1. Chapter 19, Part III Spontaneous vs. Non-spontaneous Entropy vs. enthalpy

2. How do we know a reaction occurs? Free energy—energy available to do work Spontaneous reaction—a reaction that will occur naturally and favors the formation of the product (K eq >1) All spontaneous reactions release free energy!

3. How do we know a reaction occurs? Nonspontaneous reaction—a reaction that does not favor formation of the product and does not occur naturally. Reactions can be spontaneous at one set of conditions and not at another. rea Sometimes nonspontaneous reactions can occur when coupled with spontaneous reactions.

4. NOTE: This does NOT refer to speed of reaction, only whether or not it naturally occurs.

5. LOOKING BACK: Remember that enthalpy change (  H) is the measure of change in heat content for reaction. Since all spontaneous reactions release free energy, you could expect that they would always be exothermic (release heat = -  H).

6. This is NOT the case, for example, the melting of ice to water requires energy, but it happens spontaneously. Thus, enthalpy change is not the only factor that determines whether or not a reaction will be spontaneous.

7. ENTROPY The law of disorder states that all processes occur in such a way that they move towards maximum disorder (randomness). Entropy is the measure of the amount of disorder in a system. spo A reaction that absorbs energy can be spontaneous if entropy increases.

8. How does entropy increase? Changing to a less organized phase Solid changing to liquid or gas Liquid changing to gas Substance is divided into parts NaCl is separated to Na + and Cl - in solution The product has more molecules than the reactants 2H 2H 2 O  2H 2 + O 2

9. Entropy also increases as heat is applied to a system.

10. Spontaneous or nonspontaneous So, to decide whether a reaction is spontaneous or not, you have to look at both the change in enthalpy (heat) and the change in entropy (disorder)

11. Exothermic + entropy increase = Spontaneous reaction Endothermic + entropy increase = Spontaneous reaction ONLY if entropy change is more than heat absorbed

12. Exothermic + entropy decrease = spontaneous reaction ONLY if heat change is more than increased order Endothermic + entropy decrease = nonspontaneous reaction

13. CALCULATING ENTROPY CHANGE Entropy is symbolized by S. Standard entropy (entropy @ 25 o C & 101.3kPa) is S o  S o = S o prod - S o react

14. Practice Problem #1 Calculate the standard entropy change that occurs when 1 mol H 2 O(g) condenses to 1 mol H 2 O(l).

15. Practice Problem #2 NO(g) reacts with O 2 (g) to form NO 2 (g). What is the standard entropy change for the reaction at 25 o C and 101.3kPa.

16. CALCULATING FREE ENERGY Gibbs free energy is  G. Because free energy is released in a reaction, when  G is negative (free energy is released) then the reaction is spontaneous.  G =  H – T  S (T in Kelvin) At 25 o C & 101.3kPa,  G o =  H o – T  S o Also,  G o =  G o prod –  G o react

17. Practice Problem #3 For the reaction C(s) + O 2 (g)  CO 2 (g), find  G o at 25 o C, and tell whether or not the reaction is spontaneous.

18. Practice Problem #4 Tell whether or not the following reaction is spontaneous at 25 o C using the equation  G =  H - T  S: 2Cl 2 (g) + 2H 2 O(l)  4HCl(g) + O 2 (g)