Physical Chemistry for the Chemical and Biological Sciences

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Section 8.0: CHEMICAL EQUILIBRIUM Chapter 9 in Chang Text
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…………. .the direction of spontaneous change at constant T and P is towards lower values of Gibbs Energy (G)……………. this also applies to chemical reactions
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Gibbs energy of reaction (G)……………… ……consider the reaction A  B …..if an infinitessimal amount of A turns into B then the change in the amount of A may be expressed as: d n A = - d ….and the change in the amount of B present ….is d n B = + d …. where quantity is called the extent of the reaction (its units are moles) When extent of reaction changes by finite amount  , the amount of A and B present may be expressed as……… n A -  n B + 
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Gibbs Energy of Reaction ( R G) is defined as slope of plot of Gibbs Energy (G) versus the extent of the reaction ( ) ………. . R G = [ G /  ] P,T ( here signifies derivative of slope of G wrt ) …..so if a reaction progresses by d then the change in Gibbs energy can be written…. . dG = A dn A + B dn B = - A d + B d = ( B - A ) d [ G / ] P,T = ( B - A ) so R G = ( B - A ) ….. R G is related to difference in chemical potentials of reactants and products at the specific composition of reaction mixture.
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A B R G = ( B - A ) Rxn. spontaneous in forward direction when A  B Rxn. spontaneous in reverse direction when B A At Equilibrium: R G = 0 when A = B • chemical potential varies with composition • slope of G vs. changes as rxn. proceeds G vs Figure from Atkins Text
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At constant T and P: R G 0 frwd. rxn is spontaneous Rxn. is exergonic …can be used to drive another reaction. R G 0 reverse. rxn is spontaneous Rxn. is endergonic R G = 0 at equilibrium Figure from Atkins Text
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An example of an exergonic rxn is ATP hydrolysis: ATP + H 2 O  ADP + P i + H 3 O + ATP to ADP, with Δ G ° = – 30 kJ (at 37 ° C) Δ H ° = - 20 kJ ; Δ S ° =+ 34 J /K This exergonic rxn. can couple with endergonic reactions, and help drive processes or cascades of reactions. 30 kJ is available for driving other reactions. ***since Δ S ° is large the ……. . Δ G ° is sensitive to T. We can use a favourable rxn. to ‘DRIVE’ an un-favourable one …….exergonic rxns. can be used to drive endergonic rxns…….
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Biological Energy Conversion • in cells energy released by the oxidation of foods is stored in ATP • in the cell each ATP molecule can be used to drive endergonic reactions ….where Δ G ° does not exceed 30 kJ example. Biosynthesis of proteins is highly endergonic (due to enthalpic contribution as well as decrease in entropy that accompanies assembly of amino acids into a specific sequence). Formation of a peptide link is endergonic and will typically consume approx. 3 ATP molecules for each link.
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8 - Section 8.0: CHEMICAL EQUILIBRIUM Chapter 9 in Chang...

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