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Unformatted text preview: Chem 116 Lecture 24 Notes (TC) December 12, 2008 Voltage: ‐ Standard Cell Potential = E°cell ‐ E°cell = In relation to ‐ ∆G is related to ∆H, ∆S, and T. Calculating Ecell for non‐standard conditions ‐ When it’s at 25 C but not at a standard concentrations of 1.0M: > Ecell = E°cell – (0.0257/n) lnQ ‐ or ‐ > Ecell = E°cell – (0.0592/n) logQ When it’s also not a standard temperature of 25°C: > Ecell = E°cell – (RT/nF) lnQ > F = Faraday’s constant = 96,500 coulombs/mol Electrolysis: ‐ Electrolysis reactions: reactions in which a non‐spontaneous redox reaction is brought about by the passage of a current under a sufficient external electric potential. ‐ Electrolytic cells: This is where the same process of electrolysis occurs. > contains 2 electrodes (anode and cathode). > anode is where oxidation occurs (negative side of voltage source) > cathode is where reduction occurs (positive side of voltage source) ‐ Equations: > Coulombs = amperes * seconds > Electrical work: > ∆G= ‐nFE = wmax [the maximum useful electrical work obtained from a voltaic cell] > w = nFEext [when external potential applied to cell] Absolute Entropy: ‐ Symbol = (S) ‐ More disordered = higher absolute entropy ‐ More ordered = lower absolute entropy ‐ Absolute order = Perfect crystal lattice structure with no motion at zero absolute. ‐ No such thing as (absolute entropy = 0) except at the reference state of absolute order. ‐ The more complex the molecular structure: > higher the absolute entropy of the substance. > more options for configurations, rotations, and vibrations. ‐ (S) = (k)(ln W) this is Boltzmann’s equation and it’s written on his gravestone ‐ W = degrees of freedom (a measure of how much freedom there is for particles in the system to rotate, vibrate, and move in other ways) > influenced by temperature, volume, and number of independently moving particles. > if W increases, (S) increases. ‐ ∆S > 0 = a change where the system becomes more disordered ‐ ∆S < 0 = a change where the system becomes more ordered (less disordered) ‐ Hess’s law applies to entropy because it is also a state function. Gibbs Free Energy (∆G) ‐ Measures absolute entropy change of the entire universe (actually, it’s the opposite sign of this ‐ according to the second law of thermodynamics, during any process the entropy of the entire universe increases, and during any spontaneous process the Gibbs free energy decreases) ∆G for the system is negative for any change of the system that is spontaneous ‐ Equation: > ∆G = ∆H – T ∆S > ∆G = Gibbs Free Energy > ∆H = Enthalpy change > ∆S = Entropy change > T = Absolute temperature (in Kelvin) ‐ So ∆G depends on enthalpy change and entropy change ∆G and Keq ‐ When the system is at another temperature other than standard temperature of 25°C > ∆G = ∆G° + (RT)(lnQ) ‐ At equilibrium ∆G =0 and Q =Keq > ∆G° = ‐ (RT)(lnKeq) ...
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- Fall '08