Appetizer: 3/30/15 Turn in Hess’s Law Lab. Get out your notes and a calculator and begin to work the problem. Determine the H for the reaction below. 2CO (g) + O2 (g)  2 CO2 (g) Hf (kJ/ mol) O2 (g) 0 CO2 CO (g) -110.5 CO2 (g) -393.5

Second Law of Thermodynamics "In any spontaneous process there is always an increase in the entropy of the universe" "The entropy of the universe is increasing" For a given change to be spontaneous, Suniverse must be positive Suniv = Ssys + Ssurr

18.4 Entropy For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid.Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

Entropy increases when a substance is divided into parts. 18.4 Entropy Entropy increases when a substance is divided into parts. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

18.4 Entropy Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

18.4 Entropy Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

Calculating Entropy Change in a Reaction Entropy is an extensive property (a function of the number of moles) Generally, the more complex the molecule, the higher the standard entropy value

What does G and/or G tell me?? The maximum useful work that can be extracted from a system at the given conditions Whether entropy or enthalpy is more important at the current state of the system Most importantly…If the reaction is spontaneous or not at the given conditions! * (+) G means NON-SPONTANEOUS * () G means SPONTANEOUS

Standard Free Energy Change G0 is the change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states G0 cannot be measured directly The more negative the value for G0, the farther to the right the reaction will proceed in order to achieve equilibrium Equilibrium is the lowest possible free energy position for a reaction

H, S, G and Spontaneity G = H - TS H is enthalpy, T is Kelvin temperature Value of H Value of TS Value of G Spontaneity Negative Positive Spontaneous Nonspontaneous ??? Spontaneous if the absolute value of H is greater than the absolute value of TS (low temperature) Spontaneous if the absolute value of TS is greater than the absolute value of H (high temperature)

For reactions at constant temperature: Calculating Free Energy Method #1 For reactions at constant temperature: G0 = H0 - TS0 A certain reaction takes place at 298 K. The standard enthalpy change, H, is measured as -45.2 kJ/mol and the standard entropy change, S, is found to be 62.5 J/molK at this temperature. Determine the value of the standard free energy change for the reaction.

Calculating Free Energy: Method #2 An adaptation of Hess's Law: Cdiamond(s) + O2(g)  CO2(g) G0 = -397 kJ Cgraphite(s) + O2(g)  CO2(g) G0 = -394 kJ Cdiamond(s) + O2(g)  CO2(g) G0 = -397 kJ CO2(g)  Cgraphite(s) + O2(g) G0 = +394 kJ Cdiamond(s)  Cgraphite(s) G0 = -3 kJ

Calculating Free Energy Method #3 Using standard free energy of formation (Gf0): * Gf0 of an element in its standard state is zero Use a table of standard free energy values to determine the free energy change for the reaction below at standard conditions. 2 SO2 (g) + O2 (g)  2 SO3 (g) Free Energy (kJ/mol) O2 (g) 0 SO2 (g) -300.19 SO3 (g) -371.08

Practice Quiz