Consider the following gas phase reaction. AB2 + 3 C AC3 + 2 B
What is the enthalpy of the reaction if the A-B bond energy is 130. kJ/mol and the A-C bond energy is 860. kJ/mol?

Technically, "quantum theory" is actually just the theory associated with quantum mechanics, ie; the theory of any objects isolated from their surroundings but, because it is very difficult to isolate large objects from their environments, it essentially becomes a theory of the microscopic world of atoms and sub-atomic particles.