Concept:Different types of standard enthalpy changes can be positive (endothermic) or negative (exothermic), but certain specific thermochemical processes exclusively release energy by definition.
Formula:$$ \Delta H_c^\circ < 0 \text{ (Always)} $$
Solution:- Enthalpy of combustion (\( \Delta H_c \)) is the heat released when one mole of a substance is completely burned in excess oxygen.
- Combustion is a highly energetic oxidation process that universally evolves heat due to the formation of stable oxides (like \( CO_2 \) and \( H_2O \)).
- Therefore, it is always an exothermic process (\( \Delta H = -\text{ve} \)).
Why other options are incorrect:Enthalpy of formation and solution can be either positive or negative depending on the specific substance. Enthalpy of atomization requires breaking bonds to form gaseous atoms, so it is
always endothermic.
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