what would cause entropy to decrease in a reaction
Spontaneous process This criterion can then be used to explain how it is possible for the entropy of an open or closed system to decrease during a spontaneous process. A decrease in system entropy can only occur spontaneously if the entropy change of the surroundings is both positive in sign and has a larger magnitude than the entropy change of the system:
Chemical reaction In a double replacement reaction, the anions and cations of two compounds switch places and form two entirely different compounds.[18] These reactions are in the general form:[19]