Physics · nuclei · NEET
It can be either, and the sign tells you which. Q-value is defined as (mass of reactants minus mass of products) times c^2. A positive Q means the reactants were heavier, so mass disappeared and turned into energy — energy is RELEASED (exothermic). A negative Q means the products are heavier, so you had to PUT energy in to make extra mass — energy is ABSORBED (endothermic). So Q-value is one number whose sign decides released vs absorbed.
You can use either, and both give the same answer. Method 1 (masses): Q = [ (sum of reactant masses) - (sum of product masses) ] x c^2. If masses are in atomic mass units u, then Q = (mass difference in u) x 931.5 MeV. Method 2 (binding energies): Q = (total binding energy of products) - (total binding energy of reactants). Use whichever data the question gives. If you are given atomic masses, use Method 1; if you are given binding energy per nucleon, use Method 2.
A negative Q-value means the reaction does not happen on its own. The products weigh more than the reactants, so mass had to be created, and creating mass costs energy. You must supply at least that much energy (as kinetic energy of the bombarding particle) for the reaction to occur. Such reactions are called endoergic or endothermic. Fission and fusion of light nuclei have positive Q (they release energy); many artificial bombardment reactions have negative Q.
Binding energy belongs to ONE nucleus — it is the energy needed to break that single nucleus into free protons and neutrons. Q-value belongs to a WHOLE REACTION — it is the net energy released or absorbed when reactants change into products. In fact Q = (binding energy of products) - (binding energy of reactants). Energy is released (positive Q) exactly when the products are more tightly bound, meaning their total binding energy is larger.
Because 1 atomic mass unit (1 u) is equivalent to 931.5 MeV of energy, from E = mc^2. Nuclear masses are usually listed in u, so instead of converting to kilograms and multiplying by c^2 in SI units, you just take the mass difference in u and multiply by 931.5 MeV/u. Example: a mass loss of 0.05 u gives Q = 0.05 x 931.5 = 46.6 MeV released.
It is the net energy released or taken in during the reaction. It equals the mass lost (reactants minus products) multiplied by c^2, or in u it is the mass difference times 931.5 MeV.
Q is positive when the total mass of the reactants is greater than the total mass of the products. That lost mass becomes energy, so the reaction releases energy and is called exothermic or exoergic.
Positive. In fission a heavy nucleus splits into lighter, more tightly bound fragments, so mass is lost and about 200 MeV is released per fission of a uranium nucleus, giving a large positive Q.
Yes. Q = (total binding energy of products) - (total binding energy of reactants). If the products are more tightly bound, Q is positive and energy is released.
For NEET it is almost always in MeV (mega electron volt). You get MeV directly by taking the mass difference in atomic mass units u and multiplying by 931.5 MeV/u.
