1. Chapter 20
Energy and Disorder

2. Objectives
Understand the concepts of enthalpy, entropy, and free energy and the relationship among them
Use these concepts to solve problems dealing with any of them
Understand energy as it is related to chemical systems

3. Why Reactions Occur
Exothermic reactions generally take place spontaneously
Endothermic reactions generally do not take place spontaneously
Natural processes tend to go from an orderly state to a disorderly one
High energy  low energy
Order  disorder

4. Isothermal Processes – reactions taking place at constant temperature
Isobaric Processes – reactions taking place at a constant pressure
Thermodynamics – the study of the flow of energy

6. State Functions
A state function is one whose value depends only on the current state of the system
T = T2 – T1
V = V2 – V1
P = P2 – P1

7. Internal Energy Every system has some internal energy
Internal energy, U, is a state function
Ways to transfer energy
By heating the system/surroundings
By doing work

8. U = q + w q = heat gained by the system
w = the amount of work done on the system
Neither q nor w are state functions
q has a positive sign if heat is flowing in, negative if heat is flowing out
w is negative if work is done on the surroundings, and positive if done on the system

9. Assignment 1-5 page 394 Read through the rest of chapter 20
Due: Tomorrow

10. Enthalpy Enthalpy (H) = U + PV Enthalpy is a state function
H = H2 – H1
Exothermic Rxn: H < 0
Endothermic Rxn: H > 0

11. Enthalpy Change

12. Which is Which?

13. What about this one?

14. Standard States
Standard state refers to the enthalpy substances have at K and kPa
Not the same as with the gas laws
In measuring enthalpy, set the enthalpy of any free elements to be equal to zero
A free element is one that is not in a compound

15. Enthalpy of Formation
Enthalpy of formation is the change in enthalpy when one mole of a compound is produced from free elements in their standard states
Units: KJ/mole
Symbol: Hfº
º means at standard state
Thermodynamic stability depends on the amount of energy that would be required to decompose the compound
See table A-6 in appendix for values
Thermodynamically stable compounds have large negative enthalpies of formation

16. Calculation of Enthalpy of Reaction
Hfº (products) = Hfº (reactants) + Hrº
 means summation
Hrº means change in enthalpy
If the enthalpy of formation of each product is known, you can calculate the amount of energy produced or absorbed, which then tells you if the reaction will be endothermic or exothermic
Assignment: Due at end of class
6-7 page 397

17. Hess’s Law
Hess’s Law – The enthalpy change for a reaction is the sum of the enthalpy changes for a series of reactions that add up to the overall reaction

18. Consider reaction A  C Break into two parts
(1)AB and (2)B  C
Hr(1) = Hf°B - Hf°A
Hr(2) = Hf°C - Hf°B
So, the enthalpy change for the overall change of A to C is
H° = Hr(1)° + Hr(2)°

19. Entropy
Entropy, S, is derived from the second law of thermodynamics. This law places limits on the conversion of heat into work and prohibits perpetual motion
Entropy (S) is a measure of disorder in a system
Entropy is a state function

20. Entropy cont. Examples of S > 0 = Increase in disorder-
S > 0 = Increase in disorder
S < 0 = Decrease in disorder
Assignment: Due at end of class
8-10 pg 400
10 points

21. Gibbs Free Energy
Gibbs free energy determines whether a reaction will occur or not
G = H - T S
If G < 0 the reaction is exergonic (spontaneous)
If G > 0 the reaction is endergonic
The reaction can only occur is T S is very large
If G = 0 the system is at equilibrium

22. Gibbs Free Energy Calculations
Appendix A-6
Gr° =  Gf°(products) -  Gf°(reactants)
Assignment
Problems pg

23. Chapter Review
Complete questions