What is standard state ?

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

When reaction is at standard state at equilibrium, then

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

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