Bond energies and enthalpies
Changes in enthalpy reflect the difference in energies of bonds in the reactants and products As seen below, breaking bonds requires energy. Forming bonds releases energy.
The bond enthalpy (energy) is defined as the energy required for dissociation into the component atoms in the gas phase (this may be given the symbol D).
The bond enthalpy depends on the atoms in the bond.
The bond enthalpy also depends on the number of electrons shared between the nuclei.
In general:
ΔH(reaction) = Σ(n × D (bonds broken) - Σ(n × D (bonds formed)
| ΔH°(reverse) = –bond enthalpy | ||||
| A – B(g) | exothermic endothermic | A(g) + B(g) | ||
| ΔH°(forward) = bond enthalpy | ||||
The bond enthalpy (energy) is defined as the energy required for dissociation into the component atoms in the gas phase (this may be given the symbol D).
The bond enthalpy depends on the atoms in the bond.
If the atoms are smaller, the bond is stronger (bond enthalpy higher) because the bonding electrons are closer to BOTH nuclei.
The bond enthalpy also depends on the number of electrons shared between the nuclei.
The bond enthalpy for C-C is lower than for C=C which is lower than for C≡C.
A reaction enthalpy can be estimated from bond enthalpies as a possible pathway for all reactions is one in which reactants are converted to gaseous atoms which then combine to form productsH2(g) + Cl2(g) 
2HCl(g)
Δ1H° = ??????
H2(g) 2H(g)
+ bond enthalpy (H2)
Cl2(g) 2Cl(g)
+ bond enthalpy (Cl2)
2H(g) + 2Cl(g) 2HCl(g)
- 2 × bond enthalpy(HCl) = Δ1H°
2HCl(g)Δ1H° = ??????
H2(g)
+ bond enthalpy (H2)
Cl2(g)
+ bond enthalpy (Cl2)
2HCl(g)- 2 × bond enthalpy(HCl) = Δ1H°
In general:
ΔH(reaction) = Σ(n × D (bonds broken) - Σ(n × D (bonds formed)
where n is the number of bonds of a particular type and Σ means the sum of