6.3.2Additional lessons
Homolytic Fission — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 95 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.
Section 1 of 6Homolytic Fission
Homolytic fission usually happens with non-polar bonds in the presence of heat or UV radiation.
Definition
- Definition In homolytic fission a covalent bond breaks so that each atom keeps one electron of the shared pair.
Products
- Products Two radicals form, for example Cl–Cl → Cl• + Cl•.
Fishhook arrows
- Fishhook arrows A curly arrow with a half head shows the movement of a single electron.
Energy needed
- Energy needed Energy at least equal to the bond enthalpy must be supplied, for example 242 kJ mol⁻¹ for Cl–Cl and 193 kJ mol⁻¹ for Br–Br.
Section 2 of 6Conditions for Fission
1
UV light supplies photons with enough energy to break the bond.
2
The photon energy is absorbed by one molecule and a bond breaks homolytically.
3
High temperature can also provide enough energy for bonds to break homolytically.
4
The radicals produced start a chain reaction, as in the chlorination of methane.
Section 3 of 6Comparing Bond Fission Types
Comparing Bond Fission Types
Homolytic
- Homolytic Each atom takes one of the shared electrons, so two radicals are produced.
Heterolytic
- Heterolytic One atom takes both electrons, producing a cation and an anion.
Bond type
- Bond type Non-polar bonds tend to break homolytically, while polar bonds tend to break heterolytically.
Arrow
- Arrow Homolytic uses half-arrows and heterolytic uses full-headed arrows.
| Feature | Homolytic | Heterolytic |
|---|---|---|
| Electrons | One to each atom | Both to one atom |
| Products | Two radicals | Two ions |
| Arrow | Half-headed | Full-headed |
Section 4 of 6Photochemical Reactions
Photochemical Reactions
UV and bond breaking
- UV and bond breaking UV photons have enough energy to break the Cl–Cl bond, whereas infrared light does not.
Single photon
- Single photon One photon breaks one bond, so the intensity of light affects how many radicals form.
Chain reactions
- Chain reactions Each initiation step creates two radicals, and each can start a long chain.
Bond enthalpy trends
- Bond enthalpy trends Weaker bonds such as Br–Br need less energy to break than Cl–Cl.
Section 5 of 6Worked Example
Question
- Calculate the longest wavelength of light able to break one Cl–Cl bond if the bond enthalpy is 242 kJ mol⁻¹.
- Use L = 6.02 × 10²³ mol⁻¹, h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹.
- Energy per bond = 242 000 / 6.02 × 10²³ = 4.02 × 10⁻¹⁹ J.
- λ = hc / E = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) / 4.02 × 10⁻¹⁹.
- λ = 4.95 × 10⁻⁷ m = 495 nm.
Answer
- About 495 nm;
- any shorter wavelength, including UV, can cause homolytic fission.
Section 6 of 6Exam Checklist & Tips
6.3.2 – Key Points for the Exam
- Homolytic fission Breaking of a covalent bond so each atom gets one electron.
- Photon A packet of electromagnetic radiation energy.
- Bond enthalpy The energy needed to break one mole of a bond in gaseous molecules.
- Fishhook arrow A curved arrow with half a head, showing the movement of one electron.
Common Mistakes
- Drawing a full-headed arrow for homolytic fission.
- Producing ions from homolytic fission instead of radicals.
- Forgetting that energy must be absorbed to break bonds.
- Using visible light as the typical condition in school work instead of UV.
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