19-4 Criteria for Spontaneous Change: The Second Law of Thermodynamics. ΔStotal = ΔSuniverse = ΔSsystem + ΔSsurroundings The Second Law of Thermodynamics: ΔSuniverse = ΔSsystem + ΔSsurroundings > 0 All spontaneous processes produce an increase in the entropy of the universe.

Free Energy and Free Energy Change Chemistry 140 Fall 2002 Free Energy and Free Energy Change Hypothetical process: only pressure-volume work, at constant T and P. qsurroundings = -qp = -ΔHsys Make the enthalpy change reversible. large surroundings, infinitesimal change in temperature. Under these conditions we can calculate entropy.

Free Energy and Free Energy Change For the universe: TΔSuniv. = TΔSsys – ΔHsys = -(ΔHsys – TΔSsys) -TΔSuniv. = ΔHsys – TΔSsys For the system: G = H - TS ΔG = ΔH - TΔS ΔGsys = - TΔSuniverse

Criteria for Spontaneous Change ΔGsys < 0 (negative), the process is spontaneous. ΔGsys = 0 (zero), the process is at equilibrium. ΔGsys > 0 (positive), the process is non-spontaneous.

Table 19.1 Criteria for Spontaneous Change

19-5 Standard Free Energy Change, ΔG° The standard free energy of formation, ΔGf°. The change in free energy for a reaction in which a substance in its standard state is formed from its elements in reference forms in their standard states. The standard free energy of reaction, ΔG°. ΔG° = [ p ΔGf°(products) - r ΔGf°(reactants)]

Worked Examples Follow:

Chemistry 140 Fall 2002

Chemistry 140 Fall 2002

CRS Questions Follow:

At room temperature ~290 K the reaction of H2 and O2 to form water: 1. is spontaneous because it is exothermic. 2. is non-spontaneous because DS is negative. 3. is spontaneous because DS is negative. 4. is spontaneous because DG is negative. 5. is spontaneous because DG is positive.

At room temperature ~290 K the reaction of H2 and O2 to form water: 1. is spontaneous because it is exothermic. 2. is non-spontaneous because DS is negative. 3. is spontaneous because DS is negative. 4. is spontaneous because DG is negative. 5. is spontaneous because DG is positive.

Which of the following processes would you expect to be spontaneous at all temperatures? 1. 2. 3. 4. None are spontaneous at all temperatures. 5. All are spontaneous irrespective of the temperature.

Which of the following processes would you expect to be spontaneous at all temperatures? 1. 2. 3. 4. None are spontaneous at all temperatures. 5. All are spontaneous irrespective of the temperature.

A mixture of H2 and O2 can sit in a flask almost indefinitely at 298 K without reacting. What is the best explanation for the absence of observable reaction? 1. A significant energy barrier hinders the start of the reaction. 2. The reaction is not spontaneous at this temperature. 3. The reaction is entropically unfavorable. 4. All three of these factors contribute. 5. None of the above answers is correct.

A mixture of H2 and O2 can sit in a flask almost indefinitely at 298 K without reacting. What is the best explanation for the absence of observable reaction? 1. A significant energy barrier hinders the start of the reaction. 2. The reaction is not spontaneous at this temperature. 3. The reaction is entropically unfavorable. 4. All three of these factors contribute. 5. None of the above answers is correct.

There is enough energy in lightning bolts that O2 and N2 in the atmosphere are decomposed into N and O atoms. The reaction of N and O to form NO, 1. is spontaneous at all temperatures. 2. is non-spontaneous at all temperatures. is spontaneous at high temperatures but non-spontaneous at low temperatures. 4. is spontaneous at low temperatures but non-spontaneous at high temperatures. 5. It is impossible to choose between the above responses without thermochemical data.

There is enough energy in lightning bolts that O2 and N2 in the atmosphere are decomposed into N and O atoms. The reaction of N and O to form NO, 1. is spontaneous at all temperatures. 2. is non-spontaneous at all temperatures. is spontaneous at high temperatures but non-spontaneous at low temperatures. 4. is spontaneous at low temperatures but non-spontaneous at high temperatures. 5. It is impossible to choose between the above responses without thermochemical data.