A quick web search at www.google.com yielded the answer. The energy goes to heat the acid slightly. Here is what I found at a website debunking perpetual motion machines called Go Ask Grandpa:

"Here is the attempt: Begin with two identical springs. Compress one of them and bind it with leather strapping. Note that there is potential energy in the compressed spring that is not in the other spring. Now - put each spring in an identical beaker of acid that will dissolve the springs, but not the leather. Now both springs are gone. What happened to that potential energy in the compressed spring? Did we destroy it?

The answer came later with a more careful experiment. The experiment was performed in a calorimeter. This is an insulated tank that keeps track of the heat so that none or nearly none is lost. It was found that the acid that dissolved the compressed spring got hotter than the acid that dissolved the uncompressed spring, and the extra heat was exactly the amount to account for the potential energy stored in the compressed spring. This was an amazing result at that time, and it well supported the conservation law."