Bond Breaking and Forming

In a chemical reaction, atoms do not appear or disappear. They swap partners by breaking old bonds and forming new bonds. Energy changes happen because of these two steps.

Key ideas

• Breaking bonds absorbs energy (endothermic). Energy must be supplied to pull atoms apart.
• Forming bonds releases energy (exothermic). Energy is given out when atoms snap together.

Analogy: pulling apart two strong magnets takes effort (energy in); letting them click together gives a “snap” of energy out.

Overall energy change (ΔH)

The overall energy change of a reaction depends on the balance between energy taken in to break bonds and energy released when new bonds form:

$$\Delta H = E_{\text{break}} - E_{\text{make}}$$

• If $\Delta H < 0$: more energy is released than absorbed → exothermic (surroundings get warmer).
• If $\Delta H > 0$: more energy is absorbed than released → endothermic (surroundings get cooler).

Activation energy

Even exothermic reactions need a small input to start breaking the first bonds. This is why fuel needs a spark or a flame to begin burning.

Real-world links

• Burning fuels (like methane in a stove) is exothermic because strong new bonds form in carbon dioxide and water.
• Instant cold packs feel cold because dissolving certain salts is overall endothermic.

Common misconceptions

• “Breaking bonds releases energy” is incorrect. Breaking always needs energy.
• “Exothermic always feels hot.” Small exothermic changes may not feel warm, but they still release energy overall.