- Online Text
- 1. Introduction
- 2. Microstates, Macrostates, and Entropy
- 3. The Entropy of Energy Quanta
- 4. Entropy and States of Matter
- 5. Spontaneity and Gibbs Free Energy
- 6. Coupling Reactions
- 7. Equilibrium
- 8. The Equilibrium Constant Expression
- 9. Le Chatelier's Principle
- 10. Temperature and Equilibrium
- 11. Conclusion
- 12. Further Reading
- Unit Guide (PDF)
Section 7: Equilibrium
The ΔG value of a chemical equation only tells us if the reactants are more stable than the products, or vice versa. If ΔG is negative, the products are more stable than the reactants; if ΔG is positive, the reactants are more stable than the products. If the products are more stable than the reactants, this does not mean that the reaction will continue until all of the reactants have changed into products. Some of the time, the most stable situation is when there is a mixture of reactants and products. The reaction will continue until it reaches this state of maximum thermodynamic stability (lowest Gibbs free energy).
Figure 9-9. Equilibrium
Chemical reactions spontaneously progress toward equilibrium where free energy is at a minimum, just as gravity pulls a ball to the lowest possible level.
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The situation is analogous to a ball rolling into a valley, as shown in Figure 9-9. When the ball sits at the top of the hill at position A, the reaction vessel contains 100% products, and the system has high free energy. When the ball is at position B, the reaction vessel contains 100% products. Note that position B is more stable than A because it is lower. The most stable position is in the valley between A and B. At this point, the system has reached minimum free energy—maximum thermodynamic stability.
This position is called "equilibrium." At equilibrium, the reaction appears to have stopped, because the amounts of reactants and products are constant. They are constant not because the reactions have stopped, but because the forward and reverse reactions are happening at the same rate. At equilibrium, products are formed via the forward reaction and destroyed by the reverse reaction. Because the rates of formation and destruction are equal, the amount of products remains constant. The same is true for the reactants; the forward reaction destroys the reactants, and the reverse reaction creates them, but the amount remains constant.
When the value of ΔG is negative, we say that the reactants are "favored" in the reaction. When ΔG is a positive number, the products are favored. When the products are favored, the mixture of chemicals at equilibrium will have a high percentage of products and a low percentage of reactants, and chemists will say that the equilibrium position "lies to the right." If the reactants are favored, the equilibrium state has a high percentage of reactants and a low percentage of products, and the equilibrium position "lies to the left." (Figure 9-10)
Figure 9-10. Energy Diagrams
Energy diagrams for two reactions. Notice that the equilibrium point lies toward the product side when ΔG is negative and toward the reactant side when ΔG is positive.
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To quantify exactly where the equilibrium lies, we can calculate the ratio of products to reactants in a very specific way. The next section will describe how this is done.