Source of hydrogen in reduction of oxygen in electron transport?
The equation you've shown is similar to other cellular respiration equations that I've seen, in that, it is trying to simplify a pretty complex process.

My guess is that you're doing first year cell biology like I am, so you probably know that there is a difference between the pH of the mitochondrial matrix (7.8 - making it alkaline) and the inter membrane space (roughly that of the cytosol - usually about 7). The lower the pH of a solution, the greater the concentration of H+. My point is that the solutions on either side of the inner mitochondrial membrane both have a concentration of H+.

So even though, when NADH is oxidised, it loses a H+ to solution, thereby contributing to the pH of the mitochondrial matrix, what is the likelihood that that particular H+ will be the H+ that is part of the molecule that takes the electrons at the end of the electron transport chain? I think low.

Indirectly, NADH does contribute H+ to the overall pH of the mitochondrial matrix but it doesn't directly give the H+ to "half an oxygen gas molecule".

Check the video below which is from the publisher of the textbook my course uses.

http://www.garlandscience.com/garlandscience_resources/resource_detail.jsf?landing=student&amp;resource_id=9780815344544_CH14_QTM02