I believe exothermic chemical reactions are how all fossil fuel energy works.

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Yeah, kind of the definition of those types of energy. Gas plants, coal, for the large end, as well as some trash plants that create energy. On the small end is gasoline or diesel generators, etc. We could build wood fired plants, if anybody wanted to deal with the high level of maintenance and pollutants created.

Anything burning is an exothermic reaction.

I get what OP is getting at, they saw a demonstration in science class where sodium was dropped in water and it created tons of heat and realized it could maybe boil water that runs a turbine.

The problem is that anything that reactive like that has already reacted, so you aren't going to find it in pure form. There aren't just rich deposits of pure sodium sitting around on earth, that sodium demonstration is using sodium that was created in a lab. Creating that pure sodium took as much or more energy to make than you'd get out of it.

The reaction from limestone with water is exothermic. As an example. Limestone is plentiful, water is plentiful. I guess the point of a chemical reaction is that it eventually we just run out of reactants. And that is not sustainable.

I wonder if there was some kind of recycling loophole you could design for a very low loss energy production cycle. Seems like it should be possible at least somehow.

Law of energy states energy is neither created nor destroyed, only changed in form. You aren't yielding additional energy from an exothermic reaction, the energy being given off was stored in the reagents. I think you're talking about cold fusion maybe?

Exothermic reaction is just a general thermodynamics term for reactions with negative delta H. This is like asking if the sun is a good avenue for energy. Your question is so general that it doesn’t really make sense, sorry.

Isn't this essentially what batteries and hydrogen cells are?

The hardest part of creating a perpetual motion machine is figuring out where to hide the motor.

Problem is, the only materials that fit your description and are cheaply available in the massive quantities needed are fossil hydrocarbons.

That checks out

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Like lye and water heating up? We could try but we'd have to make an awful lot of soap to neutralize the lye. Imagine a car running on that but every day you'd have to remove 10 lbs of soap

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I think OP is imagining a perpetual energy generating reaction with no downside (such as reactants getting consumed). As /u/chemchris says, and I extend a bit... mass-energy does not get created or destroyed (noting Einstein's E=mc^2 converts mass to energy, as we see in nuclear fission).

I was literally about to be like "but... gas...?" lol

Damn Hydrocarbons and their deliciously dense energy, walkin' around all caked up like that. Double cheeked up on a Thursday afternoon. Damn.

No you need to find something that is at higher potential energy to have an exothermic reaction. Only known natural source of a substance that has high potential energy is fossil fuels. All other sources are made by man where energy is put in to create these substance

Aside from what everyone else has said, I had a college professor who had a great side business designing chemical plant "heat reclamation" systems where heat would be taken from exothermic reactions and joined in system with endothermic reactions occurring in other parts of the plant, thus eliminating waste from separate heaters and coolers.

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The reaction from limestone with water is endothermic, it consumes energy. Limestone is made by tiny fossil shells piling up on the ocean bottom.

OP about to discover fire

Yeah! That’s what I’m talking about!

I guess what i’m saying is, you need fuel for an exothermic reaction. There are a million possibilities here. Which fuel is OP referring to?

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Except, you know… wood.

Actually this isn’t quite correct. The limestone must first be burned to make calcium oxide (that is energy in) and then when it’s mixed it creates an exothermic reaction when hardening.

Limestone is itself already chemically similar to hardened concrete.

Right. But that is not limestone reacting with water, it is CaO (aka quicklime) reacting with water.

Yeah. That’s what I said.

Needs detail as it comes off as an r/askreddit post

In the business, we call them "Economizers". Often times our waste streams are needlessly hot and our input streams are too cold, so we'll have the 2 trade some heat to reduce out outlet cooling + inlet heating needs.

People have started doing the same thing with home water lines actually' pre-heat the cold water going to your water heater by taking heat from the shower drain essentially.

Also Town of Holland, MI uses waste heat for heated sidewalks in the winter.

Lots of ways to re-use heat if you look hard enough.
Source: 12 yr Chemical Engineer

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Yeah - that's the point I think. Maybe OP thinks an ecothetmuc reaction is creating free energy!!