State function of a system is the thermodynamic function that depends on the state but not on the path along which the system is carried out. example: Pressure, Temperature, Volume.

The amount of heat evolved or absorbed in a chemical reaction when the number of moles of the reactants as represented by the chemical equation have completely reacted, is called the enthalpy of reaction.

Hess's law states that the change in enthalpy for a reaction is the same whether the reaction takes place in one or a series of steps.

Standard free energy of formation is the free energy change that occurs when 1 mole of a compound forms from its component elements at standard conditions.

The standard change in free energy, ΔG, for a reaction is related to its equilibrium constant, K, by the equation ΔG = -RTlnK.

State variables of a system are physical quantities whose change in value only depends on initial and final state of system and doesn't follow on the path which system follows during change.

Internal Energy of a system is a sum of all kinds of energies possessed within it.