Bond Properties

1. Bond Length

Bond length is the equilibrium distance between two bonded nuclei. It decreases as bond order increases (triple < double < single) and as the bonded atoms get smaller. Longer bonds are weaker because the shared electrons are farther from the nuclei.

2. Bond Enthalpy (Bond Energy)

Bond enthalpy is the energy required to break one mole of a specific bond in gaseous molecules, e.g.:

\[ H_2(g) \longrightarrow 2H(g) \qquad \Delta H = +436\text{ kJ/mol} \]

Breaking always absorbs energy (positive), forming always releases energy (negative). Stronger bonds have larger bond enthalpies.

3. Estimating Reaction Enthalpy from Bond Enthalpies

A chemical reaction breaks the bonds of the reactants and forms the bonds of the products. Because breaking absorbs energy and forming releases it, the net enthalpy change is:

\[ \Delta H \approx \sum (\text{bond enthalpies of bonds broken}) - \sum (\text{bond enthalpies of bonds formed}) \]

How it works

1. Write the balanced equation and draw the Lewis structure of each reactant and product.
2. List every bond in the reactants (these are broken) and every bond in the products (these are formed).
3. Multiply each bond enthalpy by the number of such bonds, using the equation coefficients.
4. Sum the broken-bond enthalpies and the formed-bond enthalpies.
5. Subtract formed from broken to estimate \(\Delta H\).
6. Interpret the sign: negative → exothermic, positive → endothermic.

Common confusions

Memory aids

"Break it, then Make it — B minus M." Energy to break reactant bonds minus energy released when you make product bonds gives ΔH. And "shorter is stronger" links bond length to bond energy.

Quick review

Topic Recap

Bond length and bond enthalpy are inverse measures of the same idea: short bonds are strong bonds, and multiple bonds are both shorter and stronger than single bonds. Because breaking bonds absorbs energy and forming bonds releases it, the enthalpy change of a reaction can be estimated by subtracting the energy of the bonds formed from the energy of the bonds broken — a fast, approximate alternative to calorimetry.

Knowledge Check

1. Is energy absorbed or released when a bond forms?
2. Which C–C bond is shortest and strongest: single, double, or triple?
3. Using \(D(H\!-\!H) = 436\) and \(D(Br\!-\!Br) = 193\text{ kJ/mol}\), and \(D(H\!-\!Br) = 366\text{ kJ/mol}\), estimate ΔH for \(H_2 + Br_2 \to 2HBr\).
4. Why are bond-enthalpy calculations only estimates?
5. For the reaction \(N_2 + 3H_2 \to 2NH_3\), which bonds are broken and which are formed?

Answers and Rationales

1. Energy is released when a bond forms; forming a bond is always exothermic, while breaking one always absorbs energy.
2. The triple bond. Higher bond order means more shared pairs pulling the nuclei together, giving a shorter, stronger bond.
3. Bonds broken: \(1\,H\!-\!H + 1\,Br\!-\!Br = 436 + 193 = 629\text{ kJ}\). Bonds formed: \(2\,H\!-\!Br = 2 \times 366 = 732\text{ kJ}\). \(\Delta H \approx 629 - 732 = -103\text{ kJ/mol}\) (exothermic).
4. Because tabulated bond enthalpies are average values across many different molecules, not the exact energy of each specific bond in the reacting molecules.
5. Broken: one \(N\equiv N\) triple bond and three \(H\!-\!H\) bonds. Formed: six \(N\!-\!H\) bonds (three per \(NH_3\), times two molecules).