Thread: Noob questions about silver nitrate photosensitivity

I'm taking Chem I at community college. We just did a lab on ionic compounds and solubility. We combined a bunch of aqueous solutions with others and watched for precipitation. I asked my professor why the silver nitrate solution is in a dark bottle, and the answer was that it decomposes when exposed to light. I've done some poking around trying to understand that more, but all I've found is an equation like:

2AgNO₃ -> 2Ag + O₂ + 2NO₂

But that leaves me with questions: if the bottle were kept closed, would the decomposition be slowed? If the bottle were left open and in the dark, would decomposition occur anyway, but maybe more slowly? And is it visible light only? Presumably it's a range of wavelengths that would cause the decomposition; is it a broad range or very narrow? Thanks.

In a clear bottle the reaction would not be slowed as it is not dependent on anything but light, the only way it would be affected in a bottle is if the decomposition was an equilibrium, if this were the case in a closed bottle the reaction would reach equlibrium and the amounts of reactants and products would undergo no further change (this is not the case for this reaction AFAIK). As for the wavelength range I can't give you any numbers but in silver halides and nitrate the photon excites an electron into a delocalised conduction band so I'm guessing there is a reasonable large range of photon energies above a critical threshold that will do the job.

Originally Posted by PhDemon

In a clear bottle the reaction would not be slowed as it is not dependent on anything but light, the only way it would be affected in a bottle is if the decomposition was an equilibrium, if this were the case in a closed bottle the reaction would reach equlibrium and the amounts of reactants and products would undergo no further change (this is not the case for this reaction AFAIK). As for the wavelength range I can't give you any numbers but in silver halides and nitrate the photon excites an electron into a delocalised conduction band so I'm guessing there is a reasonable large range of photon energies above a critical threshold that will do the job.

No Le Chatelier's Principle at play then do you think? I imagine that there would be a pressure increase on decomposition in a closed bottle. I'm not sure how far this principle can be stretched.

Le Chateliers principle will change the position of the equilibrium (compared to what it would be at atmospheric pressure) but equilibrium will still be reached at a certain pressure.