why does my iphone 5 heat up and battery lose power much too quickly

My IPhone 5 heats up and loses power very quickly. It will go from full charge to 31% within a couple of hours! I've had the phone only six months or so and it was a replacement for my first IPhone 5 which never worked at all after two weeks! I admit I am getting very frustrated with what seems like a total lack of quality control on the IPhone. I replaced my IPhone 4 due to this same problem. Are there certain apps that cause this and if so, why can't Apple warn me or tell me about them? Every time I talk to tech support I get a different answer so I am reluctant to go down that road again. I had enough of it a few months ago trying to get the bad IPhone 5 replaced!