CAIE A-Level Chemistry 3.4.3 Bond Energy and Bond Length

Practise defining gas-phase bond energy and internuclear bond length and using both to compare molecular reactivity.

Exam points

define bond energy as energy required to break one mole of a specified covalent bond in gaseous species
define bond length as the internuclear distance between two covalently bonded atoms
link shorter stronger bonds to larger bond energies and greater energy needed for reaction initiation

[Maximum number: 1]

Nitrogen and phosphorus are elements in Group 15 of the Periodic Table.

At very high temperatures, phosphorus can form $\mathrm{P}_{2}$ molecules.
$\mathrm{P}_{2}$ contains a triple bond, $\mathrm{P} \equiv \mathrm{P}$ .

The bond energy of $P \equiv P$ is $485 \mathrm{~kJ} \mathrm{~mol}^{-1}$ . The bond energy of $N \equiv N$ is $944 \mathrm{~kJ} \mathrm{~mol}^{-1}$ . Compare the reactivity of $\mathrm{P}_{2}$ and $\mathrm{N}_{2}$ . Explain your answer.

$\mathrm{P} \equiv \mathrm{P}$ is much weaker so $\mathrm{P}_{2}$ is more reactive (than $\mathrm{N}_{2}$ )

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3.4.3—Terms question 1