If state 1 is at standard conditions, in which c 1 is unity (e.g., 1 atm or 1 M), it will merely cancel the units of c 2. We can, therefore, write the entropy of an arbitrary molecule A as S ( A ) = S ⊖ ⊖ ( A ) − − k ln ⁡ ⁡ [ A ] , where S ⊖ ⊖ is the entropy at standard conditions and [A] denotes the concentration of A. The change in entropy for a reaction