From the table above, ∆Hºf(C6H12O6) = -1260.0 kJ/mol: This is shorthand for: This illustrates: Hess’s Law:∆Hºrxn= This same principle applies to Free Energy and to Entropy: ∆Gºrxn= ∆Sºrxn=
17 - 9 Example: Determine ∆Hºrxn, ∆Gºrxnand ∆Sºrxnfor the combustion of one mole of methane, CH4. ∆Hºrxn= ∆Gºrxn= ∆Sºrxn= Example: How could you determine ∆Gºrxnfor this reaction using a different approach? Four other important things to notice from the table of thermodynamic data: 1. The table does not give ∆Sº, but rather2. There’s no subscript “f” in the entropy column:3. Notice the units: 4. Compare the magnitudes of ∆Gº and ∆GImportant: ∆H, ∆S, and ∆G all depend on ___________________________ ________________________For example, changing the pressure will change _______________. Since conditions must be ___________________, we often use _________________ ___________ ________________.
17 - 10 Standard state conditions: • solids and liquids:• gases:• solutes:• temperature must ______ _____________________ ∆Gº -- is the free energy change that occurs when _________________ in their standard states are _______________ ___________________ to _______________________ in their standard states. Example: N2(g) + 3 H2(g) → 2 NH3(g) ∆Gº = -33.0 kJ When is ∆G = -33.0 kJ for this reaction? Only when _______________ N2(g) at a partial pressure of ___________ reacts with _______________ H2(g) at a partial pressure of ___________ to form _______________ NH3(g) at a partial pressure of _________________, and when all reactants and products are at __________________. Thus, ∆Gº is often an ________________ _______________ or a ________________ __________________.So, an important question: What is the relationship between this “idealized value” (∆Gº) and the “real world” free energy change (∆G) for a real reaction?∆G = where ∆G =∆Gº =R = T = Q = Example: 2 NH3(g) + CO2(g) → NH2CONH2(aq) + H2O(l) Given that the standard free energy change for this reaction is ∆Gº = -13.6 kJ, what is the value of ∆G when PNH3= 10. atm, PCO2= 10. atm, [NH2CONH2] = 1.0 M?
17 - 11 Free energy and equilibrium Five-star equation:***** or ***** Three-star equation:*** If the system is at equilibrium, then: ∆G =Q = and
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