The second law makes it much worse. If you take concentrated oxygen and concentrated carbon (soot), you can burn them in a sealed chamber into pure CO2. If you

The second law makes it much worse. If you take concentrated oxygen and concentrated carbon (soot), you can burn them in a sealed chamber into pure CO2. If you are very careful about extracting the energy, you might get most of it. But already the entropy has increased, because now the carbon and oxygen have mixed together. Attempting to reverse the reaction will need extra energy to deal with that. It gets much worse though, if you open up our chamber and vent the gas into the atmosphere. The entropy goes through the roof as the CO2 concentration drops to a few hundred PPM, and the cost of reversing the reaction goes up even more.