I have been told in class that while the $Q$ value of a nuclear reaction is positive the process is exothermic and if this parameter is negative the reaction is endothermic.

all of the fussion process are exothermic? and the fission ones are endothermics?

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    $\begingroup$ The preamble about the calculation of Q value is irrelevant. Your question is "Are all nuclear fusion reactions exothermic and fission reactions endothermic?" Probably you just wanted to pad out your question. It would be better to tell us what you think the answer is and why, and what searching you have done to find an answer. (Think about nuclear bombs or nuclear energy.) $\endgroup$ – sammy gerbil Mar 1 '17 at 5:32
  • $\begingroup$ The Hiroshima and Nagasaki atom bombs used the fission of Uranium and Plutonium to generate energy. The nuclear power plants are using fission to generate energy. $\endgroup$ – Yashas Mar 1 '17 at 5:40
  • $\begingroup$ Related, possible duplicate, to the question in the final paragraph: physics.stackexchange.com/q/125977/44126 $\endgroup$ – rob Mar 1 '17 at 6:44
  • $\begingroup$ The fusion of elements lighter than iron creates energy. The fusion of elements inclusively heavier than iron absorb energy. $\endgroup$ – BillDOe Mar 14 '17 at 19:17

The term endothermic process describes a process or reaction in which the system absorbs energy from its surroundings; usually, but not always, in the form of heat. The term was coined by Marcellin Berthelot from the Greek roots endo-, derived from the word "endon" (ἔνδον) meaning "within" and the root "therm" (θερμ-) meaning "hot." The intended sense is that of a reaction that depends on absorbing heat if it is to proceed. The opposite of an endothermic process is an exothermic process, one that releases, "gives out" energy in the form of heat. Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs, though in reality it only refers to where the energy goes, without necessarily being in the form of heat.

italics mine.

What makes me wonder is, all of the fussion process are exothermic?, and the fission ones are endothermic?

In both cases , fission and fusion, energy is released so both are exothermic, according to the definition of the words (remember heat and kinetic energy are related in statistical thermodynamics)


What makes me wonder is, all of the fussion process are exothermic?, and the fission ones are endothermics (I dont see how, but is this the case)?


First of all, the first type of fission to be technologically achieved was exothermic fission. Nuclear reactors use exothermic fission to generate power. However, examples of endothermic fission exist, as well as endothermic fusion.

All elements heaver than Iron are thought to have been produced by stepwise endothermic fusion reactions in supernovas (exothermic fusion ends with the element Iron). https://en.wikipedia.org/wiki/Nuclear_fusion.

Meanwhile, fission of Lithium-7 into Helium-4 and Hydrogen-3 by a high-energy neutron is an example of an endothermic fission reaction https://en.wikipedia.org/wiki/Tritium#Lithium. It is used in thermonuclear weapons to breed significant quantities of the rare Tritium isotope needed for the highly exothermic deuterium-tritium fusion reaction that supplies most of the weapon's power.


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