A light sensitive substance in black and white photographic film is AgBr (silver

Question

A light sensitive substance in black and white photographic film is AgBr (silver bromide). Photons provide the energy necessary to transfer an electron from the anion, Br- to the cation Ag+ to produce elemental Ag and Br and thereby darken the film.

A. If a minimum energy of 2.00x10^5 J/mol is needed for this process, what is the minimum energy needed from each photon?

B. Calculate the wavelength of light necessary to provide photons of this energy

C. Explain why this film can be handled in a darkroom under red light.

Explanation / Answer

(a)

minimum energy = 2.00x10^5 J/mol = 1.25*10^24 eV/mol

For 1 mol = 6.022*10^23 particles 1.25*10^24 eV is needed

For 1 particle = 1.25*10^24 eV/6.022*10^23 = 2.075 eV

minimum energy needed from each photon = 2.075 eV

(b)

Wavelength = hc / energy = 1242/2.075 = 598.55 nm

(c)

Under red light we have wavelength around 700 nm

So energy is lower than minimum and the Films are not effected.

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