Chemical Thermodynamics Dr. Andrea Knorr Chapter 16

The First Law of Thermodynamics Energy cannot be created or destroyed, only ____________________ between a system and the surroundings. The energy in the universe is constant. Energy is conserved!

Spontaneous Processes Spontaneous processes occur without _______________________________________. Reactions are generally spontaneous in one direction and non-spontaneous in the other direction. This spontaneity is related to the thermodynamic path the system takes from the initial state to the final state.

Reversible vs. Irreversible Reversible process – both the system & the surroundings can be restored to their original states by exactly reversing the process. Irreversible processes – cannot return to the original process without a permanent change. in the surroundings.

Spontaneous Processes Any spontaneous process is irreversible (it’s not spontaneous in the other direction). An iron nail can react with H2O and O2 to form Fe2O3, but the reverse does not happen. Spontaneous processes may be fast or slow. The study of reaction rates is called _________________________________.

1. 2. 3. 4. 1. 2. 3. 4. Examples of Processes Spontaneous Non-spontaneous

The Second Law of Thermodynamics In any spontaneous process, there is always an increase in the entropy of the universe. Entropy is a measure of _______________________________________________. The change in entropy of the universe, ΔSuniv = ΔSsystem + ΔSsurroundings In an irreversible, (spontaneous) process, ΔSuniv > 0. In a reversible process, ΔSuniv < 0. If ΔSuniv = 0 __________________________________.

Entropy Changes in Surroundings _______________ _________________ At constant pressure Units are ___________

Entropy and Temperature Entropy changes in the surroundings are primarily determined by heat flow. Exothermic reactions in a system at constant temperature _______________________ the entropy of the surroundings. Endothermic reactions in a system at constant temperature _______________________ the entropy of the surroundings. The transfer of a given quantity of energy as heat either to or from the surroundings has a greater impact at lower temperatures.

Spontaneity can depend on the temperature.

Entropy’s Driving Force The driving force for a spontaneous process is an increase in the entropy of the universe. Entropy is a thermodynamic function describing the number of arrangements that are available to a system. Nature proceeds towards the states that have the __________________ probability of existing.

Molecular Motion Molecules can undergo 3 types of motion: A particular combination of motions and locations of the atoms in a system at a particular instant is called a microstate.

Positional Entropy The probability of occurrence of a particular state depends on the number of ways (microstates) in which that arrangement can be achieved. Entropy generally increases when liquids or solutions are formed from solids gases are formed from either solids or liquids the number of molecules of gas increases during a chemical reaction.

Constant Temp and Pressure In reactions involving gaseous molecules, the change in a positional entropy is dominated by the relative numbers of molecules of gas reactants and products 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g) 9 molecules 10 molecules ΔS increases

Gibbs Free Energy Gibbs Free Energy (G) is a thermodynamic state function that combines enthalpy and entropy. For a process at constant temperature & pressure, the sign of ΔG relates to the spontaneity of the process. ΔG <0, the process is spontaneous ΔG >0, the process is not spontaneous, but the reverse process is spontaneous ΔG =0, the process is reversible and ________________________________________

Free Energy Free energy is the amount of useful work that can be obtained from a process at constant temperature and pressure. In any spontaneous process at constant temperature and pressure, the free energy always decreases.

Gibbs Free Energy (G) Calculating Free Energy Change (G) (constant temperature and pressure) ΔG = ΔH – TΔS ΔG is the change in free energy in _______ Δ H is the change in enthalpy in _________ ΔS is the change in entropy in ________ T is temperature in __________

Spontaneity Can Be Predicted

Third Law of Thermodynamics The entropy of a perfect crystalline solid at 0 K is zero. Notice the column heading is S, not DS

Calculating Entropy Change in a Reaction ΔS°reaction = ΣnS°products - ΣnS°reactants Entropy is an ___________________ property (a function of the number of moles) Generally, the more complex the molecule, the higher the standard entropy value.

Example Calculate the entropy change for combustion of liquid ethanol {2(126.8) + 3(205)} = 314.6 J/K CO2(g) 214

Calculating Free Energy Change in a Reaction ΔG°reaction = ΣnΔG°products - ΣnΔG°reactants What is ΔG°rxn for combustion of gaseous propane? ΔG°reaction = {3(-394.4) + 4(-228.6)} -{1(-23.4) + 5( )} = -2074.2 kJ/mole ΔG°reaction can also be calculated in a manner similar to Hess’s Law

Free Energy and Work The maximum possible useful work obtainable from a process at constant temperature and pressure is DG. wmax = DG Processes are never 100% efficient, so you always get less work than the maximum possible For a spontaneous process, DG is the energy that is __________________________to do work. For a non-spontaneous process, DG is the energy that is __________________ to make the process occur.

Potential energy and Free energy – an analogy between the gravitational-PE change in a boulder rolling down a hill (position a) and the free energy change in a spontaneous reaction (b). The equilibrium position in (a) is given by the minimum gravitational PE available to the system. The equilibrium position in (b) is given by the minimum free energy available to the system.

Free Energy and Equilibrium

Free Energy as a Function of Pressure ΔG = ΔG° + RT ln Q R is the ideal gas constant, 8.314 J/mol-K Q is the reaction quotient that corresponds to the reaction mixture of interest Under standard conditions the concentrations of all the reactants and products are equal to 1. Therefore Q = 1, ln Q = 0, and ΔG = ΔG°. The “RT lnQ” term is like a correction factor that accounts for the non-standard pressure.

Free Energy as a Function of Pressure Let’s calculate DG for synthesis of methanol at 0.3 atm DGf° = -29kJ (see value of DGf° for CO, CH3OH in Appendix 4) Q = 1/{(PCO)(PH2)2}] = 1/{(0.3)(0.3)2}= 37 ΔG = ΔG° + RT ln Q ΔG = -29,000 + 8.314*298 ln (37) = -20kJ/molrxn **Note that ΔG is less negative than ΔG°, implying the reaction is less spontaneous at reactant pressures less than 1atm. This is expected, as lower pressures would lead to fewer molecule collisions.

Free Energy and the Equilibrium Constant The standard free energy for any reaction is related to the equilibrium constant. At equilibrium, ΔG = 0 and Q = K, the equilibrium constant. Therefore at equilibrium: ΔG° = -RT ln K

Free Energy and the Equilibrium Constant ΔG = ΔG° + RT ln Q vs. ΔG° = -RT ln K Case 1: ΔG° = 0 System is at equilibrium Case 2: ΔG° < 0 (K>1) System will shift right Case 3: ΔG° > 0 (K<1) System will shift left

Dependence of K on Temperature y m x b

Summary of Important Thermodynamic Quantities Quantity Change in Enthalpy Change in Entropy Change in Free Energy Symbol ΔH ΔS ΔG Unit kJ/mol J/mol K k J/mol Definition Heat gained by a system Change in randomness of a system Available useful work Comments + for endothermic - for exothermic + for increasing randomness - for decreasing randomness + for nonspontaneous for spontaneous ΔG = 0 at equilibrium