What is standard state ?

What is the enthalpy of formation of the most stable form of an element in its standard state ?

Enthalpies of all elements in standard states are-

When reaction is at standard state at equilibrium, then

When reaction is at standard state at equilibrium, then

When reaction is at standard state at equilibrium, then

The standard absolute entropy of a substance, (S^(Theta)) is the entropy of the substance in its standard state at 1 atm, temperature being

The standard absolute entropy of a substance, (S^(Theta)) is the entropy of the substance in its standard state at 1 atm, temperature being

The quantity which is not zero for an element in its standard state at 298 K is