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Why do some chemical reactions occur and others don't? Some reactions are spontaneous — once the reaction is started it will proceed on its own. Others are nonspontaneous — the reaction needs a constant supply of energy. The free energy change (DG) is the measure of the spontaneity of a reaction.

Two factors determine the spontaneity of a reaction, the enthalpy change (DH) and the entropy change (DS). When a reaction is exothermic (DH < 0), spontaneity is likely, but a negative DH alone is not sufficient to predict spontaneity with certainty.

A process that becomes more disordered (DS > 0) has a tendency toward spontaneity, because disorder is the natural tendency of the universe. However, a positive entropy change alone is not sufficient to predict spontaneity with certainty.

Both DH and DS must be examined together, along with the reaction temperature, in order to determine if a process is (or is not) spontaneous. This equation shows the relationship between DH, DS, and the Kelvin temperature.

Unlike DH and DS individually, DG is a definitive predictor of spontaneity. A reaction is spontaneous if DG < 0. A reaction is nonspontaneous if DG > 0.

This was a condensed summary of material covered in lecture on Monday, March 24 and Wednesday, March 26. If reading the preceding paragraphs was like reading Greek (or geek), you may wish to review your lecture notes and read section 7.3 of the textbook before continuing. Understanding these concepts and terminology is very important to successful completion of this assignment.

DH = – 198 kJ and DS = – 0.187 kJ/K

10.	According to this information, do diamonds spontaneously turn into graphite? Explain.

Since we don't observe diamonds turning into graphite, something else must be going on. A negative value for DG indicates that a reaction is spontaneous — once started the reaction will proceed on its own. What DG does not tell us is how quickly the reaction will occur. It just so happens that diamond turning into graphite is an incredibly slow reaction. Monday's (3/31) lecture is on kinetics, the study of the rate (speed) of chemical reactions.