Concept:A chemical reaction involves two distinct thermochemical steps: breaking old bonds (which always requires absorbing energy) and forming new bonds (which always releases energy). The overall enthalpy (\( \Delta H \)) is the net sum.
Formula:$$ \Delta H = \Sigma H_{\text{bond breaking (absorbed)}} - \Sigma H_{\text{bond making (released)}} $$
Solution:- In a combustion reaction, fuel molecules and oxygen molecules are broken apart. This requires an initial input of energy (endothermic).
- However, the newly formed products (usually \( CO_2 \) and \( H_2O \)) have exceedingly strong, stable covalent bonds.
- The formation of these highly stable bonds releases a massive amount of energy.
- Because the energy released in bond making is much greater than the energy required for bond breaking, the net reaction is highly exothermic.
Why other options are incorrect:Bond breaking always absorbs energy, it never releases it (disproving option C). Bond making releases energy, it never absorbs it (disproving option B). If energy absorbed in bond breaking were greater (Option A), the reaction would be endothermic.
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