Bond Enthalpies
Introduced| English |
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| bond enthalpy/bɒnd enˈθælpi/ |
Adding up the cost of connections
- Every reaction breaks some links and forges new ones.
- Breaking costs energy; forging pays it back.
- Tally the two and you estimate the heat of the whole reaction.
- It is chemistry by bookkeeping.
The energy in a bond
- A bond enthalpy 键焓 is the energy to break one mole of a bond.
- Breaking a bond absorbs this energy; forming it releases the same.
- Stronger bonds have larger bond enthalpies.
Breaking a chemical bond...
Breaking a bond always absorbs energy.
Forming a bond ____ energy.
Bond formation releases energy, the reverse of breaking.
Estimating the reaction's heat
- $\Delta H \approx (\text{bonds broken}) - (\text{bonds formed})$.
- Sum the energies of all bonds broken, then subtract all bonds formed.
- A positive result is endothermic; a negative one is exothermic.
The estimate for $\Delta H$ is...
$\Delta H \approx$ (broken) minus (formed).
Making the sign make sense
- Break more than you make, so net energy goes in, and it is endothermic.
- Make more than you break, so net energy comes out, and it is exothermic.
- The strongest new bonds drive exothermic reactions.
A reaction is exothermic when the bonds formed are...
Stronger new bonds release more energy than breaking cost -- exothermic.
Break bonds worth $800\ \text{kJ}$; form bonds worth $950\ \text{kJ}$.
- $\Delta H \approx 800 - 950 = -150\ \text{kJ}$.
- More energy came out than went in, so it is exothermic.
Bonds broken and formed
In the combustion of methane, sort each bond change by whether it takes in or gives out energy.
Bonds broken total $600\ \text{kJ}$; bonds formed total $700\ \text{kJ}$. Estimate $\Delta H$ (in kJ).
$\Delta H \approx 600 - 700 = -100\ \text{kJ}$ (exothermic).
Bond enthalpies give only an estimate of $\Delta H$ because they are averages.
Average bond enthalpies vary by molecule, so the result is approximate.
The formula is broken minus formed, in that order -- reversing it flips the sign. Bond enthalpies give an estimate, because they are averages; an exact $\Delta H$ needs formation data. And remember: breaking absorbs energy while forming releases it.
A bond enthalpy is the energy to break one mole of a bond. Estimate a reaction's heat with $\Delta H \approx (\text{bonds broken}) - (\text{bonds formed})$: breaking absorbs energy, forming releases it. Making stronger bonds than you break gives an exothermic reaction.