5.1 The First Law of Thermodynamics, Enthalpy, and Phase Changes

Want a better handle on the 1st Law of Thermodynamics? Or maybe help with which phase changes are endothermic vs exothermic? Not to worry, Chad can help!
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[@hubercats]

This is the best explanation of these concepts I've ever seen. Thanks so much for sharing this content!

[@ChadsPrep]

Glad it was helpful!

[@sciencenerd7639]

If we were to compare the heat released from 1) a piece of wood rotting over the years until it is completely gone 2) same size piece of wood that gets burned up completely in a fire would it be the same amount of heat? Seems to me it would be the same amount of heat. In the case of the rotting wood, pretty much all of the cellulose & lignin ends up being converted to carbon dioxide and water as microbes use it up in cellular respiration. With the combustion, pretty much all of the wood becomes carbon dioxide and water, except some ash.

[@ChadsPrep]

interesting question! Are we talking about Carbon released here or heat? If you are comparing the heat of combustion with heat generated in respiration - these are not equivalent because a significant amount of energy is stored in organic compounds during the multi-step enzyme controlled respiration reactions. In terms of the carbon cycle, bacteria and other decomposers release a significant amount of methane as well as converting carbon into other organic compounds. Other detritivores such as termites, earthworms etc break down and feed on the wood in addition to decomposition by bacteria and fungi so there are too many different factors to take into account to make a simple comparison. In both cases all the matter and energy is accounted for in some form or other just dispersed into various different forms.

[@hubercats]

Hi Chad - I've been studying air conditioning systems lately and trying to understand the associated thermodynamics. Am I correct in viewing the refrigerant enclosed in the system to be "the system"? The basic cycle seems to be one where work is added to "the system" by the compressor (PV work). Then the compressed refrigerant gas is fed through a small orifice causing a pressure drop in the aptly named evaporator coil. During its passage through the evap coil the refrigerant picks up a bunch of latent heat on account of the refrigerant boiling. The refrigerant next passes through a condensing coil where the refrigerant condenses and hence throws off latent heat to outside of the system. Summarizing, it seems that PV work is added to the system by whatever motor is turning the compressor, then heat is absorbed in the evaporation phase and then rejected in the condensing phase. This cycle then repeats continuously. Am I correct to describe the refrigerant as "the system" in this example. - Thank you! - Jim PS: Since the refrigerant is enclosed (i.e., not in contact with the surroundings, it seems that the expansion that occurs just prior to the evaporation coil does not correspond to energy leaving the system. I think it's this step that is most confusing for me. I'm also confused about the energy balance in the system. I.e., if the pump is adding energy to the system, is there anything that removes this energy given that the system is closed.