WHAT REACTIONS GIVE OFF ENERGY?

This applies to chemical and nuclear rections and physical changes such as condensation.

One way to answer this question is in terms of the forces involved. If A attaches itself to B and they are attracted to each other as A approaches B, then energy must be given off. Listen for the sucking sound, speaking metaphorically. A magnet attracts a chunk of iron. Then in order to stick together after collision, some kinetic energy must be converted to heat. Other cases like that include chemical reactions like burning, and physical changes: gas to liquid or liquid to solid. If there are competing forces, go with the one that does the most work as A approaches B. If the attractor does more work, energy must be given off.

You could rig up two toy train cars on a track with magnets arranged so that they repel each other. Now if you had some kind of latching mechanism, it is possible to have a collision such that you end up with magnetic potential energy after the collision. This is the mechanical equivalent of an endothermic reaction, one that absorbs heat (as opposed to an exothermic, which gives off heat).

In the case of nuclei of atoms, there is an attractive and a repulsive force. The attractive force is greater at short distances, like the size of a nucleus, but drops off very rapidly with distance. The question is, which force involves more energy, the nuclear strong force (attractive) or the Coulomb force (repulsive)? It turns out that the attraction is the big deal for small nuclei, and the repulsion is boss for the large. Hence if you jam the little ones together, there is energy given off. A hydrogen bomb uses one of various possible schemes to build the helium nucleus out of smaller nuclei.

A helium nucleus consists of 2 protons and two neutrons (really 12 quarks- 6 up and 6 down. The protons and neutrons do not retain their individual identities in the nucleus?? I don't think anyone knows. Email me at fredrick.gram@tri-c.edu if you have evidence about this, because I want to know.).

In the sun, hydrogen is gradually being converted to helium. The metabolic rate (power output divided by mass) is very low; your own metabolic rate is about a watt per kilogram, or about 5000 times that of the sun. (Obviously we do not want to mimic the sun in a controlled fusion process- the output would be so very feeble.)

For big nuclei like uranium, energy is given off if they are split. This is because of the large Coulomb (electric) repulsion between the fragments. The nuclear attraction takes away energy as they separate, but the Coulomb force does more than enough work to make it exothermic (energy given off).

To calculate the energy given off by nukes we do not need to know the details of the forces involved in these nuclear reactions if we know the masses of the particles before and after. Mass and energy are really one thing, mass-energy. Or you could say energy has mass or mass has energy according to E = mc². Then we look at the mass of the end product (helium nucleus, for example) and the masses of the nuclei it was built out of. If the final mass is less than the initial, then energy must have been given off during the reaction. Some people use this as an explanation of why it works: mass is "converted" to energy. This promotes a misunderstanding of it. It is really no more of an explanation than to say that the reason a burning log gives off energy is that mass is being converted to energy. More about mass-energy here. Or go back to the other energy here. Or to the main physics page. If you don't, I will steal your kryptonite and your energy will be kaput.

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email me at fredrick.gram@tri-c.edu