http://www.ck12.org Chapter 20. Entropy and Free Energy
- The sign of∆G predicts the behavior of a chemical reaction at a constant temperature and pressure. If∆G is
negative, the forward reaction will proceed spontaneously, if∆G is zero, the reaction is at equilibrium, and if
∆G is positive, thereversereaction will be spontaneous. - ∆Grxncan be calculated at 25°C using∆G°fvalues.
- ∆Grxncan be calculated at other temperatures from the values of∆Hrxnand∆Srxn.
Lesson Review Questions
- What thermodynamic quantities are used to define Gibbs free energy?
- Define each component of the equation describing Gibbs free energy.
- What is true for all spontaneous processes regarding Gibbs free energy equation?
- What is the change in Gibbs free energy for the formation of any element in its most stable state at 25degC?
- What can be inferred from a very large∆Grxn? From a very small∆Grxn?
- What does a∆G value of zero imply?
- Describe how it is possible that a reaction is spontaneous at some temperatures but not at others.
- Use the table at http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/The-Free-Energy-629.html to
calculate∆G°rxnfor the following reactions at 25°C:
a. CaCO 3 (s)→CaO(s) + CO 2 (g)
b. 2Mg(s) + O 2 (g)→2MgO(s)
c. 2SO 2 (g) + O 2 (g)→2SO 3 (g)
d. 2C 2 H 6 (g) + 7O 2 (g)→4CO 2 (g) + 6H 2 O(l)
Further Reading/Supplementary Links
- Gibbs free energy: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/gibbs.php
- ∆G values: http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/The-Free-Energy-629.html
Points to Consider
- How does the equilibrium of a reaction affect the free energy?
- Can we use free energy calculations to determine equilibrium constants?
- Can we use equilibrium constants to determine free energy changes?