The surrounding is the rest of the universe. A reversible reaction is a reaction occurring at or very near equilibrium. This means that the concentrations of reactant and product are at those that cause Q to equal K eq . There is a dynamic equilibrium and the reaction is occurring in the forward and reverse reaction at equal rates. It can be said that an irreversible reaction for the forward direction is spontaneous in the forward direction.
Thermodynamic quantities are extensive properties.
This means the value will depend on how much material there is. Thermodynamic quantities are functions of state. Their value depends only on the current state, not on the path taken to get there. The internal energy of the molecule is the total energy of the molecule. This includes translational, rotational, vibrational and electronic energy.