Bond energy is the energy required to break 1 mol of bonds in the gas phase (always positive). Reaction heat can be estimated from the energy difference between bond breaking and bond formation.
Basic Knowledge
Main bond energies [kJ/mol]: H-H=436, O=O=498, N≡N=945, C-H=413, C-C=348, C=C=614, C≡C=839, O-H=463, C=O(in CO2)=799。
Formula for estimating reaction heat: ΔH = Σ(bond energies of reactants) - Σ(bond energies of products)
Bond breaking is endothermic (positive), while bond formation is exothermic (negative). Therefore, the weaker the bonds in the reactants and the stronger the bonds in the products, the greater the heat released.
Bond energies are average values and vary somewhat among molecules, so calculated and experimental values differ by several percent.
Bond energy（energy required to break 1 mol of bonds in the gas phase [kJ/mol]）
Dissociation energy（energy required to break a specific bond in one step）
Estimation formula（ΔH = Σ(bonds broken) - Σ(bonds formed)）
Single, double, and triple bonds（stronger in that order）
Bond dissociation heat（H2→2H, ΔH=+436 kJ/mol）
Enthalpy of sublimation（C(s)→C(g), ΔH=+715 kJ/mol. Essential for gas-phase bond calculations）
Further Study (Principles and Applications)
Bond energies explain why the triple bond in N2 (945 kJ/mol) is extremely stable and why nitrogen reacts only with difficulty even though it is abundant in the air. The need for high temperature, high pressure, and a catalyst in ammonia synthesis (the Haber process) is also due to the high energy barrier for breaking N≡N.
The large heat released by hydrocarbon combustion results from the much stronger C=O and O-H bonds compared with C-H and C-C bonds; rearranging these bonds releases a large amount of energy.
- Bond energy is always positive (required for breaking bonds)
- ΔH = Σ bonds broken - Σ bonds formed
- Triple > double > single in bond strength
- N≡N=945 kJ/mol is especially strong
- When solid C is used, sublimation heat +715 is required
- Estimates use average values and have several percent error
- Heat from combustion comes from forming C=O and O-H bonds
Points of Caution (Common Confusions and Frequent Mistakes)
① Bond energy is always positive. Assign signs when constructing ΔH. ② Calculations assume the gas phase. When condensed phases such as solid C(s) are included, add the enthalpy of sublimation. ③ C-H bond energy varies slightly, such as CH4(439) and C2H6(420), so tabulated values are averages. ④ Count the number of bonds accurately, such as the number of C-H bonds.
Practice
- Find ΔH for H2(g)+Cl2(g)→2HCl(g) using bond energies H-H=436, Cl-Cl=243, and H-Cl=432.
- How many C-H bonds are in a CH4 molecule? Find ΔH for CH4(g)→C(g)+4H(g) using C-H=413.
- Explain using bond energies why N2 has low reactivity even though it makes up most of the air.