Cracking the SAT Physics Subject Test

Disintegration Energy

Nuclear reactions not only produce new nuclei and other subatomic particles but
also involve the absorption or emission of energy. Nuclear reactions must conserve
total energy, so changes in mass are accompanied by changes in energy according to

Einstein’s equation ∆E = (∆m)c^2.

A general nuclear reaction is written

A + B → C + D + Q

where Q is the disintegration energy.

If Q is positive, the reaction is exothermic and the reaction can occur
spontaneously; if Q is negative, the reaction is endothermic and the reaction cannot
occur spontaneously. The energy Q is calculated as follows:

Q = [(mA + mB) − (mC + mD)]c^2

No Tricks Prefix

An exothermic reaction

RELEASES heat (energy).

An endothermic reaction

ABSORBS it. Exo-goes

outward, endo- goes

inward.

For spontaneous reactions—ones that liberate energy—most of the energy is
revealed as kinetic energy of the least massive product nuclei.