I was reading this answer and I saw an energy diagram:
Does the energy belong to the system or to the outside?
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$\begingroup$ I mean "the system" (~reactants) $\endgroup$– Marouane RASSILICommented Mar 18, 2015 at 10:49
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$\begingroup$ What "energy" are you referring to here? $\endgroup$– Kyle KanosCommented Mar 18, 2015 at 12:56
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$\begingroup$ The energy mentioned in the vertical Axis. $\endgroup$– Marouane RASSILICommented Mar 18, 2015 at 13:23
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$\begingroup$ That bar represents the direction of an increase in energy, much the same way the axis lines for an $x$-$y$ plot. $\endgroup$– Kyle KanosCommented Mar 18, 2015 at 13:30
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$\begingroup$ it represents the energy change of the reactants to its final state, no? $\endgroup$– Marouane RASSILICommented Mar 18, 2015 at 13:38
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1 Answer
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The big arrow labelled energy is just an axis. It's supposed to indicate the upwards is positive, i.e. energy is being added to the system, and downwards is negative, i.e. energy is being emitted from the system.
Re the extra question:
where does the energy released in a nuclear reaction go, is it kinetic energy or heat?
Heat is a collective phenomenon. It doesn't make sense to ask about how much heat a single object has. At the individual particle level all reactions produce either photons or kinetic energy. In the latter case the energy is released as the kinetic energy of the reaction products. There is a discussion of this in my answer to Can we make usable energy from subnuclear particles?.
answered Mar 19, 2015 at 9:20