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Keep hydrogen and chlorine together in the dark and very little happens. Put the same mixture in sunlight and it can explode. The US government safety database CAMEO Chemicals warns that hydrogen and chlorine mixtures from 5% to 95% "are exploded by almost any form of energy (heat, sunlight, sparks, etc.)". The chemist who first explained why light has such an outsized effect was Max Bodenstein.
Who was Max Bodenstein?
Bodenstein was a German physical chemist, born in Magdeburg on 15 July 1871 and died in Berlin on 3 September 1942, according to the Complete Dictionary of Scientific Biography on Encyclopedia.com. In 1923 he succeeded Walther Nernst as head of the Institute of Physical Chemistry at the University of Berlin.
In 1913 he published Photochemische Kinetik des Chlorknallgases with Walter Dux, on how the hydrogen–chlorine mixture reacts in light, and Eine Theorie der photochemischen Reaktionsgeschwindigkeiten, a theory of the rates of light-driven reactions.
Why does one photon make so many HCl molecules?
Einstein's law of photochemical equivalence says one absorbed photon activates one molecule, so you would expect only a couple of HCl molecules per photon. Bodenstein found the yield was about 10⁴ times larger than that law predicts (Encyclopedia.com).
Quantum yield (Φ) is the number of molecules that react for each photon absorbed. For hydrogen and chlorine, one photon can produce as many as 10⁶ HCl molecules (Wikipedia: Chain reaction).
Bodenstein's answer was a chain reaction: a step makes product plus an unstable, reactive intermediate, which reacts with fresh molecules to make more product and another intermediate (Wikipedia: Photochlorination). In 1918 Nernst proposed that the carriers are free atoms, his atomic chain theory (Wikipedia: Walther Nernst; Nernst's 1918 paper). The steps you write in Class 12 are that atom chain, built on Bodenstein's idea.
What are the initiation, propagation and termination steps?
This is the mechanism as set out on Purdue University's chemistry demonstration sheet:
Initiation: Cl₂ + hν → 2Cl•. Light breaks the Cl–Cl bond, giving two chlorine atoms (free radicals).
Propagation, step 1: Cl• + H₂ → HCl + H•.
Propagation, step 2: H• + Cl₂ → HCl + Cl•.
Termination: 2Cl• → Cl₂, 2H• → H₂, or H• + Cl• → HCl.
Add the two propagation steps and the radicals cancel: H₂ + Cl₂ → 2HCl. The Cl• that went in comes back out, so the loop runs again and again until two radicals meet.
Why does that end in a bang? The overall reaction releases 184.6 kJ for every two moles of HCl formed (Purdue). Thousands of loops per photon can release heat faster than the gas can lose it, and hotter gas reacts faster still. That runaway is the explosion.
Which light sets it off?
Breaking Cl₂ into atoms takes 243.36 kJ per mole, which matches the energy of photons of wavelength 491.5 nm, in the blue-green part of the visible spectrum; the Purdue sheet notes that light toward the violet end sets the reaction off (Purdue). Check the energy yourself; it is good practice for photon-energy questions:
E per photon = hc/λ = (6.626 × 10⁻³⁴ J s × 2.998 × 10⁸ m/s) ÷ (491.5 × 10⁻⁹ m) ≈ 4.04 × 10⁻¹⁹ J.
Per mole of photons: 4.04 × 10⁻¹⁹ J × 6.022 × 10²³ ≈ 243 kJ/mol.
Shorter wavelengths (violet and ultraviolet) carry more energy than that per photon; red light carries less.
Worked example: calculating a quantum yield
Suppose a flask of H₂ and Cl₂ absorbs 2.0 × 10⁻⁶ mol of photons and forms 0.40 mol of HCl.
Definition: Φ = moles of product formed ÷ moles of photons absorbed.
Substitute: Φ = 0.40 ÷ (2.0 × 10⁻⁶) = 2.0 × 10⁵.
Interpret: Φ is far greater than 1, so each photon must start a chain.
Chain length: one photon gives 2 Cl atoms and each loop gives 2 HCl, so each chain runs about 2.0 × 10⁵ ÷ 4 = 5 × 10⁴ loops.
The reasoning in steps 3 and 4 matters more than the arithmetic. The Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer. If free radicals or quantum yield still feel shaky, the Fermi Concept Library gives each concept its own lesson and its place in the map.
Where did the chain idea go next?
Nikolay Semyonov built a quantitative theory of chain reactions and shared the 1956 Nobel Prize in Chemistry with Cyril Hinshelwood (Wikipedia: Chain reaction); read how Semyonov explained branching chains. For the maths behind short-lived intermediates like Cl• and H•, see our guide to the steady-state approximation, and revise the difference between order and molecularity alongside it.
Next time you write Cl₂ + hν → 2Cl•, remember the 1913 puzzle behind it: too much product for too little light.
