Fire is energy. We need energy. Can we use this as an energy source?
No.
A guy in a lab shoots a radio wave (using energy) at a beaker of salt water and fire starts atop it (creating energy). Unfortunately, the radio wave (or whatever method you use to break the bonds) consumes more energy than the fire releases. You spend two gallons of gas getting one for free. Therefore, it can’t be used as an energy source, because you haven’t actually found a source of energy. You’ve found a terrible exchange rate.
It costs energy to split the hydrogen and oxygen. Putting them back together does release energy but not as much as you spent breaking them apart. That is just a fact of nature.
If you can’t create energy or destroy it, how can a reaction end up with more or less energy than it started with?
The salt-water-on-fire reaction is a negative energy equation. Negative because you lose some energy to the system around you, or for processes that don’t pay off in the energy release, or just because you took three lefts to make a right. Breaking those bonds might cost more energy than they give back, depending on how you break them. Even if the reaction were perfect, though, you could only get as much energy out as you put in because you're simply reversing the action.
There are such things as positive energy equations. These exist when energy has been stored in something prior to the start of the reaction, and often involve a catalyst (something to lower the energy needed to make the reaction happen). The best example: gasoline.
Lighting gasoline on fire gives off more energy than the person who ignited it put in. The energy comes from the breaking of chemical bonds. They key is that nature put those bonds together, not us. We’re taking advantage of someone else’s work. It’s what we do when we eat food: the plant got energy from the sun, stored it in a little energy piñata, and we came along, broke it open and took all the candy. So to speak. The earth and millions of years under pressure put energy into oil and coal. The sun could heat things for us. The wind could move things for us. There are ways to get energy out without putting it in, but breaking apart salt water and putting it back together is not one of them.