General Question

julia999's avatar

Why is PbI2 precipitate yellow?

Asked by julia999 (343points) July 25th, 2009

In the reaction:
Pb(NO3)2 + 2KI—-> 2KNO3 + PbI2
A yellow precipitate is formed, PbI2.

Why? I can see that Lead reacts with the iodine ions and the physical property changes, but what is it that causes the colour change?

Thanks in advance
p.s. I tried googling it and found a video of the experiment:

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3 Answers

grumpyfish's avatar

Colors are a fairly complicated subject. Essentially, a molecule or compound will be a particular color due to numerous factors, but it comes down to what colors are absorbed and what colors are reflected from the material.

Unlike mixing paint, you can’t just react yellow and blue molecules and expect to get green out (you might!), for instance Colbalt Chloride is blue:

julia999's avatar

Perfect answer, thanks grumpyfish!

JPP47's avatar

PbI2 precipitates as crystalline platelets. Crystalline PbI2 is a semiconductor with bangap of 2.3 eV. The color of a semiconductor is related with its band structure (the energy distribution of electrons in the material). A bandgap of 2.3 eV means that photons with energy higher than 2.3 eV (or wavelength shorter than about 540 nm) are absorbed while photons with energy lower than 2.3 eV (or wavelength longer than 540 nm) are transmitted. So if you illuminate PbI2 with white light, the only photons which are transmitted (those you see) are a few green, the yellow and red ones. As a result the material looks yellow.

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