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## Chemical Thermodynamics

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**Chemical Thermodynamics**BLB 12th Chapter 19**Chemical Reactions**• Will the reaction occur, i.e. is it spontaneous? Ch. 5, 19 • How fast will the reaction occur? Ch. 14 • How far will the reaction proceed? Ch. 15**energy**heat work pathway state function system surroundings Review terms**exothermic**endothermic enthalpy enthalpy change standard state std. enthalpy of formation 1st Law of Thermodynamics Review terms, cont.**19.1 Spontaneous Processes**• Spontaneous – proceeds on its own without any outside assistance • product-favored; K > 1 • not necessarily fast • the direction a process will take if left alone and given enough time. • Nonspontaneous • opposite direction of spontaneous • reactant-favored; K < 1 • not necessarily slow**Spontaneity and Energy**• Examples of spontaneous systems: • Brick falling • Ball rolling downhill • Hot objects cooling • Combustion reactions • Are all spontaneous processes accompanied by a loss of heat, that is, exothermic? • Spontaneity is temperature-dependent.**Reversible & Irreversible Systems**• Reversible – a change in a system for which the system can restored by exactly reversing the change – a system at equilibrium ex. melting ice at 0°C • Irreversible – a process that cannot be reversed to restore the system and surroundings to their original states – a spontaneous process ex. melting ice at 25°C • See p. 788-790 (last paragraph of section)**19.2 Entropy and the 2nd Law of Thermodynamics**• Entropy, S – measure of randomness • State function • Temperature-dependent • A random (or dispersed) system is favored due to probability. “Entropy Is Simple – If We Avoid the Briar Patches” Frank Lambert, Occidental College, ret. http://entropysimple.oxy.edu**Entropy Change**• ΔS = Sfinal − Sinitial (a state function) (isothermal) as for phase changes. • ΔS > 0 is favorable**Problem 26 The freezing point of Ga is 29.8°C and the**enthalpy of fusion is 5.59 kJ/mol. • Is ΔS + or − for Ga(l) → Ga(s) at the freezing point? • Calculate the value of ΔS when 60.0 g of Ga(l) solidifies.**System & Surroundings**Dividing the universe: • System – dispersal of matter by reaction: reactants → products • Surroundings – dispersal of energy as heat**2nd Law of Thermodynamics**• The entropy of the universe increases for any spontaneous process. • ΔSuniv > 0 • ΔSuniv = ΔSsys + ΔSsurr • For a “reversible” process: ΔSuniv = 0. • For an irreversible process: • Net entropy increase ►spontaneous • Net entropy decrease ► nonspontaneous**19.3 The Molecular Interpretation of Entropy**• Molecules have degrees of freedom based upon their motion • Translational • Vibrational • Rotational • Motion of water (Figure 19.8, p. 796) • Lowering the temperature decreases the entropy.**Boltzman & Microstates**• S = klnW (W = # of microstates) • If # microstates ↑, then entropy ↑. • Increasing volume, temperature, # of molecules increases the # of microstates.**Examples of systems that have increased entropy**Entropy increases for: • Changes of state: solid → liquid → gas (T) • Expansion of a gas (V) • Dissolution: solid → solution (V) • Production of more moles in a chemical reaction (# of particles) • Ionic solids: lower ionic charge S° (J/mol·K) Na2CO3 136 MgCO3 66**2 NO(g) + O2(g) → 2 NO2(g)**S° + or −?**Problem 22 TNT (trinitrotoluene) Detonation**4 C3H5N3O9(l) → 6 N2(g) + 12 CO2(g) + 10 H2O(g) + O2(g) Spontaneous? Sign of q? Can the sign of w be determined? Can the sign of ΔE be determined?**3rd Law of Thermodynamics**• The entropy, S, of a pure crystalline substance at absolute zero (0 K) is zero.**19.4 Entropy Changes in Chemical Reactions**Standard molar entropy values, S° (J/mol·K): • increase in value as temperature increases from 0 K • have been determined for common substances (Appendix C, pp. 1059-1061) • increase with molar mass • increase with # of atoms in molecule**Calculating ΔS°sys**ΔS°sys = ∑nS°(products) - ∑mS°(reactants) (where n and m are coefficients in the chemical equation)**Entropy Changes in the Surroundings**• Heat flow affects surroundings. • As T increases, ΔH becomes less important. • As T decreases, ΔH becomes more important.**Calculating ΔS°univ**• ΔSuniv = ΔSsys + ΔSsurr by obtaining ΔSsys and ΔSsurr • If ΔSuniv> 0, the reaction is spontaneous. But there is a better way – one in which only the system is involved.**19.5 Gibbs Free Energy**• The spontaneity of a reaction involves both enthalpy (energy) and entropy (matter). • Gibbs Free Energy, ΔG makes use of ΔHsys and ΔSsys to predict spontaneity. • ΔGsys represents the total energy change for a system. • G = H – TS or ΔG = ΔH – TΔS • or, under standard conditions: ΔG° = ΔH° – TΔS°**Gibbs Free Energy**• If: • ΔG < 0, forward reaction is spontaneous • ΔG = 0, reaction is at equilibrium • ΔG > 0, forward reaction is nonspontaneous • In any spontaneous process at constant temperature and pressure, the free energy always decreases. • ΔG is a state function. • ΔGf° of elements in their standard state is zero.**Calculating ΔG°sys**ΔG°sys = ΔH°sys − TΔS°sys or ΔG°sys = ∑nΔG°f(products) - ∑mΔG°f(reactants) (where n and m are coefficients in the chemical equation)**Driving force of a reaction**For a reaction where ΔG < 0: • Enthalpy-driven – if ΔH < 0 and ΔS < 0; at low temp. • Entropy-driven – if ΔH > 0 and ΔS > 0; at high temp. • “cross-over point” is where ΔG = 0**19.7 Free Energy and K**• If conditions are non-standard: ΔG = ΔG° + RT lnQR = 8.3145 J/mol·K • If at equilibrium: ΔG = ΔG° + RT lnQ = 0 ΔG° = −RT lnK