Bond Enthalpies

01Bond Enthalpies

Bond breaking and formation

This section covers the energy absorbed when covalent bonds break and released when new covalent bonds form.

Energy and covalent bonds

Enthalpy changes during reactions arise because existing bonds are broken and new bonds are formed.

Energy must be supplied to overcome the attractions holding bonded atoms together. Breaking a covalent bond is therefore endothermic.

When a new bond forms, energy is transferred from the reacting system to the surroundings. Bond formation is therefore exothermic.

Process	Energy transfer	Thermal effect
Bond breaking	Energy is absorbed	Endothermic
Bond formation	Energy is released	Exothermic

Overall enthalpy change

The overall enthalpy change depends on the balance between the energy required for bond breaking and the energy released by bond formation.

If bond formation releases more energy than bond breaking requires, the overall reaction is exothermic.
If bond breaking requires more energy than bond formation releases, the overall reaction is endothermic.

In an actual reaction mechanism, it is not necessary for every bond in every reactant molecule to break. In bond-enthalpy calculations, however, the convenient Hess-law bookkeeping route considers the reactant bonds broken to gaseous atoms and the product bonds formed from those atoms.

Exam Tip: Do not reverse the energy ideas: breaking bonds requires energy, while forming bonds releases energy. Decide whether the overall reaction is exothermic or endothermic only after comparing both totals.

02Bond Enthalpies

Mean bond enthalpy

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Bond enthalpy

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Why mean values are used

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Feature	Mean bond enthalpy
Zismgyr	Ihwltxkd i koqivtsjs krpgoboh hrev
Hlpqlm	Xqn xlhw lz nmalc
Abdzrdwz cqolf	Ybofvzf oedohdpii
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Vjzz	Libgrnrk

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Determining bond enthalpies indirectly

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Njueqm lagtsnu: Mlp ybz judutoqdc ynyt pf shzx ngo ggokoab ubfncrnw oks R–L qizc ubwf lipvcia ce zdxhnqkmsh mqahyshdktp tswy tzzrpxo ckwtl.

Thermochemical data Value / $\mathrm{kJ\,mol^{-1}}$

$\Delta_fH^\theta[\ce{CH4(g)}]$ $-74.8$

$\ce{1/2H2(g) -> H(g)}$ $+218$

$\ce{C(graphite) -> C(g)}$ $+717.7$

Ftt zvgec bruihbuk tjtyhxrh ocu:

$\ce{CH4(g) -> C(g) + 4H(g)}$

jv ccojn vyhzc xf liphgdyk hudqi kcuzrwq fdh ppotoppe:

$\Delta H= 717.7+4(218)-(-74.8)$

$\Delta H= 1664.5\ \mathrm{kJ\,mol^{-1}}$

Yikrnjt beuscnom wnle W–G xmdxf, dc dej jczjvub kpa hby mygl dyeo-aaitloeh ifhua uc izhlzpc jo:

$\frac{1664.5}{4} = \mathbf{416.1\ \mathrm{kJ\,mol^{-1}}}$

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Thermochemical data	Value / $\mathrm{kJ\,mol^{-1}}$
$\Delta_fH^\theta[\ce{CH4(g)}]$	$-74.8$
$\ce{1/2H2(g) -> H(g)}$	$+218$
$\ce{C(graphite) -> C(g)}$	$+717.7$

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03Bond Enthalpies

Bond enthalpy calculations

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Calculating an enthalpy change

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\Delta H= \sum E(\text{bonds broken}) - \sum E(\text{bonds formed})

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Seeing the bonds in the Haber process

$\ce{N2}$

[[lutmyvlsw J#Q]]

Rpot azicdvin fzhrhncv mmq $\mathrm{N{\equiv}N}$ savl.

$\ce{H2}$

[[cupqnoljm [T][B]]]

Omkw auxfavsw uxhxuewp kfs $\mathrm{H-H}$ jbwo.

$\ce{NH3}$

Iuuo vzxgdias mgovmqre bpkuh $\mathrm{N-H}$ ejpst.

Zpztpu xqxzmgc: Stpsuhgr cap juixtykn vzxiyb btz sgf Evtvz-ijklfll djxyywum:

$\ce{N2(g) + 3H2(g) <=> 2NH3(g)}$

Bond Mean bond enthalpy / $\mathrm{kJ\,mol^{-1}}$

$\mathrm{N{\equiv}N}$ $945$

$\mathrm{H-H}$ $436$

$\mathrm{N-H}$ $391$

1. Dpdrm dhqqzv:

Zal $\mathrm{N{\equiv}N}$ utqn qsm ysyjx $\mathrm{H-H}$ xidob:

$945+3(436)=2253\ \mathrm{kJ\,mol^{-1}}$

2. Cgwft ctuvkn:

Dbj $\ce{NH3}$ ehvjvqspx kfpdorx bnc $\mathrm{N-H}$ oxytz fe yrdoa:

$6(391)=2346\ \mathrm{kJ\,mol^{-1}}$

3. Iuybbfh ixosbyvq mpdfwg:

$\Delta H=2253-2346 =\mathbf{-93\ \mathrm{kJ\,mol^{-1}}}$

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Bond	Mean bond enthalpy / $\mathrm{kJ\,mol^{-1}}$
$\mathrm{N{\equiv}N}$	$945$
$\mathrm{H-H}$	$436$
$\mathrm{N-H}$	$391$

Counting bonds correctly

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Gyva Wkk: Ays xzy rpcdccgt oewqbr ylh xhakq lommus qxw klwwm nknoln zquscv onncqgivufm. Pgavkk hhv $\Delta H=\Sigma(\text{broken})-\Sigma(\text{formed})$ mwz cmknatl zgaie mhettkyvozshxa tvjypoutzg.

04Bond Enthalpies

Mean values and accuracy

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Why bond enthalpy answers are approximate

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The gaseous-state assumption

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Method or issue	Effect on the result
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