How do both fusion and fission bombs create an exothermic reaction?


Doesn’t fusion release energy, while fission requires it?

In: Chemistry

Not a nuclear physicist, but here is my understanding.

For fission, you normally have a nucleus that, if hit with a neutron, can absorb it and become another, very unstable isotope that will split in two on its own, releasing more neutrons and a lot of energy.
If on average, less than one neutron hits another atom and most exit your lump of material, you have a subcritical lump that will just sorta chill there.
If on average exactly one neutron hits, you have a critical mass that will sustain its own reaction. This is the state nuclear reactors balance in.
If it’s more than one, the reaction will continuously accelerate and basically you have a nuclear bomb. The principle there is to take two subcritical lumps, and jam them together, e. g. with explosives, to make a supercritical lump.

Overall, not much energy invested in starting the reaction itself.

Fusion makes use of the fact that, at temperatures and pressures like the core of our Sun, light nuclei will fuse together and make bigger nuclei, along with an even larger ton of energy than fission. Problem is, to achieve this first you need to actually get to these huge temperatures and pressures. Thermonuclear bombs do it by detonating a nuclear bomb around the hydrogen. For power plants, we’re kinda still working on it.

Overall, a lot of energy to start the reaction, but a whole lot more released by the reaction once started.

Short answer: no, both fusion and fission can be exothermic.

Long answer: whether a reaction is exothermic or endothermic depends on the deltaH value. Regardless of whether a reaction is fusion or fission, if the deltaH value for the reaction is negative, you have an exothermic reaction; if the deltaH value is positive, you have an endothermic reaction.

That chart explains the values of the Gibbs free energy equation in relation to spontaneity (rightmost column.) In order for a reaction to naturally occur, it needs to be spontaneous so the bottom three possibilities are what we are looking for – as you can see, while it is easier for exothermic reactions to happen when deltaH is negative (generally associated with breaking bonds) it is also possible when deltaH is positive (generally associated with making bonds.)

Both fusion and fission can either require or release energy, it simply depends on which elements are being fused / fissioned. Lighter elements are easier to fuse together than to break apart, while heavier elements are easier to break apart than to fuse together. [This chart]( demonstrates this.

It works because you use different elements.

In general element before Iron on the periodic table release energy if you put them together and element after iron release energy if you break them apart. Iron is element number 26. You can’t extract energy from iron from either fusion or fission

So the fission reaction in nuclear weapons if from uranium or plutonium that is element 92 and 93 you get energy. A fusion reaction as a hydrogen isotope and lithium so element numbers 1 and 3. So we use the last element that exists naturally on earth for fission bombs and the first and third element for a fusion bomb. This is not a coincidence.

This is also the explanation of why iron is so common, A start can burn lighter material but when the output is iron there is no way to get out more energy out. Iron and element before it is created in the normal life cycle of a star but element after is is only created when a star explodes in a supernova. The result is that Iron is the 9th most common element in the solar system and the 6th most common in the Milkey way,

In both types of reaction, there is less mass of matter left after the reaction than was present before the reaction. This ‘lost’ mass is released as energy, as described by Einstein in his equation E = mc^2

Adding oxygen to hydrogen to make water is exothermic. Adding oxygen to water to make peroxide is endothermic. How can adding oxygen be both?

With nuclear reactions, like chemical ones, whether energy is absorbed or released depends on the components of the reaction. Typically, small nuclei release energy when fusing, large ones release energy while fissioning (fizzing?). Fission bombs use uranium and plutonium, very large nuclei, while fusion bombs use hydrogen, the smallest one.