There is another aspect of chemistry that appears with chemical reactions: a rather physical concept that describes the changes in chemical energy in another way, a non molecular, macroscopic way.
You don't look at it on a particle level.
We here talk about this theory just in a global and limited way. We look at the most important concepts whereby the physical concept 'work' is left out.
The next concepts are to be acknowledged: energy (chemical, cinetical, electrical, potential and radiation); activating energy, endo and exothermic, bondings energe.
The main law of the thermodynamics is: energy never gets lost, and nothing appears out of nothing.
In fact something like the law of conservation of mass.
We know the first main law as 'the law of Hess', formulated long time ago, long before the thermodynamics.
Loss or gain of energy by substances during a chemical reaction depend only on the energy levels of the reactants and the products, not on the way we walk from reactants tot products.
But, of course, one form of energy can transfer into another form, and that is also true for the physical concept of 'work'.
This 'work' (p x ΔV) is considered as the manifest energy. Say: you can observe that energy.
Energy without 'work' gets the symbol: U.
The energy got with work has the symbol H (also called enthalpy, but in this course we leave out this concept).
So: U is the sum of H and 'work'.
During a chemical process, there is transfer of energy: ΔH and ΔU. U and H do change.
ΔU is the reaction energy (with negative sign of the system loses energy, and positive when the system gains energy).
Here a new kind of energy appears, and we call that ENTROPY with the symbol S.
S is not a positive form of energy, like heat or electricity that is helpfull for men.
No, the entropy is in fact a kind of negative energy, lost energy.
S is connected to disorder. Every system has a certain amount of disorder: the better organised the system, the less entropy is in it (like in cristals).
As soon as you dissolves such a cristal in water, the whole order gets lost. All ions of the cristal are loose, are freely swimming, the disorder of the system has increased: the entropy S increases.
All this on cost of the useful forms of energy; you cannot ignore this process. The entropy must be included, in one way or another, in the total amount of system energy.
That's how we came to define another concept: the energy that includes all forms of energy:
the free energy, with the symbol G
So G contains H as well as S.
A changing chemical system (mostly in a reaction) suffers changes in G: ΔG
and this change means also change in H and in S.
The mathematical formula that indicates these changes:
ΔG = ΔH - TΔS
ΔS gets a negative sign because it represents a kind of negative energy.
Entropy is directly connected to temperature, what can be seen as T (Kelvin) in the formula.
Maybe you can imagine that particles at higher temperature are going to move much faster, and that the disorder thus increases.
Spontaneous and non spontaneous reactions.
You can consider the process "kitchen salt dissolves in water" as a changing system.
First you have salt(s) and water(l), and afterwards a salt solution appears:
NaCl(s) NaCl(aq) ΔH > 0
This is not really a chemical reaction, but something happens with the particles:
The ionic lattice is broken
The ions are being hydrated (surrounded by water molecules)
The total process is endothermic (costs energy), and still the total process is spontaneous.
The reason for that is dhe enormous increase of entropy during this process.
This yes or no occurrence of a chemical reaction has everthing to do with the thermodynamical data.
there is a so called second main law of thermodynamics, saying:
Processes are spontaneous when there is increase of entropy.
You can also say: if the amount of disorder increases during a chemical process, than this process is spontaneously, even if the process is endothermic.
If ions in an ionic lattice are removed from each other, and can freely move throughout the solution, than the amount of disorder has increased as also the entropy.
Dissolving a salt in water is clearly a spontaneous process.
Now imagine an explosion: a solid, with lots of order (little entropy) changes completely into gases (lots of disorder, lot of entropy); Explosion must be very spontaneous.