Bond Energy & Length (Cambridge (CIE) A Level Chemistry)
Revision Note
Energy & Length of Covalent Bonds
Bond energy
The bond energy is the energy required to break one mole of a particular covalent bond in the gaseous states
Bond energy has units of kJ mol-1
The larger the bond energy, the stronger the covalent bond is
Bond length
The bond length is internuclear distance of two covalently bonded atoms
It is the distance from the nucleus of one atom to another atom which forms the covalent bond
The greater the forces of attraction between electrons and nuclei, the more the atoms are pulled closer to each other
This decreases the bond length of a molecule and increases the strength of the covalent bond
Triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the two atoms
This increases the forces of attraction between the electrons and nuclei of the atoms
As a result of this, the atoms are pulled closer together causing a shorter bond length
The increased forces of attraction also mean that the covalent bond is stronger
Comparing the length of carbon-carbon covalent bonds
Triple bonds are the shortest covalent bonds and therefore the strongest ones
Reactivity of covalent molecules
The reactivity of a covalent bond is greatly influenced by:
The bond polarity
The bond strength
The bond type (σ/π)
Worked Example
Bond lengths & bond energies
Bond lengths and bond energy for various hydrogen halides is shown in the table below.
Hydrogen halide | Bond length (nm) | Bond energy (kJ mol-1) |
|---|---|---|
HCl | 0.127 | 431 |
HBr | 0.141 | 366 |
HI | 0.161 | 299 |
Suggest why the bond energy values of hydrogen halides decrease in the order HCl > HBr > HI.
Suggest a value for the bond length in HF.
Suggest which hydrogen halide is the most reactive.
Answer 1:
Going down the halogen group, the atoms are bigger
This means that the attractive force between the bonding electrons and the nucleus gets smaller
So, less energy is needed to break the atom
Answer 2:
Going down the group the increase in bond length is approximately 0.14 - 0.20 nm
Fluorine is smaller than HCl, so a value between 0.09 and 0.11 nm is acceptable for the bond length
Answer 3:
The hydrogen halide with the longest bond length and therefore smallest bond energy is the most reactive as it takes the least energy to break apart the hydrogen and halide atoms apart
Therefore, HI is the most reactive hydrogen halide
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