# Q value (nuclear science)

In nuclear physics and chemistry, the ** Q value** for a reaction is the amount of energy released by that reaction. The value relates to the enthalpy of a chemical reaction or the energy of radioactive decay products. It can be determined from the masses of reactants and products. Q values affect reaction rates.

## Definition

Considering the energy conservation of the simple reaction, enables the general definition of Q based on mass-energy equivalence, where K is kinetic energy and m is mass:

A reaction with a positive *Q* value is exothermic, i.e. has a net release of energy, since the kinetic energy of the final state is greater than the kinetic energy of the initial state.
A reaction with a negative *Q* value is endothermic, i.e. requires a net energy input, since the kinetic energy of the final state is less than the kinetic energy of the initial state.^{[1]}

## Applications

Chemical Q values are measurement in calorimetry. Exothermic chemical reactions tend to be more spontaneous and can emit light or heat, resulting in runaway feedback(i.e. explosions).

Q values are also featured in particle physics. For example, Sargent's rule states that weak reaction rates are proportional to *Q*^{5}. The *Q* value is the kinetic energy released in the decay at rest. For neutron decay, some mass disappears as neutrons convert to a proton, electron and antineutrino:^{[2]}

where *m*_{n} is the mass of the neutron, *m*_{p} is the mass of the proton, *m*_{ν} is the mass of the electron antineutrino and *m*_{e} is the mass of the electron; and the K are the corresponding kinetic energies. The neutron has no initial kinetic energy since it is at rest. In beta decay, a typical Q is around 1 MeV.

The decay energy is divided among the products in a continuous distribution for more than 2 products. Measuring this spectrum allows one to find the mass of a product. Experiments are studying emission spectrums to search for neutrinoless decay and neutrino mass; this is the principle of the upcoming KATRIN experiment.

## See also

## Notes and references

- ↑ K.S. Krane (1988).
*Introductory Nuclear Physics*. John Wiley & Sons. p. 381. ISBN 0-471-80553-X. - ↑ B.R. Martin and G. Shaw (2007).
*Particle Physics*. John Wiley & Sons. p. 34. ISBN 0-471-97285-1.

## External links

- Nuclear Structure and Decay Data – IAEA with query on decays'
*Q*-values