SPONTANEOUS REACTIONS

Spontaneity 1 st Law of Thermodynamics- energy of the universe is ________. Spontaneous Rxns occur without any outside intervention. They may be fast; they may be slow What determines if a rxn is spontaneous?

Examples of spontaneous reactions A ball rolls down hill. Steel rusts An ice cube melts A gas fills a container A precipitate forms

Entropy (S) = Disorder 2 nd Law of Thermodynamics- reactions occur that produce an increase in the disorder of the universe. Reactions occur that favor disorder Where Δ S is + (disorder is ______) Gaseous CO 2 is more disordered than solid CO 2 Your organized locker becomes messy

Entropy Increases from solid to liquid to gas b/c more positions are available Mixing two substances increases entropy b/c it increases ____-, increases the positions that are available Heat has a greater impact on entropy at ____ temperatures (rich guy-poor guy)

Entropy Questions: #1 Which has the greater entropy? a) solid CO 2 or gaseous CO 2 b) N 1 atm or N atm #2 Predict the sign of entropy. a) Solid sugar is dissolved in water. b) Iodine vapor condensed on a cold surface to form crystals.

Entropy of the Surroundings The impact of the transfer of a given quantity of energy as heat to or from the surroundings will be greater at lower temp. ( rich guy- poor guy ) The sign of  S surr depends on the direction of heat flow (exo or endo) The size of  S surr depends on temperature (high temp vs. low temp)

 Ssurr  S surr = -  H T (-) = changes POV from system to surroundings  H = change in enthalpy of system

ENTROPY

Spontaneous rxns are favored… if the rxn is __________ (ΔH=-) if the rxn has ___________ (ΔS=+) And ultimately, it depends on GIBBS FREE ENERGY if Gibbs FREE ENERGY of the rxn is decreasing (ΔG = -), then the rxn is SPONTANEOUS

GIBBS FREE ENERGY ΔG = ΔH-TΔS See Appendix A-21 NOTE: ΔH and ΔG are in kJ ΔS is in J T is in Kelvin ΔH and ΔG for elements is zero 3 rd Law of Thermodynamics The entropy of a pure perfect crystal at 0K is zero (as the temp increases so does the entropy)

Reactions ΔH rxn = Σ(H f products )– Σ(H freactants ) ΔS rxn = Σ(S f products )– Σ(S f reactants ) ΔG rxn = Σ(G f products )– Σ(G f reactants )

Factors influencing spontaneity 1. Enthalpy ΔH 2. Entropy ΔS 3. Gibbs free energy ΔG

ΔG = ΔH-TΔS Case 1.+  S, -  H (-  G) 2.+  S, +  H (-  G) 3.-  S, -  H (-  G) 4. -  S, +  H (+  G) Result 1.Spontaneous at ___ temperatures. 2.Spontaneous at ____ temperatures. 3.Spontaneous at ____ temperatures. 4.NOT spontaneous at ANY temperature

Free Energy and Chemical Reactions  G˚ - Standard free energy change –The  G˚ when reactants˚ are converted to products˚ –Cannot be measured but can be calculated from measured quantities Method #1  G˚ =  H˚ - T  S˚ Method #2 Solve for  G using Hess’s Law Method #3 Calculate from G f ˚

Dependence of Free Energy on Pressure A constant temp. and pressure will proceed spontaneously in the direction that lowers the free energy (equilibrium) Free energy of a reaction system changes as the reaction proceeds b/c free energy is dependent on pressure of a gas and [species] G = H –TS H, pressure independent S, pressure dependent G =  G˚+ RT ln(Q)

 G =  G˚+ RT ln(Q)  G˚ is at 1 atm R = J/K mol Q – ___________ (______/_______)

Free Energy And Equilibrium When substances undergo a chemical reaction, the reaction proceeds to the minimum free energy (equilibrium), Where G products = G reactants  G = G prod – G react = 0 Then  G° = -RT(lnk) b/c  G = 0

Three Cases  G˚ = 0, K = 1  G˚ 1 (-  G)  G˚ > 0, K < 1 (+  G) 1.Equilibrium, no shift 2.Shifts _____  G˚ prod <  G˚ react 3.Shifts ______,  G˚ prod >  G˚ react