Chlorine : Law Of Photochemical Equivalence

Planck, in 1900, assumed that the energy of light and radiation in general may be absorbed and emitted in definite quanta, the quantum, ε, for each wave-length, or frequency ν, (ν =c/λ, where c - velocity of light; λ = wave-length) being equal to the product of ν and a universal constant, h, called Planck's constant: h =6.55x10^-27 erg seconds, i.e.,

ε=hν (1)

In the case of the yellow light emitted from a sodium flame, for example, ν = 5.01x10¹¹; ε = 6.55x10^-27x5.01x10¹¹ = 3.28x10^-12 ergs. The smaller the wave-length, the larger is the energy of the quantum, and it is found that, in general, light of short wave-length (blue, violet or ultraviolet) is chemically more active than light of longer wave-length (yellow or red). There is reason to believe that a single quantum of light of wave-length 5400 A.U. can be detected by a normal eye fully adapted in darkness (Noddack, 1924).

Einstein in 1912 assumed that the primary process in a photochemical change is the absorption of one quantum of energy from the radiation by each reacting molecule, so that the energy absorbed per mol is (N₀ = Avogadro's constant, 6.06x10²³):

E = N₀hν (2)

This primary process is often succeeded by further changes, which occur spontaneously with evolution of energy, so that the yield may exceed, sometimes considerably, that calculated by the law of photochemical equivalence, (2), which is in some senses analogous to Faraday's law of electrochemical equivalence:

Q = N₀ne,

where Q is the quantity of electricity required to set free one mol (N₀ molecules) of an ion of charge ne, n being the valency and e the charge on the electron.

In the decomposition of hydrogen bromide by light, for example, Warburg (1916) found that 2HBr are decomposed per quantum. This may be explained by the assumption of a primary quantum reaction:

(1) HBr + hν = H + Br,

followed by two exothermic spontaneous reactions (which will occur in the dark),

(2) H + HBr = H₂ + Br,

(3) Br + Br = Br₂.

It may also be explained by the formation of an activated molecule (i.e., one with more than the average energy corresponding with the temperature) by a primary process:

(1) HBr + hν = HBr',

followed by a collision of this with another HBr molecule:

(2) HBr + HBr' = H₂ + Br₂.

The products of the primary reaction may also undergo a cycle of changes so as to be reproduced, in which case the quantum efficiency would be infinite unless the "chain reaction" is broken at some link by collision with a foreign molecule (e.g., oxygen;. In the union of hydrogen and chlorine, at least a million molecules of hydrogen chloride can be produced per quantum. Nernst suggests the following mechanism:

I. Primary (quantum) reaction: Cl₂ + hν = 2Cl (absorbs energy).

II. Secondary reactions (exothermic):

Taylor and Marshall (1923) found that when atomic hydrogen is added to a mixture of hydrogen and chlorine, more hydrogen chloride than corresponds with its amount is formed.

In some reactions photosensitisation occurs. Phosgene, COCl₂, is a colourless gas absorbing only in the region of the ultraviolet and therefore according to the Grotthuss-Draper law (1818, 1841) that only rays which are absorbed are effective in producing chemical change is not decomposed by visible light. If chlorine, which absorbs blue light is mixed with phosgene and the mixture exposed to ordinary light the phosgene is decomposed by the energy absorbed by the chlorine which acts as a photochemical sensitiser; COCl₂ = CO + Cl₂. The combination of hydrogen or sulphur dioxide with oxygen is also sensitised by chlorine.

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