Chemical Thermodynamics The concept of chemical thermodynamics deals with how the enthalpy change and entropy change of a chemical reaction are related. Some reactions occur automatically at a certain temperature and pressure. We call these reaction spontaneous. Spontaneous reactions are not spontaneous in reverse.

Chemical Thermodynamics Spontaneous Reactions – Proceed on their own without any assistance. o Ice melting (when T > 0°C). o The rusting of iron. o Which one of these is spontaneous? Water gets warmer if a hot object is placed into it. The decomposition of water into H 2 and O 2 at room temperature. The sublimation of dry ice at -100°C. (The freezing point of dry ice is -78°C.

Chemical Thermodynamics Entropy – A measure of the randomness of a system. o ΔS = S final - S initial (The entropy of a system depends only on the initial and final states of the system.) o For an isothermal process: ΔS = q reverse T

Chemical Thermodynamics What is the entropy change of 50.0 g of liquid mercury freezing? The freezing point of Hg = °C and ΔH fusion = 2.29 kJ/mol.

Chemical Thermodynamics Second Law of Thermodynamics – Spontaneous reactions, ones that are irreversible, always lead to an increase in the entropy of the system (ΔS = +). In other words, processes that occur naturally on their own always lead to more ‘disorder’.

Chemical Thermodynamics Entropy and Phase Changes – Predict whether each of the following phase changes produced an increase or decrease in the entropy of the system; o CO 2(s)  CO 2 (g) o CaO (s) + CO 2(g)  CaCO 3(s) o HCl (g) + NH 3(g)  NH 4 Cl (s) o 2SO 2(g) + O 2(g)  2SO 3(g)

Chemical Thermodynamics Third Law of Thermodynamics – The entropy of a pure crystalline solid at absolute zero is zero. At absolute zero, all molecular motion, rotational and translational, would cease.

Chemical Thermodynamics Entropy Changes in Chemical Reactions Standard molar entropies (ΔS°) – The value of the molar entropy of a substance at 298 K and 1 atm. They are experimentally determined. The ΔS° for an element in their free states is not zero. The ΔS° for gases are higher than the ΔS° of liquids or solids of the same element. ΔS° usually increases with increasing molar mass. ΔS° usually increases with increasing number of atoms in a a compound.

Chemical Thermodynamics Entropy Changes in Chemical Reactions ΔS° = Σ nΔS° (products) - Σ mΔS° (reactants) The standard entropy change of a chemical reaction is equal to the difference of the the standard entropy change of the products minus the standard entropy change of the reactants.

Chemical Thermodynamics Using the standard entropies in appendix C of your textbook, calculate the standard enthalpy change, (ΔS°), for the following reaction at 298K; Al 2 O 3(s) + 3H 2(g)  2Al (s) + 3H 2 O (g)

Chemical Thermodynamics Gibbs Free Energy The problem - Some endothermic reactions are spontaneous while others are not. Some exothermic reactions are spontaneous reactions while others are not. The entropy and enthalpy of a chemical Reaction determine its spontaneity. So how can we tell if a reaction will be spontaneous or not?

Chemical Thermodynamics Gibbs Free Energy (ΔG) – The amount of useable energy either released or absorbed in a chemical reaction. ΔG = ΔH – TΔS If ΔG > 0, reaction is not spontaneous If ΔG = 0, reaction is at equilibrium If ΔG < 0, reaction is spontaneous

Chemical Thermodynamics Calculate the standard free energy change for the formation of NO (g) from N 2(g) and O 2(g) at 298K. 2NO (g)  N 2(g) + O 2(g) Given that ΔH° = kJ and ΔS° = 24.7 J/K. Is the reaction spontaneous under these conditions?

Chemical Thermodynamics A particular reaction has ΔH° = 24.6 kJ and ΔS° = 132 J/K at 298 K. Calculate ΔG° to determine the spontaneity of the chemical reaction.

Chemical Thermodynamics Another way to calculate the ΔG° of a chemical reaction: ΔG° = Σ nΔG f ° (products) - Σ mΔG f ° (reactants) The change in the free energy of a chemical reaction is equal to the sum of the change of the sum of the free energy of the products minus the sum of the free energy of the reactants.

Chemical Thermodynamics Calculate the standard free energy change for the following chemical reaction at 298 K: P 4(g) + 6Cl 2(g)  4PCl 3(g)

Chemical Thermodynamics What is the standard free energy change for the reverse reaction at 298 K: P 4(g) + 6Cl 2(g)  4PCl 3(g)

Chemical Thermodynamics Free Energy and Temperature ΔHΔS-TΔSΔG Reaction Characteristics -+-- Spontaneous at all temperatures Nonspontaneous at all temperatures or + Spontaneous at low T Nonspontaneous at high T ++-- or + Spontaneous at high T Nonspontaneous at low T

Chemical Thermodynamics The Haber process for the production of ammonia involves the equilibrium N 2(g) + 3H 2(g)  2NH 3(g). Assume that ΔH° and ΔS° for this reaction do not change with temperature. a.) Predict the direction in which ΔG° changes with increasing temperature.

Chemical Thermodynamics The Haber process for the production of ammonia involves the equilibrium N 2(g) + 3H 2(g)  2NH 3(g). Assume that ΔH° and ΔS° for this reaction do not change with temperature. b.) Calculate ΔG° for this reaction at 25°C and 500°C.

Chemical Thermodynamics Free Energy and The Equilibrium Constant Most chemical reactions occur at temperatures other than 298K and pressure other than 1 atm. ΔG = ΔG° + R T ln Q ΔG = Free Energy at nonstandard conditions ΔG° = Standard Free Energy R = J/K.mol T = Temperature (K) Q = Reaction Quotient Q is calculated by setting up an equilibrium expression and plugging in either the concentration or pressures of the reactants or products at the current conditions.

Chemical Thermodynamics Free Energy and The Equilibrium Constant Most chemical reactions occur at temperatures other than 298K and pressure other than 1 atm. ΔG = ΔG° + R T ln Q ΔG = Free Energy at nonstandard conditions ΔG° = Standard Free Energy R = J/K.mol T = Temperature (K) Q = Reaction Quotient Q is calculated by setting up an equilibrium expression and plugging in either the concentration or pressures of the reactants or products at the current conditions.

Chemical Thermodynamics Free Energy and The Equilibrium Constant If a chemical reaction is at equilibrium, then we know that ΔG = 0. 0 = ΔG° + R T ln K (at equilibrium) or ΔG° = - R T ln K (at equilibrium) or K = e -ΔG°/RT (on equation sheet)