MODULE 16

Key Concepts

Unit 1: Spontaneity and Second Law of Thermodynamics

Unit 2: Free Energy

Unit 3: Free Energy and Equilibrium

Unit 4: Third Law of Thermodynamics

In the earlier chapter on Thermochemistry, we looked at the energy requirement for a chemical reaction.  We explored different methods to determine how much energy will be needed or released in a chemical reaction.  Thermodynamics adds another dimension to this concept.  One of the most critical roles of thermodynamics is predicting if a chemical reaction can occur on its own under a specific set of conditions.

Leave an apple slice out at room temperature, and within an hour or two, the white flesh turns brown. This is an excellent example of a spontaneous chemical reaction between the apple and the air. The apple’s pale flesh turns brown and does not turn white again. The reaction is spontaneous only in one direction and not in other direction. Have you ever wondered why?

Iron rusting is another example with similar observation. Iron combines with Oxygen to make rust spontaneously. At the same time, rust does not change back to Iron and Oxygen gas on its own.  What makes this reaction spontaneous in one direction and not in the other?  We will try to answer this question with the help of the laws of thermodynamics in this chapter.

The First Law of Thermodynamics says energy cannot be created or destroyed.  Using this law, we can predict if energy will be absorbed or released in the overall reaction. The release of energy favors the spontaneity of the reaction, bringing the system to an overall lower state of energy.  However, this factor alone is not enough to predict if a particular reaction will take place spontaneously or not under the given set of conditions.