For academic reasons I find separating energy into three distinctive issues; that is, Kinetic Energy, Potential Energy (or, Positional Energy) and Internal Energy. I must confess that I'm a chemistry professor, not a physics prof, so my distinctions between these energy 'types' is chemistry oriented. Here's my definitions and examples ...
Kinetic Energy is the mechanical energy of motion, Potential energy is mechanical stored energy and Internal energy is the energy content of a system due to its
state of existence and chemical structure. That is, state of existence meaning solid, liquid or gas and chemical structure meaning the substances molecular geometry. This is generally a macro molecular interpretation as opposed to a kinetic micro molecular particle level interpretations, but does give a more physical illustration of changes in internal energy.
Consider Water (good old H
2O)... It can exist as solid (ice), liquid (water) or gas (steam) but its chemical structure is always bent angular with two covalent bond and two non-bonded pair of electrons around the central element oxygen. To change the structure would be to change the molecule and the internal energy of water that supports this structural configuration.
https://www.google.com/search?rlz=1...TiAhXlmq0KHUmSCFAQ7Al6BAgJEA0&biw=1536&bih=722
Typically, for chemical reactions, changes in internal energy is calculated from the 1st Law equation ΔU
Rxn = Heat of Reaction (q) + Work of Rxn (w) = ΔH
Rxn + Pressure(P)⋅ΔMolar Volume of gasses (V), or simply ΔU = ΔH + P⋅ΔV
m.
Heat of Reaction (or, Enthalpy of Rxn) can be calculated using the Hess's Law Equation ΔH
Rxn = Σn⋅ΔH
fo(Products) - Σn⋅ΔH
fo(Reactants) which shows a clear differential numerically in energy content.
Work of Reaction depends upon changes in molar volumes of gasses (ΣV
m(products) - ΣV
m(reactants). If ΔV
m > 0 => work is exothermic expansion and is subtracted from the Heat of Reaction; if ΔV
m < 0 => work is endothermic compression and is added to the Heat of Reaction and if If ΔV
m = 0 => work is isothermic (w = 0) and ΔU
Rxn = ΔH
Rxn.
Example (Open System, ΔV
m ≠ 0)
3H
2(g) + N
2(g) => 2NH
3(g); ΔH
Rxn = -91.8 Kj
The reaction shows 4V
m(reactants) → 2V
m(products) => decrease in 2V
m net change in molar volume and is a compression reaction. Therefore, work is endothermic (w > 0 ) ~ +5.0 Kj. Therefore, the change in Internal Energy (ΔU) = ΔH + P⋅ΔV
m = -91.8Kj + (+5.0Kj) = -86.8Kj (net).
Considering structural differences between reactants and products, one can see a clear difference in structure between H - H and O=O giving a pyramidal structure for gaseous ammonia. That is, the internal energy of ammonia is 91.8Kj less than the internal energy of H
2(g) and O
2(g).
https://www.google.com/search?rlz=1....0.60.534.10...1...1..gws-wiz-img.xzAEwcWMe5g
Clearly different structure than H - H and O = O and thus different internal energy holding the structure in a pyramidal geometry.