3.2 Gibbs Free Energy Warm Up HH
H
HH
O
H H
O O
O
H
1. Write a balanced chemical equation (including phase subscripts) for the reaction shown in the diagram.
2. Use the values in Table A8 in the appendix at the back of this book to calculate ΔHrxn and ΔSrxn for the reaction.
3. What is the function of the candle in this process? ________________________________________________________________________________________________________ 4. Is this spontaneous process enthalpy driven or entropy driven? Explain. ________________________________________________________________________________________________________ ________________________________________________________________________________________________________
We have now seen that two factors — enthalpy and entropy — determine whether or not a physical or chemical event is spontaneous or thermodynamically favorable. Sometimes these two factors work Free Energy — Enthalpy and Entropy together. For example, when a stone wall crumbles, its enthalpy decreases and its entropy increases. Since a decrease in enthalpy and an increase in entropy both favor a spontaneous change, the two Combine factors complement one another. In other situations, the effects of enthalpy and entropy are in opposition. Such is the case in the melting of ice, the vaporization of water and even the formation of water from its elements as in the Warm Up. When enthalpy and entropy effects oppose one another, the overall reaction spontaneity is less obvious. We can determine the net effect of the two factors through another thermodynamic quantity called Gibbs free energy, G, named after Josiah Willard Gibbs (1839–1903), a U.S. scientist (Figure 3.2.1). Gibbs free energy is defined as: G = H – TS. For a chemical or physical change at constant temperature and pressure, the equation becomes: ΔG = ΔH – TΔS
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Chapter 3 Thermodynamics 191