Bond enthalpy
Bond enthalpy is the energy needed to break one mole of a particular bond in the gas phase. Breaking bonds costs energy and forming bonds releases it. ΔH ≈ Σ(bonds broken) − Σ(bonds formed). It gives only an estimate.
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Analogy
Pulling apart LEGO bricks takes effort, and snapping new ones together gives it back.
Example
Broken: H–H 436 + Cl–Cl 243 = 679 kJFormed: 2 × H–Cl 431 = 862 kJΔH = 679 − 862 = −183 kJ
In practice
A fuel is a good one when the new bonds it forms (like in CO₂ and H₂O) are much stronger than the bonds it breaks.
Practice
Flashcards
- Does bond breaking absorb or release energy?It absorbs energy. Bond forming releases it.
Questions people ask
Why are bond enthalpies only approximate?
A bond's strength changes slightly depending on the rest of the molecule. The tables give an average over many compounds. The O–H bond in water is not exactly the same as in ethanol. For accurate answers, use enthalpies of formation instead.
Does breaking a bond release or absorb energy?
Breaking a bond always absorbs energy, because the atoms are held together by attraction and you must pull them apart. Forming a bond always releases energy. A reaction is exothermic when the energy released forming bonds is greater than the energy absorbed breaking bonds.