1.1 What is Chemistry?

http://www.ck12.org Chapter 20. Entropy and Free Energy

20.2 Spontaneous Reactions and Free Energy

Lesson Objectives

• Define Gibbs free energy, and be able to calculate the change in Gibbs free energy for a given process when
  provided with the appropriate data.


• Use the value for Gibbs free energy to predict whether a reaction will occur spontaneously.

Lesson Vocabulary

• Gibbs free energy: A thermodynamic quantity that combines enthalpy and entropy into a single value in order
  to predict whether or not a process is spontaneous.

Check Your Understanding

• What must be true for a reaction to be considered spontaneous?

Gibbs Free Energy

We have learned that a spontaneous reaction must increase the total entropy in the universe:∆Suniv>0. To analyze
the entropic effects of a given process, we generally break this quantity down into two separate components. This is
expressed with the following equation:

∆Suniv=∆Ssys+∆Ssurr>0

In general, the entropy change for a chemical reaction or phase change can be easily determined from standard
entropy values. Additionally, we saw in the previous lesson that the following relationship is true:

∆Ssurr= -∆HTsys.

Substituting this into the above equation, we get the following:

∆Suniv=∆Ssys-∆HTsys>0

This equation can then be rearranged as follows:

T∆Suniv= -∆Hsys+ T∆Ssys>0

• T∆Suniv=∆Hsys- T∆Ssys<0