A reaction is exothermic overall when
Correct answer: A. the energy released in forming new bonds exceeds the energy absorbed in breaking old ones
- A. the energy released in forming new bonds exceeds the energy absorbed in breaking old ones
- B. the energy absorbed in breaking bonds exceeds the energy released in forming them
- C. no bonds are broken
- D. the products are gases
Explanation
Breaking bonds always costs energy and forming them always releases it, so the sign of the overall enthalpy change is decided by which total is larger. This is the basis of calculating an approximate enthalpy change from tabulated bond energies. Combustion is strongly exothermic because the very strong bonds in carbon dioxide and water release more than was spent breaking the fuel apart.
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About Bond Energy
Bond energy is the energy needed to break one mole of a particular covalent bond in gaseous molecules, while bond formation releases energy. Reaction enthalpy is estimated by subtracting the total energy of bonds formed from bonds broken, with attention to average bond energies and their limitations for specific molecules.
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