How are nuclear fission and fusion compatible with the law of conservation of energy? During fission $He$ splits into 2 hydrogen atoms along with enormous amount of heat energy and hydrogen also combine to form helium with enormous amount of heat energy. But in my opinion to follow the law of conservation of energy one of the reactions must be endothermic. Please clear this vagueness.
[Physics] Nuclear reactions and energy conservation
energy-conservationnuclear-physics
Related Solutions
The answer lies in this diagram
It shows the binding energy per nucleon over the number of nucleons n in a nucleus. As you can see, it has a maximum around Iron (n=56), which means that left from iron energy can be released by fusion. For elements heavier than iron you need to put in energy to fuse them together, or equivalantly that fission is energetically favorable. That is why conditions like in a supernova are necessary to produce the heavier elements.
So, fusioning ${}^2H$ to ${}^4He$ releases energy, but the reverse fission process costs energy. There is no net gain, even in a perfect aparatus.
In a fission reaction it's nuclear potential energy that is converted to kinetic energy.
The nucleons are bound together in the nucleus by the strong nuclear force. The total amount of energy it would take overcome this force and disassemble the nucleus to individual protons and neutrons is called the nuclear binding energy.
The same nucleons exist before and after a fission reaction but the total amount of nuclear binding energy depends what which nucleus (or nuclei) they are in. See the graph below.
The average binding of energy of the nucleons becomes greater when the uranium atoms undergo fission and form lighter nuclei. This is to say it would now take even more energy to disassemble all to individual protons and neutrons. It's a bit like falling further down a hole. It now takes more energy to get out.
The nuclear potential energy is mostly converted into kinetic energy of the daughter nuclei, a small fraction (about 6%) of the energy goes into gamma rays and fast neutrons.
The mass of the products is less than the mass of the original nucleus as calculated by $E=mc^2$ but this is not unique to nuclear reactions, the same would be true for burning coal or letting water flow over a dam. It's just that in most non nuclear reactions the change is mass in immeasurably small.
Best Answer
The energy released/absorbed in nuclear reactions depends on the binding energies of the nuclei.So yes,energy is released in nuclear fusion of hydrogen to helium while energy will be absorbed in the reverse fission reaction.The curve below illustrates this point precisely: