1. Chemistry 130 Chemical Equilibrium Dr. John F. C. Turner 409 Buehler Hall jturner@ion.chem.utk.edu

2. Chemistry 130 Chemical change and equilibrium Transition state theory shows that there are always two reactions associated with a chemical change – the forward reaction and the reverse reaction. The forward reaction has a rate constant k 1 and the reverse reaction k -1 We can write a rate law for these two reactions: and the precise concentrations at equilibrium will be determined by the activation energies for the forward and reverse reactions

3. Chemistry 130 Chemical change and equilibrium We understand the relationship between the rate constant k and the activation energy; it is given by the Arrhenius equation: When the rates of the forward and reverse reactions are equal, the rates of formation and destruction of products are equal – we say that the system is at equilibrium.

4. Chemistry 130 Chemical change and equilibrium When the rates are equal, and so then

5. Chemistry 130 Chemical change and equilibrium When the rates are equal, and so then

6. Chemistry 130 Equilibrium constants As the rates are equal at equilibrium, then the K eq is a fundamental constant of the particular reaction and we call it the equilibrium constant. In general, the equilibrium constant for a reaction is Often we do not know k 1 or k -1 but we can easily measure the ratio of the two through the concentrations of the reactants and the products.

7. Chemistry 130 Equilibrium constants For a general reaction the equilibrium constant is given by In general, the equilibrium constant is written in terms of concentrations, which at their most basic are given by

8. Chemistry 130 Equilibrium constants The equilibrium constant for a reaction is useful in many ways and occurs in both thermodynamics and kinetics. The first calculation that it allows us to perform is the calculation of concentration. At 698 K, hydrogen iodide decomposes via Given

9. Chemistry 130 Equilibrium constants Rules for equilibrium constants If you reverse a reaction as written, the equilibrium constant is inverted. If you add two or more reactions together, the equilibrium constants are multiplied. If you muliply a single reaction by a number n, you raise the equilibrium constant to the power n

10. Chemistry 130 Equilibrium constants A very large or very small equilibrium constant describes a reaction that has gone to completion or one that does not perceptibly occur: For which means that the combustion of hydrogen and oxygen is essentially complete. For the reverse reaction, which implies that the reverse reaction is negligibly important.

11. Chemistry 130