chemistry question #3291



Steve, a 25 year old male from Honolulu, HI asks on February 24, 2006,

Q:

I'm interested in commerical lightsticks... I understand it is hydrogen peroxide and oxalate ester reacting (and transferring energy) to a dye molecule to produce the light. My question is what is the limiting reagent in this reaction? If there were two containers of hydrogen peroxide within the single larger container of oxalate ester, could the stick be "snapped" a second time and light again produced for a certain amount of time? Would there need to be more oxalate ester? Or dye molecule? Or once the dye molecule returns to the calm state, it could be "re-excited" at any point, again producing light?

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the answer

Dr. Thomas G. Chasteen at Sam Houston State University, Huntsville, Texas answered on March 2, 2006, A:

I think (but only the manufacturer knows for sure) that the limiting reagent in these reaction is the compound that gets oxidized, the oxalate ester. I call it the fuel, analogous to the reduced carbon compounds in combustion that gets oxidized by oxygen. The phenyl oxalate ester fuel is, in general, the most expensive reagent in the mix besides, and so economics will out and the cheaper oxidant is probably added in excess. So when the fuel has been oxidized, the excited product created, and its energy transferred to the dye, added oxidant won't get you anywhere. Some of the mechanistic studies have suggested that the ratio of oxidant to fuel should be 1:1. So the excess I'm describing may not be much.

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