[shoofy2]

Perhaps I am conceptualizing this wrong --

Suppose an exothermic reaction occurs, and thus the energy of the molecules increases. This energy will then be dispersed through heat or work.
**EDIT: I am making a small leap here and concluding that all exothermic reactions don't necessarily have to result in a release of heat, rather all that results is increased energy in the molecules. My textbook describes it as a process in which heat is evolved, but surely a reaction can take place where energy transfer to the surroundings is unable to occur, and thus heat would not be evolved, nor would work be done. Pressure and temperature would merely increase as a result of increased internal energy. I could be off on this, feel free to set me straight.**

So, supposing that work can be done on the surroundings (suppose a piston-type system), and heat can also be transferred to the surroundings (suppose colder surroundings), what variable(s) determines what form of energy transfer the system will favor?

Thank you.

[renge ishyo]

Well, if you look at the *microscopic* scale then for both heat and work there is only a net transfer of energy from the higher temperature body (where the molecules are moving faster) to the lower temperature body (where the molecules are moving slower) until the relative distribution of velocities between the two bodies are equal. Whether or not this phenomenon is considered as *macroscopic* heat transfer or work depends on the effects you see on the large scale.

For example, suppose you heat up a metal cylinder filled with gas that has a piston attached to it such that the gas in the cylinder is much hotter than the surrounding air. Further suppose that you clamp the piston in place so that it doesn't move and no macroscopic "work" can be done. What happens? Well, initially the gas molecules are moving much faster than the air molecules outside the cylinder. When the gas molecules collide with the atoms on the inside face of the metal cylinder, the atoms in the metal cylinder vibrate more rapidly, and the gas molecules inside the cylinder slow down after the collision. These "sped up" atoms in the metal cylinder then smash into the slower moving air molecules that come close on the outside of the cylinder...speeding up the air outside and slowing down the vibration of the metal atoms on the cylinder. This process repeats over and over again until the average speed of molecules colliding with the metal from the gas inside the cylinder and the average speed of the molecules in the air outside the metal cylinder are the same, so that no net energy transfer is occurring between the inside and the outside. At the macroscopic level, since we observed no macroscopic work, we would conclude that the energy was entirely dissipated as heat in the process.

Now let's do the same experiment with the piston allowed to move so that the gas can "push the piston outward" thereby doing macroscopic work on it. How does the interpretation change? Well, the transfer of energy through the sides of the metal container occurs exactly as described before, but we also have some fast moving molecules colliding with the piston on one side of the cylinder and these molecules lose energy when they collide with the piston that speeds up the molecules in the piston "outward" causing the piston to "push" outward against the atmosphere (or whatever you put next to the piston). This moving piston then pushes against many air molecules which causes the speed of the air molecules to increase while the molecules in the piston are slowed down by the collision. At the macroscopic level you would interpret the situation as some heat being lost to the air, but also that some of the heat was "converted" into work since we saw the piston move. The interpretation at the microscopic level is that both scenarios are more or less the same, molecules just collide and release energy to other molecules until the net speed of all of the surrounding molecules is the same.

At the macroscopic level we subdivide these energy transfers into different things like heat and work to help us organize our discussion about how we interpret the energy transfer to the surroundings (did we see only a rise in temperature? or did we see macroscopic movement as well?). It may also be necessary to keep the distinction between heat and work for some technicians or even engineers who may not have developed a modern understanding at the processes occurring at the molecular level. These professionals are still very capable of interpreting things macroscopically in their work without considering what the individual molecules are up to, and hence there is justification for keeping the distinctions between work and heat there based on their macroscopic differences. Knowing that these processes are essentially the same thing at the microscopic level wouldn't really help them perform their tasks any easier, but a chemist or physicist needs to think about such things!.

Hope this helped.

[shoofy2]

Perfect. Exactly what I needed.

Thanks so much.