Bond energy calculations
Bonds and energy
- A reaction breaks the bonds in the reactants and makes new bonds in the products.
- Bond breaking takes energy in (endothermic step).
- Bond making gives energy out (exothermic step).
- The bond energy is the energy to break one mole of a bond.
$$\Delta H = (\text{energy to break bonds}) - (\text{energy released making bonds})$$
Practice
Breaking chemical bonds:
Bond breaking is endothermic (takes energy in); bond making is exothermic (gives energy out).
Worked example
- For $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$ (bond energies kJ/mol: H–H = 436, Cl–Cl = 242, H–Cl = 431):
- bonds broken: $436 + 242 = 678$,
- bonds made: $2 \times 431 = 862$,
$$\Delta H = 678 - 862 = -184 \text{ kJ/mol}$$
- Negative → exothermic (more energy out making bonds than in breaking them).
Practice
Bonds broken total 678 kJ/mol and bonds made total 862 kJ/mol. What is ΔH (in kJ/mol)?
ΔH = energy in − energy out = 678 − 862 = −184 kJ/mol.
Practice
A ΔH of −184 kJ/mol means the reaction is exothermic.
A negative ΔH means more energy was released (making bonds) than absorbed (breaking bonds) — exothermic.
You've got it
Key idea
- breaking bonds takes energy in; making bonds gives energy out
- $\Delta H$ = (energy in to break) − (energy out making)
- if more is released than absorbed, $\Delta H$ is negative → exothermic