Thermodynamics Chapter 19 Liquid benzene Production of quicklime Solid benzene ⇅ CaCO 3 (s) ⇌ CaO + CO 2

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

Fig 19.1 Spontaneous and nonspontaneous processes

spontaneous nonspontaneous Spontaneous Physical and Chemical Processes

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.

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

Entropy (S) - measure of randomness or disorder of a system orderS disorder S  S = S final - S initial For an isothermal process:  S = q rev T State functions - properties that are determined by the state of the system, regardless of how that condition was achieved. at constant T

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

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.  S univ =  S sys +  S surr > 0 Spontaneous process:  S univ =  S sys +  S surr = 0 Equilibrium process:  S univ =  S sys +  S surr ≥ 0 For any process:

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.

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 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 Change in entropy for a process:  S = k lnW final  k lnW initial  S = k ln W final W initial

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

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.

 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 Fig 19.11

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

Fig Entropy as a function of temperature