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Ch_322a_10.06

Ch_322a_10.06 - CH 4(g Cl 2(g CH 3 Cl(g HCl(g S o standard...

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The Gibbs Free Energy Change The chemical driving force for a reaction actually is the Gibbs Free Energy Change, ! G . A negative value indicates conversion of reactants to products is thermodynamically favorable. Of course, this parameter does not indicate the rate of the reaction. Although ! G is the parameter that describes reaction spontaneity, the more readily calculated Standard Free Energy Change, ! G o , often is used. ! G = ! G o + 2.303RT(Log Q ) where Q is the reaction quotient Components of the Standard Free Energy Change ! G o = ! H o - T ! S o In many reactions, the Standard Entropy Change, ! S o , is small, so the term T ! S o is small relative to ! H o , and ! G o ! H o . ~ ~ ! S o typically is small in reactions where the molecularity (number of different species) of the reactants and products is the same .
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An Analysis of the Chlorination of Methane Reaction The entropic contribution ( ! S o ) to the ! G o for the chlorination of methane is very small.
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Unformatted text preview: CH 4(g) + Cl 2(g) CH 3 Cl (g) + HCl (g) S o standard entropy 44.50 (cal/deg mol) 53.2 55.97 44.62 For the above reaction, the change in Standard Entropy is ! S o = " S o " S o products reactants ! S o = (55.97 + 44.62) - (44.50 + 53.2) cal/deg ! S o = +2.8 cal/deg mol At 25 o C (298 K), ! G o = ! H o- T ! S o = (-24.5 kcal/mol) - (298 deg)(2.8 cal/deg mol) ! G o =-24.5 kcal/mol - 0.834 kcal/mol = -25.3 kcal/mol therefore, for the chlorination reaction ! H o ! G o ~ ~ General Observations When the molecularity of reactants and products is the same, in reactions involving molecules of comparable size, the standard entropy change, ! S o , is close to zero. Because of the convenience in evaluating reaction mechanisms using the standard enthalpy change, ! H o , rather than the standard free energy change, ! G o , entropy changes will generally be ignored....
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Ch_322a_10.06 - CH 4(g Cl 2(g CH 3 Cl(g HCl(g S o standard...

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