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Dispelling a Myth

Dispelling a Myth. (tetrahedral carbon). (aromatic carbon). …diamonds last forever…. Diamond . Graphite. time favors graphite formation!. Take Home: If you want to buy something that lasts forever, avoid diamonds and buy a pencil…but don’t use it. Energy Transformations.

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Dispelling a Myth

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  1. Dispelling a Myth (tetrahedral carbon) (aromatic carbon) …diamonds last forever…. Diamond Graphite time favors graphite formation! Take Home: If you want to buy something that lasts forever, avoid diamonds and buy a pencil…but don’t use it.

  2. Energy Transformations Concept of Free Energy Enthalpy Energy locked in chemical bonds Entropy Energy of molecular organization DG The energy differential between products and reactants TAKE HOME: Energy cannot be realized unless there is a change in chemical form

  3. G(rxn) = H(rxn) - TS(rxn) What is Free Energy? Rule: Nature acts to minimize the energy and to maximize the entropy of all processes. Rule: A reaction in which the total energy of the products is less than the total energy of reactants is energy favored Rule: If there is little or no change in entropy (S ~0) then the reaction is considered enthalpy driven. Little or no change in the enthalpy means the reaction is entropy driven. Rule: When both enthaply change and entropy change are taken into account, the compromise function is called the free energy of the reaction.

  4. A B There are 3 quantities in the above expression. What are they? 1. The absolute value of A 2. The absolute value of B 3. The difference in the value of B relative to A, i.e., B - A Free energy determinations rest on differences between products and reactants, as symbolized by a Δ

  5. C6H12O6 + O2 2CO2 + 2C2H5OH Enthalpy driven Entropy driven C2H5OH + 3O2 2CO2 + 3H2O G = -1326 KJ/mol Enthalpy driven N2O5 2NO2 + 1/2O2G = -30 KJ/mol Entropy driven G(rxn) = H(rxn) - TS(rxn) G = -218 KJ/mol H = -82KJ/mol TS = 136KJ/mol H = -1367KJ/mol TS = -41KJ/mol H = +110KJ/mol TS = +140 KJ/mol

  6. G(rxn) = H(rxn) - TS(rxn) G(rxn) < 0 Reaction is spontaneous and energy can be used or stored to drive endergonic reactions (negative) G(rxn) > 0 Reaction is not spontaneous and no additional products can form without energy input (positive) G(rxn) = 0 Reaction is at equilibrium and no further net change occurs (neither)

  7. Standard State value (-/+) A B [B] DG = DGo + 2.303RT log [A] DG Measures distance from equilibrium True or False: Free energy changes during the course of a reaction (We either add to or subtract from the Standard State Go) 1. Free energy change at any time depends on [B]/[A] 2. Large [A] = large negative log term 3. Large [B] = large positive log term 4. When [B] = [A] set at one molar, DG = DGo 5. When [B]/[A] reaches equilibrium conc., DG = 0 The further away from equilibrium, the greater the value (either + or -) for DG A reaction at equilibrium yields no free energy

  8. B A [B] [B]eq DG = DGo + 2.303RT log Keq = [A] [A]eq See Biochemical Strategies, p.170 Standard Free Energy in Chemical Change At Equilibrium : G = 0 and … Go = –2.303RT log Keq If at equilibrium [B] > [A] DGo is negative Exergonic If at equilibrium [B] < [A] DGo is positive Endergonic If at equilibrium [A] = [B] DGo is 0 Only when the products have less energy than the reactants can energy be made available

  9. [B] DG = DGo + 2.303RT log [A] Problems will be worked at recitation sessions Sample Problems R = 8.314 Joules/mol x degree T = K (degrees Kelvin where 273 K = 0oC) Suppose equilibrium for a reaction is reached when B attains 10 times the concentration of A. Calculate the following at 37oC. (1) Go = _______ (2) log of [B]/[A] = _______ (3) G when [B]/[A] is 10:1= ___ (4) G when B is 0.1A= _____ (5) G when A is 0.01B = ____ (6) B/A to yield is -30.5 KJ/mol (energy needed to make ATP)

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