Skip to notes
wise otter.Chemistry
Contents
K324 / 2027
Chemical Energetics

Lesson 4 of 4

Breaking bonds costs energy; making bonds releases it

The net change depends on both parts of the rearrangement.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only
  • I can explain bond-breaking/making energy.Syllabus

    Syllabus K324, 9(c). Explain bond-breaking/making energy

  • I can explain overall enthalpy qualitatively.Syllabus

    Syllabus K324, 9(d). Explain overall enthalpy qualitatively

Breaking bonds costs energy; making bonds releases it

The net change depends on both parts of the rearrangement.

Make a guess. You are not marked.

A molecule of hydrogen, H-H, is split into two hydrogen atoms. Does breaking this bond take in energy or give it out?

  1. It takes in energy to pull the atoms apart
  2. It gives out energy as the atoms are freed
  3. Neither, because no new substance forms
Show the answer

It takes in energy to pull the atoms apart

Atoms in a bond attract the shared electrons. Pulling them apart works against that attraction, so energy must be supplied.

Breaking a covalent bond separates atoms that attract one another, so energy must be supplied: it is endothermic. Forming a bond releases energy as atoms become more strongly held together: it is exothermic. A chemical reaction usually does both; never explain combustion by saying that breaking fuel bonds releases energy.

Compare the two energy contributions
  1. Break reactant bonds

    Energy is absorbed.

  2. Form product bonds

    Energy is released.

  3. Compare the amounts

    More released than absorbed gives an exothermic reaction; more absorbed than released gives an endothermic reaction.

Worked exampleExplain hydrogen combustion qualitatively

Why can hydrogen reacting with oxygen release energy even though bonds must first break?

Try it first. Then show the worked stepsHide the worked steps
  1. H-H and O=O bonds must be broken, requiring energy.
  2. O-H bonds form in water, releasing energy.
  3. The energy released in making product bonds exceeds that needed to break reactant bonds.
Answer

The net change is exothermic. This explanation compares totals, not just whether a bond is strong.

Check your understanding

A student says, "An endothermic reaction forms no bonds." Which correction is right?

  1. Bonds do form, but bond making takes in energy, so it is what makes the reaction endothermic.
  2. Bonds do form, but less energy is released making them than is taken in breaking bonds.
  3. The student is right, because forming any bond releases energy and would make it exothermic.
Show the answer

Bonds do form, but less energy is released making them than is taken in breaking bonds.

Every reaction breaks reactant bonds (taking in energy) and makes product bonds (releasing energy). Endothermic means the energy taken in is greater than the energy released.