The free energy for the oxidation of glucose to CO₂ and water is -686 kcal/mol and the free energy for the reduction of NAD+ to NADH is +53 kcal/mol. What is the main reason that only two molecules of NADH formed during glycolysis when it appears that as many as a dozen could be formed?

The main reason why only 2 molecules of NADH is formed during glycolysis when it appears that as many as a dozen could be formed is because during glycolysis,most of the free energy that are available from the oxidation of glucose remains in one of the products of glycolysis known as pyruvate which acts like glucose but doesn't undergo metabolism.

Glycolysis is the splitting of sugar molecules into useable/ manageable forms,it can also be seen as the aerobic catabolism of sugar inorder to produce energy in the form of ATP,NADH and PYRUVATE.The following are the products of glycolysis; pyruvate,2 molecules of ATP and 2molecules of NADH.

Pyruvate are used in aerobic respiration or anaerobic respiration if oxygen is not available.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom