Bonds: pay to break, get paid to make

By the end of this lesson

By the end you can explain why a reaction releases or absorbs energy in terms of bonds broken and bonds formed, and state the sign conventions for each.

The one rule that explains the whole topic

Every chemical reaction can be split into two halves:

Break the bonds in the reactants. This always costs energy. Energy is needed to pull apart atoms that are attracted to each other.
Form the bonds in the products. This always releases energy, because the atoms are moving to a lower, more stable arrangement.

Compare the two totals:

released more than it cost → energy is left over → exothermic;
released less than it cost → the reaction is short of energy → endothermic, so it must take the difference from the surroundings.

Definition

Bond energy (bond enthalpy)

The energy needed to break one mole of a particular bond in the gaseous state, in $\text{kJ mol}^{-1}$ .

Because breaking bonds needs energy, bond energies are quoted as positive numbers. Bond breaking is endothermic: $\Delta H$ is positive. Bond making is the reverse, so it releases the same amount of energy.

Why breaking costs and making pays

In a molecule, the atoms sit at a minimum of potential energy — the bottom of a well. The nuclei attract the shared electrons, and the atoms are happiest where the attraction and the nuclear repulsion balance.

To break the bond you must drag the atoms up out of that well, so you put energy in.

To form the bond, the atoms fall back into the well, releasing energy — exactly like a ball rolling downhill.

This is why an isolated atom is high in energy and a bonded molecule is low. Chemistry is mostly the story of atoms falling into wells and climbing out of others.

Worked example: Worked example: does it warm or cool?

In a reaction, breaking the reactant bonds costs $1200\ \text{kJ mol}^{-1}$ and forming the product bonds releases $1500\ \text{kJ mol}^{-1}$ .

Calculate $\Delta H$ and state whether the reaction is exothermic.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

CalculateQuestion 3 · Apply

Answer the 3 questions above to continue.