AP® Chemistry · Unit 6 · Topic 6.7
Bond Enthalpies
Bond enthalpies let you estimate delta H for a reaction by adding up the energy to break all the bonds in the reactants and subtracting the energy released forming all the bonds in the products.
Breaking a bond always costs energy, you have to pull two atoms apart against their attraction. Forming a bond always releases energy, the opposite process. A reaction is just bonds breaking in the reactants and new bonds forming in the products, so the overall energy change is whatever is left over after you pay for the breaking and collect for the forming. If the new bonds are stronger than the old ones, forming them releases more energy than breaking the old ones cost, and the extra energy escapes as heat. That is why product bonds being stronger than reactant bonds means an exothermic reaction.
The calculation turns that idea into arithmetic: delta H equals the sum of bond enthalpies for every bond broken, minus the sum of bond enthalpies for every bond formed. Breaking is energy in, so it is added as a positive cost; forming is energy out, so it is subtracted. You need to know exactly which bonds exist in reactants and products, including how many of each, which means drawing out the structures first.
This sits next to formation-enthalpy calculations as a second way to estimate delta H, useful especially for reactions involving gas-phase molecules where bonds are easy to count. It connects forward to Hess's law style thinking: both methods add up pieces to get a whole reaction's energy, just using different pieces, bonds versus whole compounds.
Key terms
- Bond enthalpy
- The average energy needed to break one mole of a specific type of bond in gas-phase molecules.
- Bonds broken minus bonds formed
- The rule that delta H equals total energy to break reactant bonds minus total energy released forming product bonds.
- Exothermic
- A reaction that releases heat because the bonds formed are stronger than the bonds broken.
- Formation enthalpy
- The directly measured heat change when one mole of a compound forms from its elements in their standard states.
Watch it explained
- How it worksHow does summing bonds broken minus bonds formed estimate the enthalpy of a reaction?
- Worked exampleFind delta H for H2 plus Cl2 forming 2 HCl using bond enthalpies of 436, 243, and 431 kJ per mol.
- Common mistakeWhy isn't a bond enthalpy calculation exact the way a formation enthalpy calculation is?
- Going furtherWhat happens to the estimate when the reaction involves a liquid or a solid?
Check yourself
End of topic.