• summary_type: application
• word_count: 160

The Q-value of a nuclear reaction equals (total mass of reactants - total mass of products) x 931.5 MeV. If Q > 0, the reaction is exothermic (energy released); if Q < 0, it is endothermic (energy must be supplied). For endothermic reactions, the threshold kinetic energy of the projectile exceeds |Q| because momentum must also be conserved: KE_threshold = |Q| x (1 + m_projectile/m_target). Conservation laws in nuclear reactions: charge (Z), mass number (A), energy, and momentum are all conserved. To identify unknown products: balance Z and A on both sides. JEE problems typically give masses and ask for Q-value, or give the reaction and ask for energy released. When using atomic masses, the electron masses cancel for nuclear reactions that conserve charge. For beta-plus decay, subtract 2m_e*c^2 from the atomic mass difference to get the correct Q-value.