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IB DP Chemistry – S2.2.2 Single, double and triple bonds- Study Notes

IB DP Chemistry - S2.2.2 Single, double and triple bonds- Study Notes - New Syllabus - 2026, 2027 & 2028

IB DP Chemistry – S2.2.2 Single, double and triple bonds – Study Notes – New Syllabus

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Structure 2.2.2 — Single, Double and Triple Bonds

Structure 2.2.2 — Single, Double and Triple Bonds

Covalent bonds form when atoms share electrons. The number of shared electron pairs determines whether a bond is a single, double, or triple bond.

  • Single bond: 1 shared pair of electrons (e.g. \( \text{H}_2 \), \( \text{CH}_4 \))
  • Double bond: 2 shared pairs of electrons (e.g. \( \text{O}_2 \), \( \text{CO}_2 \))
  • Triple bond: 3 shared pairs of electrons (e.g. \( \text{N}_2 \), \( \text{HC≡CH} \))

Bond Order

Bond order is the number of chemical bonds between a pair of atoms. It reflects the stability of a bond.

  • Single bond: Bond order = 1
  • Double bond: Bond order = 2
  • Triple bond: Bond order = 3
  • In delocalised systems (e.g. benzene), bond order can be fractional (e.g. 1.5) 

Note: Higher bond order generally indicates a stronger and shorter bond.

Bond Length and Bond Strength

There is an inverse relationship between bond length and bond strength:

  • Bond Length: Distance between the nuclei of two bonded atoms
  • Bond Strength (Bond Enthalpy): Energy required to break a bond in 1 mole of a gaseous substance

General Trend:

  • Single bond: Longest and weakest
  • Double bond: Shorter and stronger than a single bond
  • Triple bond: Shortest and strongest

Reason: More electron density between the nuclei leads to greater electrostatic attraction between the nuclei and the bonding electrons, pulling the atoms closer and requiring more energy to break the bond.

Illustrative Example (Bond Enthalpies):

  • \( \text{C–C} \): 348 kJ/mol, 154 pm
  • \( \text{C=C} \): 614 kJ/mol, 134 pm
  • \( \text{C≡C} \): 839 kJ/mol, 120 pm

Example

Identify the type of bond (single, double, or triple) between the carbon atoms in the molecule ethyne (\( \text{HC≡CH} \)).

▶️Answer/Explanation

Ethyne contains a triple bond between the two carbon atoms.
The triple bond consists of one sigma bond and two pi bonds — so the bond order is 3.
This bond is very strong and short compared to single or double C–C bonds.

Example

Compare the bond lengths and strengths of the following: \( \text{O}_2 \), \( \text{N}_2 \), and \( \text{F}_2 \).

▶️Answer/Explanation
  • \( \text{O}_2 \): Double bond → medium length and strength
  • \( \text{N}_2 \): Triple bond → shortest and strongest
  • \( \text{F}_2 \): Single bond → longest and weakest

Thus, bond strength increases and bond length decreases in the order:
\( \text{F}_2 \lt \text{O}_2 \lt \text{N}_2 \)

Example

Which molecule has a bond order of 1.5? Explain why it is not an integer.

▶️Answer/Explanation

Benzene (\( \text{C}_6\text{H}_6 \)) has a bond order of 1.5 between carbon atoms.
This is because electrons are delocalised across the ring, creating resonance structures with alternating single and double bonds.
The average bond order is calculated as:
\( \text{Bond order} = \frac{\text{Total shared pairs}}{\text{Number of bonds}} = \frac{9}{6} = 1.5 \)

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