Hello, I asked this similar questions on a different thread but it may be more appropriate for the chemistry forum. What is the reason we can only measure energy "changes" and not the actual energy value associated with a system? Absolute energies and enthalpy cannot be determined but I am not sure why... I guess we can still say that a system has more or less energy than another system but we cannot know the actual energies of each system, only their difference... However, when a car moves at a certain speed, we can certainly express the car's kinetic energy without a problem... Thanks!