Why does each NADH give 2.5 ATPs but each FADH2 only gives 1.5.
Answer: When FADH2 is taken up by Complex II, it undergoes the same transfer of electrons through iron-clusters and then to Coenzyme Q, and QH2 is formed however, this is all not coupled to a pumping of protons out at this point. (It is only involved in the Q protons while NADH is involved in the Q protons AND the pump at Complex I)