6.3.2Additional lessons
Homolytic Fission — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 148 of 172
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 5Equal Bond Breaking
The bond breaks evenly, which is typical of nonpolar bonds.
Definition
- Definition In homolytic fission a covalent bond breaks so that each atom takes one electron from the shared pair.
Products
- Products Two radicals are formed, for example Cl–Cl → Cl• + Cl•.
Arrow
- Arrow A single-headed curly arrow, called a fishhook, shows the movement of one electron.
Conditions
- Conditions The bond is broken by ultraviolet light or high temperature, and the process occurs in the gas phase or nonpolar solvents.
Part 2 of 5Why UV Light Works
1
A photon of light must carry at least as much energy as the bond enthalpy to break the bond.
2
The Cl–Cl bond enthalpy is 242 kJ mol⁻¹, which equals about 4.0 × 10⁻¹⁹ J per molecule.
3
The wavelength is found from E = hc/λ, so a higher energy needs a shorter wavelength.
4
UV photons have more energy than visible photons and can break many covalent bonds.
5
This is why chlorine radicals form quickly in sunlight but not in the dark.
Part 3 of 5Where Homolytic Fission Occurs
Where Homolytic Fission Occurs
Nonpolar bonds
- Nonpolar bonds Bonds between identical atoms like Cl–Cl or Br–Br split evenly because neither atom attracts electrons more.
Bond strength
- Bond strength The weaker the bond, the longer the wavelength of light that can break it, so Br–Br splits more easily than Cl–Cl.
Link to mechanism
- Link to mechanism Homolytic fission is the initiation step of free-radical chain reactions such as halogenation of alkanes.
Comparison
- Comparison Heterolytic fission is the alternative, producing ions instead of radicals.
| Feature | Homolytic fission | Heterolytic fission |
|---|---|---|
| Electron split | One to each atom | Both to one atom |
| Products | Two radicals | A cation and an anion |
| Arrow | Single-headed | Double-headed |
Part 4 of 5Worked Example
Question
- Calculate the longest wavelength of light able to break the Cl–Cl bond, given a bond enthalpy of 242 kJ mol⁻¹.
- (h = 6.63 × 10⁻³⁴ J s;
- c = 3.00 × 10⁸ m s⁻¹;
- L = 6.02 × 10²³ mol⁻¹)
- Energy per bond = 242 000 J mol⁻¹ ÷ 6.02 × 10²³ mol⁻¹ = 4.02 × 10⁻¹⁹ J.
- λ = hc ÷ E = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) ÷ 4.02 × 10⁻¹⁹ m.
- λ = 4.95 × 10⁻⁷ m, which is 495 nm.
Answer
- About 495 nm;
- light of this wavelength or shorter can break the Cl–Cl bond.
Part 5 of 5Exam Checklist & Tips
6.3.2 – Key Points for the Exam
- Homolytic fission Breaking a covalent bond so that each atom keeps one bonding electron.
- Fishhook arrow A single-headed curly arrow showing movement of one electron.
- Bond enthalpy The energy needed to break one mole of a covalent bond in the gas phase.
- Photon A packet of electromagnetic energy with E = hc/λ.
Common Mistakes
- Using a double-headed arrow for homolytic fission, which shows movement of a pair of electrons.
- Forgetting the dot on the radicals formed.
- Assuming visible light is always too weak to break bonds.
- Mixing up the energy per mole and the energy per molecule.
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