Bond breaking

Bond breaking is the stage of a reaction that absorbs energy, because energy must be supplied to pull apart the bonds in the reactants. Malay: pemutusan ikatan · Chinese: 断键.

Every chemical bond holds atoms together, so energy has to be put in to break it. Breaking bonds is therefore always an endothermic step. In a reaction, the reactant bonds break first so that the atoms are free to rearrange, and this stage takes energy from the surroundings or from the energy of the reactants themselves. How much energy is needed depends on how strong the bonds are; stronger bonds need more energy to break.

Example. When hydrogen burns in oxygen, the H–H bonds in hydrogen molecules and the O=O bonds in oxygen molecules must be broken before water can form. That breaking step absorbs energy. Only afterwards do the O–H bonds of water form and release energy. Whether the overall reaction is exothermic or endothermic is decided by comparing the energy absorbed in breaking with the energy released in forming.

Confusion to avoid. Do not think bond breaking releases energy, it is the reverse. Students often say “breaking bonds gives out energy” by loose analogy with breaking a spring, but chemically the opposite is true: breaking always takes energy in, and forming always gives energy out. Keeping this the right way round is essential for explaining ΔH.

Bond breaking and bond forming are a pair, and the balance between them is what makes a reaction exothermic or endothermic. A simple way to hold it steady is: breaking needs energy in, forming gives energy out, so a reaction is exothermic only when the energy given out is the larger of the two. This idea underlies the whole energy level diagram, so our teachers make sure SPM Chemistry answers state clearly that bond breaking absorbs energy and never releases it.