H = U + PV  

You do not need to know this equation or anything about internal energy                  

Total enthalpy of a system (H) is equal to the system’s internal energy (U) added to the work done against the surroundings to establish the system’s physical dimensions. That last ‘work done’ term equates to the system’s volume multiplied by its pressure. Internal energy can be thought of as all the work being done by a substance as its particles obey the natural laws of physics. The work being done is the relative movement of all the particles, including subatomic particles, and the potential energy of the particles due to their position relative to each other. 

 ΔG =  ΔH + TΔS                   

Gibbs energy is the energy available in a system to do work. Thus a drop in Gibbs energy means that a reaction does work going from reactants to products. This tendency to do work means that it is a feasible or spontaneous reaction. 

 The enthalpy of a substance (H) is only easy to calculate for an ideal gas. Real substances do not have enthalpy values but they do have entropy (S) values - you can look up S for ammonia under standard conditions but you cannot look up H. One reason for this is that S has a zero reference point - recall that a perfect crystal at absolute zero has zero entropy. It is completely ordered. There is no mess in this crystal. S = 0. Compare this to enthalpy, where we find that most substances still have energy, known as zero point energy, at absolute zero, H ≠ 0. The closest thing to measuring the enthalpy of a substance is measuring how much heat is lost or gained when the substance is formed from a given starting state or reagents, under specified conditions. In other words, we can measure ΔH but not H.

Literature values are usually empirical values. They are measured by experiment, even if the experiment uses Hess' Law and other known data to calculate the value. As long as the known data is obtained by observation rather than a model, it is empirical. Theoretical values are measured by eg. modelling particles as point charges and using what is known about their interactions to make predictions.