how is hess's law used to calculate the enthalpy of a reaction?

In the 1800s, one of the statements in john dalton's atomic theory was that atoms are indivisible. later experimental evidence led to the discovery of subatomic particles such as neutrons, electrons, and protons. what happened to the indivisible atom part of dalton's atomic theory, and why?

Ais a mountain created from eruptions of lava, ash, rocks, and hot gases.

If anyone would be able to me out with these three questions it would be these are from the chem 2202 course.