Using bond energies to compare reactivity

5 minAS Level3.4.3

By the end of this lesson

By the end you can use bond energies and bond lengths to compare how reactive covalent molecules are.

The laziest gas in the room

You breathe in about 78% nitrogen with every breath, and it does almost nothing. Fluorine, by contrast, attacks nearly everything it touches.

A big part of the reason is in their bonds. Reactions usually start by breaking a bond, and some bonds are far harder to break than others.

When a reaction needs a particular bond to break:

high bond energy (usually a short bond) → hard to break → less reactive;
low bond energy (usually a long bond) → easy to break → more reactive.

Only compare similar reactions, where the same kind of bond is breaking.

molecule	bond	bond energy / kJ mol⁻¹	reactivity
$\text{N}_2$	N≡N	944	very unreactive
$\text{O}_2$	O=O	496	reactive
$\text{F}_2$	F–F	158	extremely reactive

Nitrogen's triple bond is so strong that $\text{N}_2$ is used to give an unreactive atmosphere, for example in food packaging.

Quick check

Multiple choiceQuestion 1 · Recall

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