Bond Energy: MDCAT Chemistry notes

Definition

Bond energy is the energy needed to break one mole of a particular bond in gaseous molecules into separate atoms. It is measured in $\mathrm{kJ\,mol^{-1}}$. The same amount of energy is released when that mole of bonds forms.

Energy change in reactions: reactants' bonds are broken, which absorbs energy, and products' bonds are formed, which releases energy. The balance of these two is the enthalpy change of the reaction.

$$\Delta H \approx \sum(\text{bonds broken}) - \sum(\text{bonds formed})$$

Potential energy and bond formation

As two atoms approach, attraction between each nucleus and the other atom's electrons lowers the potential energy. At the bond length, attraction dominates and the potential energy is at a minimum, so the bond is stable. If the atoms are pushed closer, nucleus–nucleus repulsion takes over and the potential energy increases steeply.

Important values

Bond	Bond energy / $\mathrm{kJ\,mol^{-1}}$
H–H	436
H–F	about 565
H–Cl	431
H–Br	366
H–I	299
Cl–Cl	242
C–H	414
C–C	348

Note that 242 is Cl–Cl and 431 is H–Cl. Do not mix them up with H–H (436).

Factors affecting bond energy

• Atomic size / bond length: smaller atoms form shorter, stronger bonds. So HF > HCl > HBr > HI, and C–F > C–Cl > C–Br > C–I. The C–Br bond is weaker than C–Cl, with a textbook value of about 290 $\mathrm{kJ\,mol^{-1}}$.
• Electronegativity difference: more polar bonds have extra ionic attraction and are stronger. This is one reason HF is so strong.
• Bond order: C≡C > C=C > C–C.
• Among C–H, C–C, C–N and C–O single bonds, C–H has the highest bond energy, because H is small.

Halogen exception: among $\mathrm{F_2, Cl_2, Br_2, I_2}$ the strongest bond is Cl–Cl. F–F is unexpectedly weak because the atoms are so small that their lone pairs repel each other strongly. Order: $\mathrm{Cl_2 > Br_2 > F_2 > I_2}$.

Bond length

Bond length is the distance between the nuclei of two bonded atoms, roughly the sum of their covalent radii. Some typical values are C–C 154 pm, C–Cl about 177 pm and Cl–Cl 199 pm. It depends on:

• the size of the atoms
• bond multiplicity, since multiple bonds are shorter (a C=O double bond is shorter than a C–O or C–N single bond)
• hybridization, since more s character gives a shorter bond
• electronegativity difference, since a larger difference shortens the bond

Ionization energy is not a factor in bond length.

Common MDCAT traps

• $\mathrm{F_2}$ does not have the strongest halogen bond. $\mathrm{Cl_2}$ does.
• In hydrogen halides the order is simple: HF is the strongest and HI the weakest.
• Energy change in a reaction comes from both bond breaking and bond making.
• At the equilibrium bond length the potential energy is at its minimum, not its maximum.
• Bigger halogen means a longer and weaker bond to carbon, so C–Br is lower than C–Cl.

Quick revision

• H–H bond energy is 436 $\mathrm{kJ\,mol^{-1}}$.
• Bond breaking is endothermic and bond formation is exothermic.
• HF has the highest bond energy of the hydrogen halides.
• C–H is the strongest of the common single bonds to carbon.
• Shorter bonds are stronger bonds.