2.2.3Additional lessons
Multiple Bonds — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 24 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Bond Order
Atoms sometimes need to share more than one pair to complete their octets.
Single bond
- Single bond One shared pair of electrons, as in H–H or the carbon-carbon bond in ethane.
Double bond
- Double bond Two shared pairs of electrons, as in O=O and in the C=C of ethene.
Triple bond
- Triple bond Three shared pairs of electrons, as in N≡N and in ethyne HC≡CH.
Bond order
- Bond order The number of shared pairs between two atoms is called the bond order, which is 1, 2 or 3.
Part 2 of 6Common Examples
Common Examples
Oxygen, O₂
- Oxygen, O₂ The two oxygen atoms share two pairs, and each atom keeps two lone pairs.
Nitrogen, N₂
- Nitrogen, N₂ The two nitrogen atoms share three pairs, and each atom keeps one lone pair;
- the strong triple bond makes N₂ unreactive.
Carbon dioxide, CO₂
- Carbon dioxide, CO₂ Carbon forms a double bond to each oxygen, so the molecule is O=C=O with two lone pairs on each oxygen.
Ethene, C₂H₄
- Ethene, C₂H₄ The two carbon atoms are joined by a double bond and each carbon also has two bonds to hydrogen.
Part 3 of 6Effect on Length and Strength
1
Increasing the number of shared pairs increases the electron density between the two nuclei.
2
The greater electron density attracts the nuclei more strongly towards each other.
3
The nuclei are pulled closer, so the bond length decreases as bond order increases.
4
Breaking the bond requires more energy, so bond enthalpy increases as bond order increases.
5
However, the triple bond is less than three times as strong as the single bond, so the relationship is not exactly proportional.
| Bond | Length / pm | Enthalpy / kJ mol⁻¹ |
|---|---|---|
| C–C | 154 | 346 |
| C=C | 134 | 614 |
| C≡C | 120 | 839 |
Part 4 of 6Reactivity
Unsaturation
- Unsaturation Compounds with C=C or C≡C are called unsaturated because they can add extra atoms.
Alkene reactions
- Alkene reactions The double bond in ethene is the reactive site that allows addition reactions to occur.
Stable molecules
- Stable molecules Molecules with very strong multiple bonds, such as N₂, need a large energy input before they react.
Drawing check
- Drawing check Each carbon in an organic molecule must have exactly four bonds counted as shared pairs.
Part 5 of 6Worked Example
Question Draw the Lewis formula of carbon dioxide.
- Valence electronsC gives 4 and each O gives 6, so 4 + 12 = 16 electrons, which is 8 pairs.
- Place C in the centre with a single bond to each O (2 pairs used) and add three lone pairs on each O (6 pairs used).
- Carbon has only four electrons, so move one lone pair from each oxygen to form a second bond to carbon.
- Now each atom has eight electronsC has four shared pairs and each O has two shared pairs and two lone pairs.
Answer O=C=O with two lone pairs on each oxygen atom.
Part 6 of 6Exam Checklist & Tips
2.2.3 – Key Points for the Exam
- Double bond A bond consisting of two shared pairs of electrons.
- Triple bond A bond consisting of three shared pairs of electrons.
- Bond order The number of shared electron pairs between two bonded atoms.
- Unsaturated Containing at least one carbon-carbon double or triple bond.
Common Mistakes
- Saying that a double bond is twice as strong as a single bond; it is stronger but not exactly double.
- Forgetting that bond length decreases as the number of bonds increases.
- Leaving off lone pairs on oxygen or nitrogen in Lewis formulas.
- Drawing a carbon atom with five bonds.
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