(s) + Cl2 (g) NaCl(s)
_ Horxn = -411 kJ
At 1 atm, water freezes below 0ƒC but melts above 0ƒC.
Both processes are spontaneous, but the first is exothermic and the
second endothermic.
H2O(l) H2O(s) _ Horxn = -6.02 kJ
(exothermic; spontaneous at T < 0oC)
H2O(s) H2O(l) _ Horxn = +6.02 kJ
(endothermic; spontaneous at T > 0oC)
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