Thermodynamics

1. Production of quicklime Thermodynamics Liquid benzene ⇅ Solid benzene Chapter 19 CaCO3 (s) ⇌ CaO + CO2

2. Kinetics → How fast does a reaction proceed? Thermodynamics → Does a reaction proceed? Three Laws of Thermodynamics 1st Law: Energy is conserved in any process 2nd Law: Defines a “spontaneous” process 3rd Law: Defines absolute disorder

3. Fig 19.1 Spontaneous and nonspontaneous processes

4. nonspontaneous spontaneous Spontaneous Physical and Chemical Processes

5. Reversible Processes • System changes in such a way that system and surroundings can be put back in their original states by exactly reversing the process.

6. Irreversible Processes Fig 19.5 • Irreversible processes cannot be undone by exactly reversing the change to the system • Spontaneous processes are irreversible

7. S order disorder S qrev T S = Entropy (S) - measure of randomness or disorder of a system State functions - properties that are determined by the state of the system, regardless of how that condition was achieved. DS = Sfinal - Sinitial For an isothermal process: at constant T

8. How does the entropy of a system change for each of the following processes? (a)Condensing water vapor Randomness decreases Entropy decreases (DS < 0) (b)Forming sucrose crystals from a supersaturated solution Randomness decreases Entropy decreases (DS < 0) (c)Heating hydrogen gas from 60°C to 80°C Randomness increases Entropy increases (DS > 0) (d)Subliming dry ice Randomness increases Entropy increases (DS > 0)

9. First Law of Thermodynamics Energy can be converted from one form to another but energy cannot be created or destroyed. Second Law of Thermodynamics The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. DSuniv = DSsys + DSsurr > 0 Spontaneous process: Equilibrium process: DSuniv = DSsys + DSsurr = 0 For any process: DSuniv = DSsys + DSsurr≥ 0

10. Entropy on the Molecular Scale • Molecules exhibit several types of motion: • Translational: Movement of the entire molecule from one place to another. • Vibrational: Periodic motion of atoms within a molecule. • Rotational: Rotation of the molecule on about an axis or rotation about  bonds.

11. Entropy on the Molecular Scale • Boltzmann envisioned the motions of a sample of molecules at a particular instant in time • e.g., taking a snapshot of all the molecules • This sampling ≡ a microstate of the thermodynamic system

12. Entropy on the Molecular Scale • Each thermodynamic state has a specific number of microstates, W, associated with it • Entropy ≡ S = k lnW where k is Boltzmann constant, 1.38  1023 J/K Wfinal Winitial • Change in entropy for a process: • S = klnWfinalklnWinitial S = k ln

13. Entropy on the Molecular Scale • Entropy increases with the number of microstates in system • Number of microstates and, therefore, the entropy tends to increase with increases in: • Temperature • Volume • Number of independently moving molecules

14. Entropy and Physical States • Entropy increases with the freedom of motion of molecules S(g) > S(l) > S(s) • Generally, when a solid is dissolved in a solvent, entropy increases.

15. Entropy Changes Fig 19.11 • In general, entropy increases when • Gases are formed from liquids and solids • Liquids or solutions are formed from solids • Number of gas molecules increases • Number of moles increases

16. Third Law of Thermodynamics The entropy of a pure crystalline substance at absolute zero is 0. Fig 19.13 Perfectly ordered crystalline solid at and above 0 K

17. Fig 19.14 Entropy as a function of temperature