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2.2.16Additional lessons

Sigma and Pi Bonds (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 40 of 172

📚 IB Chemistry HL Hub / Topical Revision
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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.
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Part 1 of 6Sigma Bonds

Covalent bonds can be classified by how their orbitals overlap.

Overlap
  • Overlap Orbitals overlap head-on along the line joining the two nuclei.
Electron density
  • Electron density The electron density is concentrated between the nuclei, on the bond axis.
Occurrence
  • Occurrence Every single bond is a sigma bond, and every multiple bond contains exactly one sigma bond.
Rotation
  • Rotation Atoms can rotate freely about a sigma bond.

Part 2 of 6Pi Bonds

Pi Bonds
Overlap
  • Overlap Parallel p orbitals overlap sideways, above and below the bond axis.
Electron density
  • Electron density The density lies in two regions, one above and one below the plane of the atoms.
Strength
  • Strength A pi bond is weaker than a sigma bond because sideways overlap is less effective.
Rotation
  • Rotation Pi bonds prevent rotation about the bond, which allows cis and trans isomers to exist.

Part 3 of 6Counting Bonds

1
A single bond has 1 sigma bond and no pi bonds.
2
A double bond has 1 sigma and 1 pi bond.
3
A triple bond has 1 sigma and 2 pi bonds.
4
In a molecule draw all the bonds, count each line as a sigma bond, and add the extra lines in multiple bonds as pi bonds.
5
Pi bonds are usually the more reactive part of the molecule because their electron density is more exposed.
MoleculeSigma bondsPi bonds
C₂H₄51
C₂H₂32
CO₂22
HCN22

Part 4 of 6Examples

Propene
  • Propene CH₃CH=CH₂ has 8 sigma bonds and 1 pi bond.
Benzene
  • Benzene C₆H₆ has 12 sigma bonds and a delocalised pi system from three pi bonds.
Nitrogen
  • Nitrogen N₂ has one sigma bond and two pi bonds in its triple bond.

Part 5 of 6Worked Example

Question Count the sigma and pi bonds in propene, CH₃CH=CH₂.

  1. C–H bonds3 + 1 + 2 = 6 sigma bonds.
  2. C–C single bond1 sigma bond.
  3. C=C double bond1 sigma and 1 pi bond.

Answer Total: 8 sigma bonds and 1 pi bond.

Part 6 of 6Exam Checklist & Tips

2.2.16 – Key Points for the Exam

  • Sigma bond A covalent bond formed by head-on overlap of orbitals.
  • Pi bond A covalent bond formed by sideways overlap of p orbitals.
  • Orbital overlap Region where atomic orbitals combine to form a bond.
  • Restricted rotation The inability to rotate about a bond that contains a pi component.

Common Mistakes

  • Counting a double bond as two sigma bonds.
  • Saying that pi bonds are stronger than sigma bonds.
  • Forgetting that a triple bond contains one sigma and two pi bonds.
  • Missing the C–H bonds when counting sigma bonds in a hydrocarbon.

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